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ABSTRACT
ABUNDANCE, MOVEMENTS, DIVE BEHAVIOR, FOOD HABITS,
AND MOTHER-PUP INTERACTIONS OF HARBOR SEALS
(PHOCA VITULINA RICHARDS!) NEAR MONTEREY BAY,
CALIFORNIA
Data concerning movements of radio-tagged harbor seals (Phoca vitulina
richardsi), along with abundance, food habits, and mother-pup suckling behavior
were analyzed to better understand the ecology of harbor seals in Monterey Bay,
California. Based on a mean index of relative importance (IRI) using 222 fecal
samples, Sebastes sp., flatfishes (Pleuronectidae and Bothidae) and Chilara taylori
were the primary fishes consumed. Lolig:o QPalescens and Octopus rubescens were
the primary prey species consumed before pupping. Female harbor seals
abandoned their pups during mid-lactation, presumably foraging. Diel
observations were conducted on harbor seal mother-pup pairs during the 1992
pupping season at South Fanshell Beach, Monterey Bay, California. Mean
proportion of animals suckling per hour was significantly greater for diurnal
periods (X= 0.51 h-1, SE = 0.28-1) compared with nocturnal periods
(X= 0.23 h-1, SE = 0.19-1; z = 3.35, p < 0.05). Diurnal suckling was 117% greater
than nocturnal, which may prove important in energetic studies.
Stephen John Trumble
August 1995
ABUNDANCE, MOVEMENTS, DIVE BEHAVIOR, FOOD HABITS,
AND MOTHER- PUP IN1ERACTIONS OF HARBOR
SEALS <PHOCA VITIJLINA RICHARDSD NEAR
MONTEREY BAY, CALIFORNIA
by
Stephen John Trumble
A thesis
submitted in partial
fulfillment of the requirements for the degree of
Master of Science in Marine Sciences
in the School of Natural Sciences
California State University, Fresno
August 1995
ACKNOWLEDGMENTS
Special thanks are extended to Dr. James T. Harvey for his many
invaluable contributions to this work. Acknowledgments are also given to
committee members Dr. Gregor Cailliet and Dr. David Grubbs for their assistance
with this work. Special gratitude to Dion Oxman for all of his enthusiasm, hard
work assisting me in the field, analyzing data, editing, and just being a great
friend. Special recognition is also extended to Sheila Baldridge and Alan
Baldridge for providing me with all their expertise in marine science. I am
indebted to Gail Johnston and Virgie Lopez for getting me through the CSUF
paper nightmare. Thank you to Bob Huettman and Gene Fryberger (and the Seal
Watch group) for the use of the blind at S. Fanshell Beach and for all of your
assistance and knowledge. A special thank you to the staff at MLML for the use
of the vehicles, boats, and repairing an occasional bent propeller.
Without the student body this project would have been impossible. The
students/friends who have contributed valuable time and effort into this project
are: Doreen Moser, Dion Oxman, John Mason, Patience Browne, Matt Burd, Eric
Johnson, Mike Torok, Tony Orr, Torno Eguchi, Meg Lamont, Kim Raum-Suryan,
Rob Suryan, Lisa Landon and the crew at UC Davis, Tony Bennett, Eric Dorfman,
Steve Osborne, Michele Lander, Tom Norris, Cheryl Baduini, Sal Cherchio, and
the rest whom I have forgotten to mention.
The Dr. Earl H. Myers and Ethel M. Myers Oceanographic and Marine
Biology Trust, the American Cetacean Society (Monterey Bay Chapter), Moss
Landing Marine Labs for providing needed moneys for this project. Special
v
thanks to Jay Barlow at Southwest Fisheries (NMFS) who provided the radio
tags instrumental in the movement and dive behavior aspects of this study.
I am completely indebted to Cindy Trumble for her continued support over
the past few years, financially and emot;ionally. Thank you, Kristen L. Trumble, for
inspiration.
TABLE OF CONTENTS
Page
LIST OF TABLES
LIST OF FIGURES
Vll
viii
INTRODUCTION
1
MATERIALS AND l'vffiTHODS
9
RESULTS .
17
DISCUSSION .
27
LITERATURE CITED
50
APPENDICES
62
A. TABLES
63
B. FIGURES
73
LIST OF TABLES
Table
1. Monthly mean dive informatio"n including number of dives per
tracking(# of Dives), mean dive and surface interval (SI) duration,
percentage of time spent hauled-out and diving, and percentage
time spent diving at night for individual harbor seals tagged near
Monterey Bay in 1992. .
Page
64
2. Mean dive and haul-out information for harbor seals including mean
hours hauled-out per harbor seal, mean individual haul-out time, total
hours spent diving, and hours spent diving during daytime and
65
nighttime near Monterey Bay in 1992.
3. Prey species in 65 harbor seal scats (in decreasing order) collected
during summer, 1991-1992.
66
during autumn, 1991-1992.
67
during winter, 1991-1992..
68
during spring, 1991-1992. .
69
7. Seasonal mean lengths of important prey items (em) found in harbor
seal fecal samples near Monterey in 1991-1992. .
70
8. Percent similarity indices based on prey items found in harbor seal
fecal samples among seasons. .
71
9. Estimated biomass of commercially important fishes and cephalopod
species eaten by harbor seals in Monterey Bay between 1992 and
1993 compared to total catch of these species in commercial fisheries
(Cal Fish and Game).
72
UST OF FIGURES
Figure
Page
1. A map of Monterey Bay showing the five locations of study and
haul-out sites: Cypress Point, S. Fanshell Beach, Hopkins Marine
Station, Seal Rock, Elkhorn Slough, and Davenport
74
2. Mean monthly abundances of harbor seals at offshore haul-out sites
near Monterey Bay in 1991-1992.
75
3. Movements of harbor seal #660 in Monterey Bay from October
1992 through February 1993. .
76
4. Movements of harbor seal #800 in Monterey Bay from October
1992 through February 1993. .
77
5. Movements of harbor seal #680 in Monterey Bay from February
1992 through May 1992. .
78
6. Movements of harbor seal #951 in Monterey Bay from February
1992 through March 1992.
79
7. Mean duration of dives each hour for harbor seal #660 (n=687)
during monthly 24-hr trackings, October 1992 to February 1993.
80
during monthly 24-hr trackings, October 1992 to February 1993
81
during monthly 24-hr trackings, February 1992 to May 1992.
82
during monthly 24-hr trackings for February and March 1992
83
11. Location and quantities of fecal samples collected from harbor seal
haul-out sites near Monterey in 1991-1992 (n=222)
84
12. Cumulative species curve representing number of harbor seal fecal
samples collected during autumn 1991 near Monterey Bay
85
13. Seasonal prey array indices calculated from harbor seal fecal samples
collected near Monterey Bay in 1991-1992.
86
lX
Figure
14. Length frequency histograms for Sebastes sp. and Chilara taylori
found in harbor seal fecal samples collected near Monterey Bay in
1991-1992
Page
87
15. Length frequency histograms f.or Octo.pus sp. and Loligo opalescens
88
found in fecal samples near Monterey Bay in 1991 - 1992.
16. Length frequency histograms for Citharichthys sordidus and
Porichthys notatus found in harbor seal fecal samples near
Monterey Bay in 1991-1992
89
17. Seasonal percent number of prey in fecal samples of harbor seals
collected near Monterey Bay in 1991 - 1992.
90
18. Winter frequency of occurrence (%FO) values of prey found in
harbor seal fecal samples collected in 1991-1992 versus winter
trawls conducted in Monterey Bay during the mid 1970's.
91
19. Summer frequency of occurrence (%FO) of prey found in
harbor seal fecal samples near Monterey Bay in 1991-1992 versus
summer shallow and deep trawls conducted ·in Monterey Bay
during the mid 1970's
92
20. Percent of maximum of harbor seals hauled-out at S. Fanshell Beach,
Monterey Bay, California, during the 1992 pupping season
93
21. Mean number of lone harbor seal pups (closed circles) and mother- pup
pairs (open circles) at S. Fanshell Beach, Monterey Bay during the
1992 pupping season. .
94
22. Mean abundance of harbor seal adults (open bars) and pups (closed
bars) at S. Fanshell Beach, Monterey, California, during the 1992
pupping season .
95
23. Mean hourly suckling duration of pups at S. Fanshell Beach,
Monterey, California, during the 1992 pupping season. . .
96
24. Mean duration of diurnal (open bars) and nocturnal (closed bars)
suckling sessions for each observation day throughout lactation
during the 1992 pupping season at S. Fanshell Beach. .
97
25. Mean proportion of harbor seal pups suckling per hour at S.
Fanshell Beach during the 1992 pupping season.
98
X
Figure
Page
26. Diurnal (open bars) and nocturnal (closed bars) proportion of
animals suckling per diel cycle during the pupping season at S.
Fanshell Beach, Monterey.
99
27. Diurnal (open bars) and nocturnal (closed bars) total time spent
suckling for harbor seal pups at S. Fanshell Beach, Monterey,
during the 1992 pupping season. .
100
INTRODUCTION
In general, the difficulty of finding relationships between pinnipeds and
their environment is complicated by their geographic and seasonally variable
behavior. Management and conservation issues have forced researchers to find
links between different aspects of pinniped biology, such as movements, dive
behavior, food habits, and mother-pup interactions. Although researchers have
focused on the interactions of movements, dive behavior, and food habits of
pinnipeds, few have incorporated the potential impact of food habits with the
energetics involved with lactation.
The harbor seal (Phoca vitu1ina) is one of 33 species of phocids that occur
throughout the world, and is one of the most widely distributed pinnipeds,
occurring along temperate, sub-Arctic, and Arctic coasts of the North Pacific and
North Atlantic (Thompson 1989).
The Pacific harbor seal (£. y. richardsi), one of several species of pinnipeds
that inhabit the western coast of the United States, is found in estuaries, along
rocky shorelines, and on intertidal rocks (King 1983). Before passage of the
Marine Mammal Protection Act (MMPA) in 1972, management and conservation
of pinnipeds was impossible due to heavy exploitation (Pearson and Verts 1970,
Boveng 1988). Although data concerning exploitation were anecdotal, harbor
seals in California were commercially hunted until1938, and between 1938 and
1972, sport and commercial fishermen would harass and kill harbor seals that
interfered with fishing operations (Boveng 1988, Hanan et al. 1993). The MMPA
restricted harassment and killing (takes) unless individuals obtained special
permits. Nnmbers of marine mammal takes were based on the status of stocks
2
relative to its optimal sustainable population (OSP). Optimal sustainable
population was defmed by the National Marine Fisheries Service (NMFS), as the
range between maximum net productivity level (MNPL) and the population
carrying capacity, K (historic K). This method, however, may not be applicable to
some pinniped populations due to past "overeJrploitation.
Following 1972, harbor seal populations have increased along the U.S.
west coast (Boveng 1988, Harvey et al. 1990, Hanan 1993). Between 1927 and
1991, numbers of harbor seals in California increased 7.3% (Hanan 1993). Hanan
(1992) stated the Pacific harbor seal population off California may be near MNPL.
Population increases may pose new problems for harbor seals and humans.
Harbor seals along the west coast of the U.S. damage fishing gear (Harvey 1987),
and eat salmon caught by hook and line and gillnets (Pitcher 1977, Jeffries 1984).
Pinniped mortality also has increased due to entanglement (Miller et al. 1983,
Harvey 1987, Boveng 1988, Hanan 1993). Because of increased conflicts
between commercial fisheries and pinnipeds, researchers have studied movements,
activity patterns, food habits, and population growth (pupping).
To understand the population dynamics, several researchers have
concentrated efforts on movements and activity patterns of harbor seals (Allen et
al. 1987, Allen et al. 1989, Brown and Mate 1983, Pitcher and McAllister 1981,
Slater and Markowitz 1983, Stewart 1984, Thompson 1989). Most harbor seal
research in California, however, has been limited to the Farrallon Island area
(Allen et al. 1989, Slater and Markowitz 1983) and San Miguel Islands (Stewart
1984), with few data from the Monterey Bay area.
Previous research indicated Pacific harbor seal haul-out patterns were
determined by several factors, including: weather, temperature, tidal patterns, time
of day, human proximity, pupping season, food availability, and pelage molt
3
(Slater and Markowitz 1983, Brown and Mate 1983, Stewart 1984, Watts 1992).
No single factor was responsible, and probably all of these factors operate
simultaneously in determining haul-out behavior (Schneider and Payne 1983). In
some studies, greatest number of harbors seals ashore occurred during midday
and in late spring-early summer during molt (Pitcher and McAllister 1981, Brown
and Mate 1983, Stewart 1984). Pacific harbor seals are gregarious, and spend
from 37% to 50% of the diel cycle ashore (Newby 1973, Sullivan 1979, Yochem
et al. 1987). Previous research indicated harbor seals were site specific, and
moved locally when foraging, although this may be an artifact of seasonal or
short-term studies (Thompson 1989). Questions concerning movements and
activity patterns have been answered because of advances in radio telemetry
(Brown and Mate 1983, Harvey 1987). These behavioral data need to be coupled
with food habit data to assess pinniped-fishery interactions.
Food habits of pinnipeds have traditionally involved examination of
stomach contentS from animals that have been killed or found dead (Fiscus and
Baines 1966, Pitcher 1977, Frost and Lowry 1980, Selzer et al. 1986). The method
of sacrificing pinnipeds, although valuable in determining prey items, has
associated problems such as empty stomachs, and animals sinking immediately
after being killed (Fiscus and Baines 1966, J. Harvey pers. comrn.). Frost and
Lowry (1980) killed 61 ribbon seals (Phoca fasciata), and found food present in
only 28 stomachs. Pitcher (1980) collected 548 harbor seals from 1973 to 1978, of
which 269 stomachs contained prey items.
Stomach contents of stranded animals also have been used as indicators of
prey use (Jones 1981, Selzer et al. 1986). Selzer et al. (1986) recovered more than
500 stranded harbor seals in New England, with prey items present in 53 animals.
Jones (1981) recovered 12 stranded harbor seals from the central coast of
4
California. This technique yields a partial list of prey species, but because of
stranding biases (e.g., animals not feeding before stranding), this method should
be considered only when stomach contents from healthy animals (lavage) or fecal
samples are not available.
Recently, researchers have extracted hard parts (fish otoliths and bones,
and cephalopod beaks) from fecal samples collected at haul-out sites to study
prey consumption in pinnipeds (Brown and Mate 1983, Hawes 1983, Antonelis
et al. 1984, Harvey 1987). This method is valuable when killing animals is not
feasible. Pitcher (1980) compared fecal samples with stomach contents, and
concluded that fecal samples can provide accurate information on pinniped food
habits. Several researchers subsequently used fecal samples to quantify daily fish
consumption by pinnipeds (Everitt and Gearin 1981, Brown and Mate 1983,
Jeffries 1984). Jeffries (1984) collected 436 fecal samples in Washington between
1975 and 1977, and determined harbor seals consumed commercially important
fish species such as salmon (Onchorhynchus sp.), although salmon otoliths were
not often found in fecal samples. Biases associated with determining food habits
using fecal samples include degradation of otoliths in the stomach or large
intestine, feeding on parts of the fish other than the head area, and otoliths and
beaks becoming trapped in the stomach (Hawes 1983, Jobling 1987, Harvey
1989).
Several studies have been performed on captive pinnipeds to determine
the degree of dissolution as otoliths passed through the digestive tract (Hyslop
1980, Hawes 1983, Murie and Lavigne 1986). Estimates of otolith recovery in
fecal samples range from 4% (da Silva and Neilson 1985) to 96% (Harvey 1989).
Jobling (1987) contended that fecal analysis was not an ideal method for
assessing food habits because of underestimations on the size of individual prey
5
items, but Harvey (1989) incorporated correction factors to provide more accurate
food habit data regarding prey species, number, and size. Brown and Mate (1983)
estimated size of fish consumed using a linear regression of otolith length to fish
length, and Harvey (1987) estimated prey sizes, prey mass, and annual prey
consumption of harbor seals in Oregon. ·
Few food habit studies of harbor seals have been attempted along the
central coast of California (Antonelis and Fiscus 1980, Jones 1981, Oxman 1995,
Harvey et al. in press). Between 1975 and 1977, Haniey et al. (in press) collected
30 harbor seal fecal samples from Elkhorn Slough, Monterey Bay, California. This
study provided an initial assessment of harbor seal food habits for the central
California coast. Additional data are needed on harbor seal food habits in this
region, especially for harbor seals inhabiting the rocky shoreline near Monterey.
No study to date has provided information on prey consumption for Pacific
harbor seals that inhabit rocky substrata in the North Pacific. Harkonen (1987)
compared fecal samples of Pacific harbor seals collected from rocky and slough
shores at Koster Island, Sweden. He found rocky shore inhabitants fed on a
greater number of species than slough inhabitants.
Harbor seals are opportunistic predators feeding primarily on benthic and
epibenthic prey along with schooling fishes, usually during nocturnal periods. I
hypothesized that the primary prey items for harbor seals inhabiting the central
California rocky shore would be the schooling fishes of Merluccidae, Clupeidae,
Scorpaenidae, and Embiotocidae, and some deep-sea fishes (i.e., Ophidiidae and
Myxinidae). I also hypothesized that during squid spawning season from April to
June, which coincides with the harbor seal pupping season, cephalopods would
constitute a major portion of the harbor seal diet because of their abundance in
Monterey Bay (Innes et al. 1978, Hawes 1983).
6
Understanding the diet composition of harbor seals before and during the
pupping period (lactation), along with suckling behavior and abundance of
harbor seals on the pupping site may provide insight on the female's ability to
accumulate substantial energy reserves before pupping. Harbor seals are thought
to fast or feed little during an abreviated lactation period (King 1983). Phocids
may fast from 4 days, as with the hooded seal (Cystophora cristata; Kovacs 1986),
or 2.5 months in the Baikal seal (Phoca sibirica: Popov 1979). The Pacific harbor
seal along the central California coast, has a 4 to 6 week lactation period between
April and June (Bigg 1969, Temte et al. 1991).
Duration of lactation is affected by growth rate of pup, milk composition,
precocious nature of pup, and suckling pattern (frequency and duration of
individual bouts). Oftedal et al. (1987) compared suckling of three otariids and
three phocid species and found average duration and frequency of suckling of
otariids was approximately two times that of phocids. Suckling duration and
frequency data have been used in energetic studies of northern (Mirounga
angustirostris) and southern (Mirounga leonina) elephant seals (Bryden 1968, Le
Boeuf et al. 1972), grey seals (Halichoerus grypus: Davies 1949, Fogden 1971),
harp seals (Phoca groenlandica; Kovacs 1987), and Weddell seals (Lc:;ptonychotes
weddelli: Tedman and Bryden 1979) to quantifiy maternal investment.
During lactation of otariids, which closely resembles the lactation strategies
of some large terrestrial mammals, females forage frequently during lactation
which may last 4 months to 3 years (Fedak and Anderson 1982, Bonner 1984,
Oftedal et al. 1987, Riedman 1990). Many otariid females accompany their pups
to sea during lactation (Costa et al. 1986). Extended lactation periods are
characteristic of most otariids, with 9 of 15 species suckling yearlings or older
(Bonner 1984).
7
Intraspecific differences in mean duration of suckling may be attributed to
predators, milk production, haul-out substrate, and heat (Bonner 1984, Watts
1992, Kovacs and Lavigne 1992). Although there are few data concerning
suckling duration in harbor seals,
Knu~tson
(1977) observed 23 harbor seal
mother-pup pairs in Humboldt Bay, California, and calculated a mean suckling
duration of 6.6 minutes (SD=3.4 min.). Newby (1973) also observed six harbor
seal mother-pup pairs in Washington, and calculated a mean suckling duration of
1.2 minutes. Suckling data from previous studies were not representative of the
diel cycle or entire lactation period.
The goals of this study were to examine suckling duration and proportion
of harbor seal suckling throughout the diel cycle during lactation. I hypothesized
there was no significant difference between mean suckling duration and
frequency between diurnal and nocturnal periods (Kovacs 1987, Oftedal et al.
1987). During mid and late lactation, harbor seals mothers abandoned pups to
presumably forage (pilot study 1991); therefore, I hypothesized suckling duration
significantly increased over the course of lactation to compensate for the females'
absence. I also hypothesized mothers and pups do not remain in close contact
throughout lactation, due to the females' foraging activities during the latter
phase of lactation (pilot study 1991).
Objectives
1. Estimate abundance of harbor seals near Monterey.
2. Determine daily and seasonal activity patterns, dive behavior, and
movements of harbor seals near Monterey.
3. Identify and determine relative importance of prey items consumed by
harbor seals near Monterey Bay.
8
4. Determine if seasonal variation exists in prey use.
5. Estimate percentage number (%N), percentage frequency of occurrence
(%FO), mean IRI, length, and biomass of individual prey species consumed.
6. Compare fish consumption of harbor seals with commercial fish and
cephalopod catches in Monterey Bay.
7. Compare food habits with previous data collected from trawls in
Monterey Bay.
8. Determine if significant difference exist in mean suckling duration
(diurnal and nocturnal) throughout lactation.
9. Determine if significant difference exist in mean "proportion of animals
suckling" (diurnal and nocturnal) throughout lactation.
10. Determine if suckling duration increased significantly throughout
lactation.
11. Determine if mother-pup pairs remain in close contact throughout
lactation.
MATERIALS AND METHODS
Monterey Bay, located along the central coast of California, is an open
embayment approximately 37 km wide.(north to south) with an axial length of 16
km (east to west). Although the Monterey Bay submarine canyon is the dominant
geological feature with ocean depths near 900 m, Monterey Bay is primarily
shallow with 80% of depths less than 180m (Dorfman 1991).
Harbor seals were counted weekly between Seal Rock to Hopkins Marine
Life Refuge (Fig. 1; Appendix B) during low tide from shore using binoculars.
Seal Rock and Hopkins Marine Life Refuge are rocky haul-out sites near the
southern tip of Monterey Bay, and are exposed at low, medium, and medium-high
tides during calm sea conditions.
To examine harbor seal movements near Monterey Bay, 4 adult males were
opportunisticallycaptured between February 1992 and September 1992 at Seal
Rock. Adult status was determined using Fancher (1979). Each captured harbor
seal was weighed to the nearest kilogram, measured (length and girth) to the
nearest centimeter, sex determined, and fitted with numbered flipper tags and
dorsally mounted radio transmitters (Fedak et al. 1982). Each transmitter,
designed to operate up to 9 months, operated on a discrete frequency between
164 MHz and 165 MHz allowing individual identification. An Advanced
Telemetry System receiver and four-element yagi antenna were used to track
randomly chosen harbor seal during 24-h sample periods. Strength of radio
signals was dependent on elevation and sea condition.
Movements, which were recorded on a chart, and dive behaviors of
individual harbor seals were documented between February 1992 and February
10
1993: Duration of each emergence and dive was recorded to the nearest second
using a digital stopwatch. Dive durations were limited to greater than 5 seconds
to account for signal interruptions (e.g., waves, rocks). Activity patterns, other
than dive durations, were not analyzed statistically because of the insufficient
sample size of radio-tagged harbor seals (N =4 ). Fifty randomly chosen diurnal
and 50 nocturnal dive durations for each radio-tagged harbor seal were selected
and analyzed statistically using a Student's t-test.
Between May 1991 and May 1992, harbor seal fecal samples were
collected near Monterey Bay to identify prey composition and temporal changes
in the diet. Only fresh fecal samples were collected weekly from haul-out sites
used exclusively by harbor seals. Because of the rocky substrata where all fecal
samples were found, tweezer, sponge, and water bottles were used to wash
between rocky crevasses ensuring the collection of all hard parts. Upon
collection, each fecal sample was placed into a plastic ziplock bag and frozen
until processed. To recover hard parts, thawed samples were placed into a
detergent solution and rinsed through a series of nested sieves (0.5 mm, 1.0 mm,
2.0 mm; Murie and Lavigne 1986). Fecal samples were categorized as being
collected in summer (May, June, July), autumn (August, September, October),
winter (November, December, January), or spring (February, March, April).
Seasons were chosen because of the close approximation to the climatic
seasonality of Monterey Bay in terms of air and water temperatures, rainfall, and
salinity (Yoklavich et al. 1991). To ensure adequate sample size, I tried collecting
20 fecal samples each month. I assumed fecal samples were representative of the
total adult population in Monterey Bay throughout the year.
Sufficiency of number of samples was evaluated by plotting cumulative
numbers of prey taxa against randomly chosen fecal samples for each season.
11
Curves were visually inspected to assess minimum number of samples needed
seasonally to evaluate prey composition.
Prey items were identified to the lowest taxon possible using illustrations
(Morrow 1979), and the otolith reference collection at Moss Landing Marine
Laboratories. Sagittal otoliths were used in identifying fishes, whereas,
cephalopod beaks were classified as either market squid (Loligo qpalescens) or
octopus (Octqpus rubescens; Clarke 1962, Clarke 1986). Otoliths, teleost bones,
and teeth were dried and placed into vials, whereas cephalopod beaks were
placed into vials containing 50% isopropyl alcohol. Cephalopod beaks were
initially identified by Steve Osborne and hagfish teeth identified by Eric Johnson.
Percentage number (%N) and percentage mass (%M) were calculated for
each fecal sample and averaged for seasonal values. Maximum number of left or
right sagittal otoliths and upper or lower cephalopod beaks for individual prey
taxon represented maximum counts of individual prey per fecal sample. Otoliths
and beaks were measured to the nearest 0.1 mm using either hand-held calipers or
an image analyzer. Hagfish teeth were measured at the base of the bicuspid and
tricuspid tooth using an image analyzer. Size and weight of prey consumed was
estimated using species-specific regressions of otolith-beak length to prey
standard length (Wolff 1982, Clarke 1986, Harvey et al. in press) and correction
factors for the amount of dissolution. A correction factor of 27.5% was applied to
species not found in the literature (Harvey 1989). Regressions for octopus lower
beak lengths to body lengths were provided by Steve Osborne (MLML).
Regressions for hagfish (Eptatretus sp.) were provided by Eric Johnson (MLML).
Cephalopod beak size is not significantly reduced during digestion; therefore,
correction factors were not applied (Harvey 1989). No correction factor was used
for hagfish teeth. A Kruskal-Wallis one-way ANOVA was used to determine
12
differences among estimated prey lengths and season for five prey species that
occurred in each season.
Because of the difficulty in discerning otolith differences among species of
Sebastes, length-weight regressions for .S.. jordani (shortbelly rockfish) were used
(Echeverria 1987). Using small rockfish species provided the most conservative
length-weight estimates.
A mean Index of Relative Importance (IRI, Pinkas et al. 1971) was
calculated using mean percentage number, mean percentage mass, and seasonal
percentage frequency of occurrence (%FO):
"X IR1 =("X %N + "X %M) X %FO.
Mass (%M) was substituted for volume (%V, Hyslop 1980), which was used in
the original equation (Pinkas et al. 1971). Seasonal percent number of individual
prey taxa among seasons was graphed and analyzed using percentage similarity
indices (Sanders 1960, Silver 1975). Significance level was arbitrarily placed at
75%. Differences in prey taxa among seasons were statistically analyzed using a
Kruskal-Wallis one-way ANOVA (Zar 1984). A non-parametric Tukey-type
multiple comparison test was performed on taxa significantly different among
seasons. Significance levels for all statistical tests were placed at 0.05.
The following prey array indices were used to describe differences in
seasonal prey items consumed:
Species richness (S) = # of prey species
Shannon-Weaver diversity index: H' =I (Lpi lnpi) I
Prey Evenness: J = H' I H'max, where H'max =InS
Index of Specialization: R = 1 - J
13
Prey Dominance: D = l:.pi2
Prey array indices were calculated for each fecal sample (Krebs 1989).
Mean seasonal prey array values were statistically tested using a Kruskal-Wallis
one-way ANOVA.
Comparisons using Spearman rank correlation were made between %FO
data for individual prey taxa from this study and %FO values from trawls
obtained from Monterey Bay (Cailliet et al. 1979). I assumed that trawls
conducted in Monterey Bay sampled depths common to harbor seal feeding
areas, and trawls could capture all harbor seal prey items.
To ascertain impact of harbor seal diet on the commercial fisheries in
Monterey Bay, biomass was calculated for those fishes or cephalopods deemed
commercially important by the California Department of Fish and Game. The
following equation was used in estimating biomass:
Mean biomass (kg) of X Mean number of X · 365 days =Annual
Biomass
species X/scat
seals in M.B.
of sp.X eaten
Biomass was calculated for each commercially important prey taxa and multiplied
by numbers of harbor seals counted for Monterey Bay (Hanan et al. 1993).
The assumption of one fecal sample per day per harbor seal is based on nocturnal
movements (feeding) from this study and passage rates and defecation times
reported for harbor seals (Helm 1984, Harvey 1987). Harvey (1987) reported
otoliths recovered from fecal samples represent prey items eaten within the last 24
hours.
Fecal samples were not collected from pupping grounds at S. Fanshell
Beach because of disturbances to the harbor seals, therefore, observations of
14
mother-pup behavior were conducted to determine if feeding occurred for harbor
seal cows during lactation.
Harbor seals were observed during eight diel periods from 9 April to 14
May 1992, at S. Fanshell Beach, Monterey Bay, California (Fig. 1). It should be
noted that all figures may be found in Appendix B of this study. Observation
periods were chosen haphazardly throughout the study. The study area consisted
of a sloping sandy beach. Binoculars (7x50) were used during diurnal
observations, which were conducted from an elevated blind 3 m to 20m from the
animals. A JAVELIN model226light-intensifying night scope was used for all
nocturnal observations. Observations were possible during all hours except
during inclement weather and crepuscular and twilight hours, when binocular
and night scope visibility was limited. Using a digital stopwatch, suckling sessions
were recorded to the nearest second. To minimize human disturbance, a 2-m high
fence was placed behind the blind and between the haul-out site and adjacent
road.
Counts of mother-pup pairs, lone pups, and total animals were conducted
hourly throughout the diel cycle. The maximum hourly count during a 24-h cycle
was used as the maximum count for that date. These data provided percentage of
harbor seals hauled-out throughout the diel cycle. All percentage data were
transformed using the arcsine transformation to improve equality of variance and
normality. Tidal height was separated into two groups; low G:; 1 m), and high(> 1
m), with 1m corresponding with the highest low tide mark. Differences between
mean percentage of animals on the haul-out site with respect to time (arn!pm) and
tidal height were tested using a two-factor ANOVA. Individual animals or pairs
could not always be identified, therefore, each suckling session represented a
single record. In all cases statistical significance level (a) was set at 0.05.
15
Harbor seal pups, whose behavior included frequent vocalizations and
movements, were considered abandoned and not counted as solitary. Pups were
categorized as solitary, when females entered the water leaving their presumed
pup alone.
All suckling sessions were recorded throughout a 24-h period using
continuous scan sampling (Altmann 1974). The pupping site was scanned
repeatedly each minute, usually by two observers. Timing a suckling session
would begin during the initial scan the suckling session was witnessed, and
concluded in the scan which separation of mother and pup occurred. Cumulative
on-teat durations were recorded as a "suckling session" which consisted of both
on-teat periods and short pauses, such as movement between teats (Oftedal et al.
1987). A long break (>30s) concluded a suckling session. It was possible to
record up to five mother-pup pairs suckling simultaneously. Hourly suckling
sessions were placed into two time periods: diurnal (0601 h- 1800 h) and
noctnrnal (1801 h - 0600 h).
Proportion of animals suckling for any hour was defmed as total number of
scans with suckling sessions divided by number of mother-pup pairs present on
the haul-out site. For example, If five suckling sessions were observed in 1 hour
for 10 mother-pup pairs, the proportion of individuals that suckled during that
hour would be 50%. It was assumed individual suckling sessions per hour were
randomly distributed among mother-pup pairs. There would be a bias if a few
mother-pup pairs produced most of the suckling sessions. Total time spent
suckling, for diurnal or nocturnal data, was produced by multiplying mean
duration of suckling session with proportion of animals that suckled for the
corresponding hour (Oftedal et al. 1987). Mean duration of hourly nocturnal and
diurnal suckling and "proportion of animals suckling," were tested using at-test
16
on randomly chosen records. A nonparametric two-sample test (Mann - Whitney
U test) was used if normality was violated. Differences between duration of
diurnal and nocturnal suckling sessions and "proportion of animals suckling"
were tested using the nonparametric Kruskal-Wallis one-way analysis of variance
using chi-square critical values (Zar 1984). A nonparametric Student-NewmanKeuls range test was used as a multiple comparison among means. Correlation
analyses, using date and suckling duration as variables, were used to test whether
durations of suckling sessions significantly changed during lactation. It was
assumed as the lactation period progressed, the mean age of pups at S. Fanshell
Beach also increased.
RESULTS
From June 1991 to May 1992, numbers of harbor seals between Seal Rock
and Hopkins Marine Life Refuge followed the overall trend with greatest counts
between December and March, whereas South Fanshell Beach, a pupping site,
had the greatest number of harbor seals in April and May (Fig. 2). Numbers of
harbor seals ashore were greatest in December (max = 311, SD = 42.4) and March
(max= 305, SD = 69.3; Fig. 2), whereas numbers were less during August (max=
109, SD = 13.9) and September (max= 143, SD = 17.4; Fig. 2). There was a
significant increase in mean counts of harbor seals during winter and spring
months (ANOVA,F =7.37, p =0.0001).
All movements (n = 10) recorded for harbor seal #660 from October 1992
to February 1993 were north of Seal Rock and within Monterey Bay (Fig. 3). On
one occasion, seal #660 entered Elkhorn Slough, a shallow tidal embayment and
seasonal estuary at the head of the Monterey Bay Submarine Canyon. Sixty
percent of trackings ended offshore of Sunset Beach, approximately 25 km north
of Seal Rock. During December and February, one trip was recorded during a 24h period, whereas October, November, and January recorded many smaller trips
within a 24-h period (Fig 3).
All movements (n = 13) recorded for harbor seal #800 from October 1992
to February 1993 were north of Seal Rock and within Monterey Bay (Fig. 4).
Trends revealed shorter movements with 46%
< 5 km from Seal Rock. During
February and November, seal #800 traveled offshore near Sunset Beach. During
one 24-h tracking seal #800 left and returned to Seal Rock five times, moving a
18
maximum distance of 15 km north. On all occasions seal #800 left and returned to
Seal Rock at least twice within a 24-h period (Fig. 4 ).
All movements (n = 6) recorded for harbor seal #680 from February 1992
to May 1992 were north of Seal Rock and within Monterey Bay (Fig. 5). Sixtyseven percent of all movements from Seal Rock concluded near the mouth of
Elkhorn Slough (Fig. 5). In February, Seal #680 was located at an offshore haulout site near Davenport, California, approximately 40 km north of Seal Rock (Fig.
5).
All movements (n = 2) recorded fm:harbor seal #951 during February and
March 1992 were north of Seal Rock and within Monterey Bay (Fig. 6). Seal
#951 was tracked twice before the signal was lost. During each tracking seal
#951 moved< 15 km north of Seal Rock. Radio-tagged harbor seals exhibited a
high degree of site fidelity with 92.5% found at Seal Rock during daylight hours.
Harbor seals radio-tagged at Seal Rock appeared to dive during nocturnal
hours. During 1,819 harbor seal dives, 595 dives (33%) occurred during diurnal
hours (0601 h -1800 h) and 1,224 dives (67%) occurred during nocturnal hours.
Mean duration of dives was 4.4 min. (SD
=2.4 min.).
Although dive times/haul-out records varied throughout the diel cycle,
harbor seal #660 was usually ashore during the day while diving at night (Fig. 7).
From October 1992 to February 1993, harbor seal #660 recorded 687 dives, with
mean day and night dive times ranging from 2.3 min (SE =1.7 min) to 5.2 min (SE
= 2.5 min; Table 1, Fig 7). It should be noted that all tables may be found in
Appendix A of this study. Mean duration of diurnal dives was significantly
greater than nocturnal dives for seals #660 (p = 0.033, p < 0.05). No trends were
evident between percentage of time spent hauled-out or diving for harbor seal
#660, although greater percentages of dives occurred during night (Table 1).
19
Harbor seal #800 was usually ashore or infrequently diving during the day,
whereas, most diving with greater mean dive times occurred at night (Fig. 8).
From October 1992 to February 1993, harbor seal #800 performed 702 dives,
with mean dive times ranging from 2.4 min (SE = 1.6 min) to 4.1 min (SE =2.3
min; Table 1, Fig. 8). A greater percentage of time was spent hauled-out during
each diel cycle, except during December when harbor seal #800 traveled into
Elkhorn Slough during daylight hours. There was no significant difference
between mean duration of diurnal and nocturnal dives for seal# 800 (p = 0.364).
Harbor seal #680 was usually ashore during midaftemoon, whereas most
diving bouts began late afternoon and continued throughout the night, with
greater mean dive times occurring at night (Fig 9). From February 1992 to May
1992, harbor seal #680 performed 351 dives, with dive times ranging from 3. 3
min (SE =3.6 min) to 10.3 min (SE =3.1 min). During spring dives, seal #680
performed mean dive durations greater than 7 min. Trends revealed greater
percentages of the diel cycle spent diving for seal #680 (Table 1, Fig. 9).
Harbor seal #951 was followed during two 24-h periods, diving
predominately at night, with greater mean dive times occurring at night (Fig 10).
Mean dive durations during spring for harbor seal #951 ranged from 7.8 min (SE
= 3.9 min) to 9.1 min (SE = 2.7 min, Table 1).
Mean duration ashore for harbor seals radio-tagged near Monterey Bay,
although variable, indicated diurnal haul-out periods. While at Seal Rock, mean
duration ashore (n =24) was 6.7 h (SD =2.7 h) to 12.9 h (SD =3.0 h, Table 2) for
individual animals. The mean duration on haul-out sites for an individual was
11.21 h (47% of diel cycle). Duration of single haul-out bouts ranged from <1.0 h
to 15.4 h. During diurnal periods, 50% of all haul-out bouts began between 0700
h and 1200 h, whereas 61% of haul-out bouts ended between 1600 hand 2000
20
h. Of all haul-out bouts beginning during nocturnal periods, only 8% of all haulout bouts began between 2100 h and 2400 h. Seals #680 did not haul-out from
1900 h to 0300 h, and seal #951 was not on the haul-out site from 1700 h to
0500 h. Seal #800 and #660 spent 2 hand 4.5 h, respectively, ashore in Elkhorn
Slough during December.
Fecal samples collected from harbor seals near Monterey Bay contained
mostly cephalopods and fishes. Of 222 harbor seal fecal samples collected near
Monterey (Fig. 11), from May 1991 to May 1992, 97.3% (n = 216) contained
identifiable hard parts. Twenty-two prey items were identified to species and four
to genus. Of 2,233 individual prey items, 61.6% (n = 1376) were cephalopods and
38.4% (n = 857) were fishes. Six percent of otoliths (n =54) were not identifiable
because of erosion or breakage. Octopus (%N = 31.7%) was slightly more
abundant than market squid (%N = 30.3%) in harbor seal diets. Fishes consumed
by harbor seals were mostly flatfishes (Pleuronectidae, Bothidae; %N = 31 %),
spotted cusk-eel(Chilara taylori, %N = 9.5%), and rockfishes (%N = 6.5%).
A cumulative species curve indicated approximately 30 fecal samples per
season were required to assess seasonal food habits (Fig. 12). The fewest fecal
samples were collected during winter (43); therefore, sample size was adequate
for comparing prey composition among seasons.
Seasonal prey array indices were calculated for each fecal sample. Fecal
samples contained one to nine prey taxa()( = 2.4, SE = 0.008; Fig. 13). Although
the greatest mean number of prey taxa per fecal sample()( = 2.9, SE = 0.03)
occurred in autumn, there was no significant difference in mean number of prey
taxa among seasons (Kruskal-Wallis, H = 0.273, p > 0.05). The most diverse array
of prey species was consumed during autumn (H' = 0.70, SE = 0.007); therefore,
autumn had the lowest Specialization Index (R = 0.5, SE = 0.007) and
21
Dominance Index (D =0.63, SE = 0.009). Autumn also revealed the greatest
evenness value (J) of 0.5. Spring and winter exhibited greatest percent
dominance indices (%DOM = 0.8). Mean prey array indices, however, were not
different significantly among seasons (Kruskal-Wallis; H', H = 0.093; D, H =
4.995; J, H = 3.026; R, H = 3.018; S, H = 0.273; p < 0.05; Fig. 13).
Fecal samples (n = 65) collected during summer reflected a diet consisting
mostly of fishes, with 12 prey taxa identified to species and four to genus (Table
3). Rockfishes received the highest mean lRl, whereas plainlm midshipman,
Pacific hake (Merluccius productus), spotted cusk-eel, and cephalopods ranked
two through six, respectively (Table 3). Other important prey items were northern
anchovy (Engraulis mordax) and flatfishes. Two species were observed only
during the summer, Pacific herring (Clupea pallasi), and night smelt (Spirinchus
starksi).
Fecal samples (n = 67) collected during autumn reflected a diet greatest in
prey diversity, 19 prey taxa were identified to species and three to genus (Table
4). Octopus and market squid dominated the diet, based on mean lRl values
(Table 4). Rockfishes, spotted cusk-eel, hagfish (Eptatretus sp.), and white
croaker were the principal fishes consumed (Table 4). Autumn had the greatest
number of identifiable species with starry flounder (Platichthys stellatus),
jacksmelt (AtherinQPS affinis), slender sole (Lyopsetta exilis), and sablefish
(AnQJ)lopoma fimbria) only observed during this season.
Fecal samples (n = 43) collected during winter reflected a diet dominated
by cephalopods (%N = 91% ), with 11 prey taxa identified to species and three to
genus (Table 5). Octopus had the highest mean lRl value, with market squid
second. Important fishes consumed during winter were rockfishes, cusk-eel, and
midshipman (Table 5).
22
Fecal samples (n =44) collected during spring also reflected a diet
dominated by cephalopods (%N =75%). Market squid was the most important
prey item based on mean IRI values, with octopus second in importance.
Important fishes consumed were Pacific sanddabs, English sole (Pleuronectes
vetulus), and plainf'rn midshipman (Table 6).
Length frequency distributions of rockfish and cusk-eel retrieved from
harbor seal fecal samples collected near Monterey Bay indicated harbor seals fed
on juvenile-sized fish during summer and larger fish during autumn and spring.
Summer revealed the greatest number of rockfish and cusk-eel eaten (Fig. 14).
Harbor seals ate rockfish with an estimated standard length of 11.15 em (SE =
1.10 em) and cusk-eel with an estimated standard length of 19.9 em
(SE = 0.5 em ). Frequency distributions of seasonal prey lengths for rockfish were
not statisically significant among winter and summer but increased significantly in
autumn and spring (Table 7). Frequency distributions of seasonal prey lengths for
cusk-eel increased significantly during spring and winter (Table 7).
Length frequency distributions of octopus and market squid, retrieved
from harbor seal fecal samples collected near Monterey Bay, indicate seals fed on
adult-sized cephalopods during each season. Octopus had an estimated mean
dorsal mantle length of 5.27 em (SE =0.8 em) and market squid had an estimated
mean dorsal mantle length of 11.18 em (SE =1.15 em, Fig. 15). Length of octopus
and market squid were not significantly different among seasons (Table 7).
Length frequency distributions of Pacific sanddabs retrieved from harbor
seal fecal samples collected near Monterey Bay indicated seals fed on juvenilesized fish during each season (Fig 16). Harbor seals ate Pacific sanddabs with an
estimated mean length of 20.4 em (SE =0.8 em), with sanddab lengths
23
significantly greater during spring. No sanddab otoliths were recovered during
winter (Fig. 16).
Length frequency distributions of plainfin midshipman retrieved from
harbor seal fecal samples collected near Monterey Bay indicated seals fed on
juvenile-sized fish during each season cFig. 16). Harbor seals ate plainfrn
midshipman with an estimated mean length of 20.37 em (SE = 0.9 em), which
were significantly smaller during autumn and spring (Table 7).
Although percent similarity indices (PSI) indicated harbor seal diets were
most similar between autumn and winter (72%; Table 8), diet composition (%N)
was significantly different among seasons (Fig. 17). Although cephalopods
occurred in each season, percent number of octopus and market squid recovered
from fecal samples were significantly different among seasons (octopus, H =
36.86; market squid, H =40.82, p < 0.05; Fig. 17). Percent number of octopus
sigificantly increased during winter and autumn, whereas market squid
sigificantly increased during winter and spring. Rockfish (H = 11.9, p > 0.05) and
cusk-eel ( H = 15.46, p > 0.05) occurred in each season and percent number of
both prey items decreased significantly during winter (Fig. 17). Plainfrn
midshipman showed no significant difference in percent number among seasons
(H = 6.74, P < 0.05).
Among the top nine commercially important prey species found in fecal
samples from harbor seals near Monterey Bay, white croaker, sanddabs, rockfish
sp., and market squid ranked first through fourth, respectively, in estimated
biomass eaten (Table 9). Trawls conducted in Monterey Bay indicated market
squid, rockfish, white croaker, and sanddabs ranked first through fourth,
respectively, in total catch in Monterey Bay (Bob Leos Pers. Comm).
24
Frequency of occurrence (%FO) of taxa collected in winter trawls from
Monterey Bay (Cailliet et al. 1979) and harbor seal fecal samples collected during
winter 1991 and 1992 were not significantly correlated (rs
=0.245, p > 0.05; Fig.
18). Octopus, which was the most freqqently occurring prey species in winter
fecal samples, was absent from Monterey Bay trawls (Cailliet et al. 1979; Fig. 18).
Conversely, anchovy was found frequently in trawls (Cailliet et al. 1979; Fig. 18)
and was absent from fecal samples during this study. Pacific electric ray (Torpedo
califomica) and Pacific butterfish (Peprilus sirnillirnus) were frequently found in
trawls but were absent from harbor seal fecal samples (Fig. 18).
Comparisons between summer trawl data in Monterey Bay (Cailliet et al.
1979) and harbor seals diet during summer (1991 and 1992) were significantly
correlated for deep and shallow trawls (Deep, rs
=0.501, p < 0.05; Shallow, rs =
0.432, p < 0.05; Fig. 19). Octopus was present in 8.6% offecal samples during
summer (1991 and 1992) but comprised 24.2% of summer trawls in Monterey Bay
(Cailliet et al. 1979, Fig. 19). Market squid, however, which occurred in 89% of
summer trawls from Monterey Bay (Cailliet et al. 1979) was present in only 10%
of fecal samples during the 1991- 1992 season (Fig. 19). Pacific electric ray,
Pacific butterfish, and medusafish (Icichthys lockingtoni) were frequently found
in summer trawls but were absent from harbor seal fecal samples (Fig. 19).
During the lactation period, percent maximum numbers of harbor seals
ashore on S. Fanshell Beach occurred between 1000 hand 2000 h (Fig. 20). Time
of day and tidal height did not have a significant influence on abundance of
harbor seals during the diel cycle (Time F = 1.524, Tidal height F = 1.423; p >
0.05, Fig. 20). Interaction of time of day and tidal height was not significant (F
=
3.966, p = 0.054), although generally more harbor seals were ashore during low
tide in the afternoon.
25
Mean number of mother-pup pairs on S. Fanshell Beach declined, while
mean number of lone pup increased, during nocturnal hours (Fig. 21). Mean
number of mother-pup pairs was greatest from 1000 h until 2200 h and mean
number of lone pups was greatest betw(:en 2200 h to 0300 h (Fig. 21).
Mean numbers of harbor seals at the haul-out increased until 22 April
when a maximum 85 animals (}( = 55) were observed (Fig. 22). Mean numbers of
mother-pup pairs were greatest on 29 April (max= 34) and declined steadily until
14 May 1992. Mean numbers of pups ashore peaked on 22 April and remained at
peak levels through 6 May 1992 (Fig. 22).
Harbor seal pups at S. Fanshell Beach suckled during every hour
throughout the diel cycle (Fig 23). During 184 hours of continuous scan
sampling, 630 suckling sessions were observed with 454 occurring diurnally and
176 nocturnally. There was no significant difference in duration of hourly
suckling sessions (t = 0.472, p > 0.05; Fig. 23) between diurnal (}( = 295.83 s, SE
= 233.13 s) and nocturnal periods(}(= 309.15 s, SE = 216.24 s).
Although nocturnal mean suckling durations were greater in six out of
eight observation periods, mean diurnal and mean nocturnal suckling durations
followed similar trends throughout lactation (Fig. 24). Duration of mean diurnal
(H = 12.08, p > 0.05; Fig. 24) and mean nocturnal (H = 9.96, p > 0.05, Fig. 24)
suckling sessions for each diel observation were not significant. Mean duration of
suckling sessions throughout the study was 5.0 min. (SE = 3.4 min.) with a
maximum duration of 21.45 min. Duration of suckling sessions increased
significantly as lactation progressed (r = 0.4, p = 0.0002, N = 80; Fig. 24).
Proportion of harbor seal pups suckling on S. Fanshell Beach peaked
during diurnal hours (1200 h) but remained consistant from 1300 h to 0600 h
(Fig. 25). Mean hourly "proportion of animals suckling" was significantly greater
26
during diurnal()( = 0.51 h-1, SE = 0.28 h-1) than nocturnal()( = 0.23 h-1, SE =
0.19 h-1; z = 3.35, p < 0.05, Fig. 25) periods.
Mean "proportion of animals suckling" calculated for each diel cycle was
greater during diurnal periods on all occasions, except for 12 April (Fig. 26). Total
"proportion of animals suckling" was 0.37 h-1.
Total time spent suckling during diurnal hours for harbor seals at S.
Fanshell Beach was greater than nocturnal hours on all days except 12 April
1992 (Fig. 27). Diurnal time spent suckling was 1.0 h/24 h whereas nocturnal time
spent suckling was 0.46 h/24h. Combined suckling time was 0.74 h/ 24 h (Fig.
27).
DISCUSSION
Establishing trends in pinniped abundance using land counts has inherent
problems such as the inability to count
au animals (i.e., those in water) or
disturbances (human or environmental) that may influence numbers ashore.
Observed trends in relative abundance may be caused by chance, redistribution
of harbor seals to or from a nearby site, or actual population changes. I assumed
that counts of harbor seals along the central coast of California were a valid index
of local seal abundance. All harbor seal counts should be construed as minimum
counts, because only animals on shore were counted. No variables such as air
temperature, wave intensity, or tidal height were recorded and correlated with
counts. All counts were conducted during low tide, although it was impossible to
collect data during similar weather patterns, it was my intention to count during
calm, sunny conditions during afternoon hours when human disturbance was
minimum.
After the pupping season harbor seals abandoned pupping sites, such as S.
Fanshell Beach and Cypress Point, and relocated to other locations (i.e., Hopkins
Marine Life Refuge, Seal Rock, and Elkhorn Slough). Thompson (1989) reported
marked seals in Orkney, Scotland chose different sites for pupping and
nonpupping periods. It is widely accepted that harbor seals seek areas that are
well protected from human disturbance, especially during pupping (Allen et al.
1989). Harbor seals near Monterey Bay haul-out at well-protected sites such as
Seal Rock, Hopkins Marine Life Refuge, and Elkhorn Slough.
Increases in harbor seal abundance in Monterey Bay (Hanan et al. 1993)
may be reflected in pup counts, where a 7% yearly increase in pup production
28
occurred between 1983 and 1993 (Bob Huettman pers. cornrn.). Numerous factors
such as protection, molt, redistribution, food availability, and reproductive status,
acting singly or in combination, may account for increases in relative seasonal
abundance of harbor seals (Slater and Markowitz 1983, Brown and Mate 1983,
Stewart 1984, Allen et al. 1987, Watts i992).
Previous researchers suggested protection may be responsible for increases
in relative abundance by significantly reducing harassment and killing of harbor
seals (Bonner 1984, Payne and Schneider 1984, Harvey et al. 1990). A census
conducted in Massachusetts revealed an increase of 11.9% per year since passage
of the MMPA (Payne and Schneider 1984). Harvey et al. (1990) reported harbor
seal populations, within bays and estuaries in Oregon, increased following the
MMPA. Jeffries (1986) observed similar trends in Washington. Although the
MMPA is designed to protect harbor seals from exploitation, local populations
may not increase where human disturbance is prevalent.
Harbor seals will not pup on beaches used by humans, and frequent
disturbances at or near haul-out sites adversely affect reproductive rates and site
fidelity (Newby 1973, Brown and Mate 1983, Slater and Markowitz 1983, Allen
et al. 1989). Increased harbor seal abundance along the central coast of California
from Cypress Point to Hopkins Marine Life Refuge may be the result of increased
protection during the pupping season. Since 1983, The Pebble Beach Company
along with volunteers from the American Cetacean Society, have placed opaque
fences separating humans and pupping sites, minimizing disturbances. This
protection may have increased number of pups on previously abandoned
beaches. Increases in relative abundance may result in greater competition for
haul-out space, forcing harbor seals into nearby sloughs or estuaries (Pitcher and
McAllister 1981, Jeffries 1986, Harvey et al. 1990).
29
Although many researchers have reported increases in relative abundance
of harbor seals at haul-out sites during the molt period (Slater and Markowitz
1983, Yochem et al. 1987, Allen et al. 1989, Thompson 1989), some have reported
decreases during molt with peaks during winter (Pitcher and McAllister 1981,
Harvey 1987). Pitcher and McAllister (1981) reported increased abundance
during winter was due to increased food availability. Harvey (1987) concluded
some researchers may have overlooked winter increases of harbor seals in the
open ocean by only counting in bays or estuaries. Harvey (1987) also stated that
increases in harbor seal abundance ashore during winter may be because; harbor
seals are energetically stressed during the winter and need to rest ashore, or food
availability is greater and less energy is expended foraging. Increases in harbor
seal abundance in Monterey Bay may be because of increased prey availability.
Counts of harbor seals near Monterey (max= 311), and consumption of octopus
peaked during winter, which corresponds with mating season of octopus.
Harbor seals move among haul-out sites and congregate depending on
season and activity (Pitcher and McAllister 1981, Brown and Mate 1983, Jeffries
1986). Redistribution of juvenile harbor seals from pupping sites to adjacent
sloughs or estuaries is well documented (Bonner and Whitthames 1974). Elkhorn
Slough, located approximately 10 km north of protected pupping sites, has a
population consisting of 80% subadults (Oxman pers. comm.). Decreased harbor
seal abundance near Monterey during molt (August and September), may indicate
some harbor seals moved to Elkhorn Slough, which had increases in relative
abundance during molt (Oxman pers. comm.). Unti11989, no harbor seal pups
were recorded in Elkhorn Slough, but from 1989 to 1991 seven pups were
documented (Oxman and Trumble unpubl. data). Harbor seals radio-tagged at
30
Seal Rock moved into Elkhorn Slough on two occasions, although radio
transmitters were not operative during molt.
Although radio-tagged harbor seals at Seal Rock were ashore during all
hours, daylight hours were usually spent ashore, whereas nights were spent
diving/foraging. Harbor seal numbers often increased on haul-out sites from early
morning to early afternoon (Boulva and McLaren 1979, Stewart 1984, Allen et al.
1987, Yochem et al. 1987, Thompson 1989, Watts 1992). Yochem et al. (1987)
reported 46% of all haul-out bouts began at night (1800 h - 0600 h), whereas
54% began during daylight hours (0600 h- 1800 h). Yochem et al. (1987) also
concluded 34% of all haul-out bouts began between 0800 hand 1300 h. Harbor
seals radio-tagged near Monterey had a similar pattern. The average proportion of
time ashore (47%) for harbor seals tagged near Monterey is similar to other data
reported along the open coast of California (44% and 37%; Sullivan 1979,
Yochem et al. 1987). Harvey (1987) reported that three seals in Oregon spent
10% to 19% ashore, and attributed discrepancies with previous studies to a lack
of diel tracking and infrequent trackings over long distances from the haul-out
site. Allen et al. (1987) studied harbor seals at Drakes Estero, California, and
stated harbor seals were on haul-out sites an average of 7 h throughout the diel
cycle. Stewart and Yochem (1983) reported harbor seals were ashore 35% to
65% of each day, depending on the month. Differences between this study and
previous studies may be attributed to reduced sample size of radio-tagged harbor
seals in Monterey and sex of the seals tagged. Only males were radio-tagged,
which is not a true representation of the harbor seal population in Monterey Bay.
Although harbor seals radio-tagged near Monterey had some site fidelity, I
cannot exclude the possibility that some or all seals hauled-out at other sites
during times not tracked or when seals were out of range, presumably foraging.
31
Harbor seals radio-tagged near Monterey Bay always moved north of Seal
Rock and upon entering the bay, either traversed the bay or moved along the
shore and returned to Seal Rock with 24 h. Radio-tagged harbor seals frequently
moved off Sunset Beach, which is near Soquel Canyon, along the coastline, and
less frequently into Elkhorn Slough. Topographical features, such as Soquel
Canyon, may provide increased concentrations of prey, and thus form focal
points for predators (Brown 1980, Evans 1987).
Several researchers have indicated harbor seals stay within 7 km of the
shore, feeding on benthic prey (Brown and Mate 1983, Harvey 1987), Elkhorn
Slough is approximately 20 km northeast from Seal Rock. When tagged harbor
seals moved into Elkhorn Slough, Monterey Bay was experiencing stormy, El
Nino conditions. It is possible that prevailing southwesterly storm winds and
swells made returning to Seal Rock difficult or wave disturbances at Seal Rock
made resting ashore difficult. Slater and Markowitz (1983) stated harbor seals
used alternate sites (protected) off California during inclement weather.
On one occasion, seal #680 traveled approximately 40 km north to a haulout site near Davenport, California, before returning to Seal Rock. Brown and
Mate (1983) stated five of 11 radio-tagged harbor seals in Oregon moved
distances greater than 25 km, and most harbor seals tagged returned to sites
where they were captured. Harvey (1987) reported harbor seals in Oregon moved
up to 280 km from the capture site. Allen et al. (1987) reported one harbor seal
radio-tagged in Drakes Estero moved 210 km to Hopkins Marine Life Refuge.
Harbor seals near Monterey Bay increased their duration of dives, usually
diving at night, with increased distance from Seal Rock. This may indicate
foraging on prey items in deeper waters. The most important prey items found
throughout the year, octopus, market squid, rockfish, and cusk-eel exhibit
32
nocturnal behavior which would increase their vulnerability to harbor seal
predation at night (Hobson et al. 1981).
Maximum and average dive durations for harbor seals captured at Seal
Rock were within ranges reported from previous studies (Harvey 1987, Allen et
al. 1987) and are well within aerobic liniits (Hochachka 1981). The significant
difference between day and night duration of dives for harbor seals #660, #680,
and #951 may be an artifact of sampling. Duration of dives for harbor seal #800
were not statistically significant between day and night, possibly because of
disturbances. Of the five diel trackings of seal #800, three were during very harsh
conditions (2= Beaufort 4).
Durations of dives of harbor seals #660 and #800, tracked from October
1992 to February 1993, were less than seals #951 and #600, which were followed
from February 1992 through May 1992. This may be an artifact of sample size. A
greater sample size of both males and females, tracked throughout the year, is
required.
Many researchers have indicated harbor seals feed opportunistically,
adjusting their foraging patterns to take advantage of seasonally and locally
abundant prey (Pitcher 1980, Brown and Mate 1983, Roffe and Mate 1984,
Harkonen 1987, Pierce et al. 1990, Olesiuk 1993). Based on the dive patterns of
harbor seals and activity patterns of some dominant prey species, nocturnal
foraging in harbor seals has been inferred (Brown and Mate 1983, Yochem et al.
1987). There is some debate, however, on the mechanism (visual, tactile, or
echolocation) of prey detection during low light conditions (Lavigne et al. 1977,
Riedman 1990). Habits of prey items, along with activity patterns based on radiotracking, provided some evidence of night foraging in Monterey Bay.
33
Numerous potential problems such as inability to identify all prey items,
assuming representation of all prey items in fecal samples, otolith dissolution, beak
retention, and passage or recovery rates, are present when using fecal samples to
estimate numbers and sizes of prey consumed (Prime 1979, Hawes 1983, Harvey
1989).
Otolith dissolution is dependent on passage rates, size of otoliths, and
whether animals are captive (Hawes 1983, Dellinger and Trillmich 1988, Harvey
1989). Harvey (1989), who studied captive harbor seals, reported mean lengths of
fishes estimated from otoliths in feces averaged 5.7% to 36.9% less than the
original fish lengths. To counter problems associated with otolith dissolution,
species-specific correction factors have been formulated (Dellinger and Trillmich
1988, Harvey 1989).
Cephalopod importance in the harbor seal diet may be underestimated
when using fecal samples in food habit analyses. Harbor seals often retain, then
regurgitate "wads" of beaks rather than pass them through their digestive system
(Pitcher 1980, Harvey 1989). Harvey (1989) found 37% of cephalopod beaks
from captive harbor seals were represented in feces, which may have
underrepresented cephalopods as an important prey item. No correction factor
was applied to beak lengths because researchers have shown that no significant
erosion occurs during digestion (Kashiwada et al. 1979, Harvey 1989). No
regurgitation of beaks was found at any haul-out site during the collection phase
of this study.
Fishes may be misrepresented as a prey item in the diet of harbor seals
using fecal samples for food habit analyses. Based on fecal sample analyses,
variability in passage and recovery rates can cause potential problems when
estimating biomass consumed or approximate time of feeding (Prime 1979, Murie
34
and Lavigne 1986, Harvey 1989). Murie and Lavigne (1986) stated otoliths are
either completely digested or passed within 12.5 h of feeding. Harvey (1989) also
found recovery rates for fishes with small, less robust, otoliths were less than
larger otoliths because of incomplete digestion. Complete digestion of otoliths
may underestimate number and size of nshes with small diffuse otoliths such as
salmon (Onchorhynchus sp.), anchovy, or cartilaginous fishes. Although absolute
recovery rates of hard parts found in fecal samples were affected, Dellinger and
Trillmich (1988) concluded that proportions of prey items may not change
significantly. Using this assumption, the relative importance of prey items found in
harbor seal fecal samples may be made with confidence.
Most researchers quantifying food habits have collected hard parts during
months or years, only to lump data giving an incomplete picture of seasonal
predation upon different prey items (Brown and Mate 1983, Green and Burton
1987). Few researchers have quantified seasonal changes in prey consumption
(Everitt and Gearin 1981, Olesiuk 1993), with fewer studies incorporating indices
such as IRI values (Pitcher 1980). Hyslop (1980) indicated using IRI values
creates additional biases by confounding two sources of error and variation.
Factors which may bias biomass estimations is the variation inherent in the
regression analyses. Factors contributing to this variation include natural
variability of similar sized prey items, variation caused by sex differences or
reproductive status, and biases due to sampling procedures. I believe regression
variation was minimized by collecting a large sample size of fecal samples.
Other potential problems incurred during this study are, assurniog trawls
conducted in Monterey Bay during 1979 (Cailliet et al. 1979) can capture all prey
available to harbor seals, and assurniog fish abundance and diversity has remained
the same from 1979 to 1992.
35
Many prey items found in harbor seal fecal samples near Monterey Bay
also were identified as important prey items for other pinnipeds along the west
coast. Several nocturnally active species, such as market squid, octopus, cusk-eel,
and rockfish found in fecal samples near Monterey Bay, were previously reported
as important pinniped prey items (Kenyon 1965, Pitcher 1977, Pitcher 1980,
Antonelis et al. 1984, Lowry et al. 1990, Harvey et al. in press). Harvey et al. (in
press), collected otoliths from Elkhorn Slough and identified several nocturnally
active prey species of harbor seals common to this study such as night smelt,
cusk-eel, and white croaker. Antonelis et al. (1984) collected fecal samples from
California sea lions (Zalophus californianus) at San Miquel Island, California and
identified Pacific hake, market squid, octopus, and rockfishes as important prey
items during spring and summer. A preliminary food habit study of California sea
lions in Monterey Bay also indicated Pacific hake, market squid, and rockfishes
were among the prey items found in fecal samples (Nicholson 1986, John
Douglas, pers. comm.). Common prey items among growing populations of
pinnipeds in Monterey Bay may lead to increased future competition.
Determining prey length and weight is crucial when estimating the relative
importance of prey items in diets of pinnipeds, and assessing the trophic impact of
foraging on fish and cephalopod populations. Determining prey size also can
provide evidence on pinniped feeding behavior, such as time and depth of prey
capture.
Rockfishes were an important prey item found throughout the year in the
diet of harbor seals near Monterey Bay during 1991 and 1992. Rockfishes were
found in harbor seal fecal samples throughout the year, and had the greatest IRI
values of fish species during summer, winter, and autumn. Harbor seals near
Monterey Bay primarily ate juvenile rockfishes (11 = 11.51 em), which coincides
36
with their movement into shallower waters (Mary Yoklavich pers. comm.). Cailliet
et al. (1979) found increased abundances of rockfishes in both shallow and deep
water trawls during summer in Monterey Bay; whereas their abundance in winter
trawls dramatically declined, it was not mentioned if juveniles were prevalent.
Harvey et al. (in press) also identified tliese prey items from fecal samples or
sediment samples at harbor seal haul-out sites in Elkhorn Slough, near Monterey
Bay.
Plaintm midshipman and cusk-eel were important prey items in the diet of
harbor seals residing near Monterey Bay. Plainfin midshipman and cusk-eel are
benthic, nocturnal feeders found in offshore waters (Eschmeyer 1983, Wang
1986). They spawn in late spring and early summer (Fitch and Lavenberg 1970),
which corresponds with greater mean IRI values in the harbor seal summer diet.
In this study, plaintm midshipman declined in mean IRI value during autumn,
which coincides with their movement from an inshore habitat during winter, to an
offshore deep water habitat during autumn (Fitch and Lavenberg 1973). Cailliet
et al. (1979) reported higher occurrences of plaintm midshipman in summer
shallow water trawls in Monterey Bay, when compared with summer deep water
and winter trawls.
Summer spawning of cusk-eel in deeper waters off the Monterey
Submarine Canyon may explain its appearance in fecal samples, but not in trawls
conducted in Monterey Bay (Cailliet et al. 1979). Harbor seals fed on larger sizes
during winter and spring. Harvey et al. (in press) stated cusk-eel was an important
prey item for harbor seals in Elkhorn Slough, and was caught in trawls
exclusively between 2100 h and 0300 h. This indicated nocturnal foraging by
harbor seals near Monterey Bay.
37
Harbor seals fed on the commercially important white croaker throughout
the year, although few numbers collected in fecal samples during spring and
winter precluded statistical testing. White croaker is a nocturnal-feeding
epibenthic fish usually found
inshor~
over sand and gravel bottoms (Wang 1986).
Although white croaker spawns throughout the year in Monterey Bay (Emmett
et al. 1991), most spawn in shallow water from September to May, with juveniles
moving offshore during summer and autumn. White croaker was not found in
trawls conducted in Monterey Bay (Cailliet et al. 1979), probably because this
species is found just beyond the surfzone (Wang 1986). White croaker has been
identified in fecal samples of California sea lions at San Clemente Island,
California (Lowry et al. 1990), and fecal and sediment samples at haul-out sites of
harbor seals in Elkhorn Slough (Harvey et al. in press).
Pacific sanddab, a nocturnal feeder which starts spawning in late summer
and continues ~ntil early autumn (Love 1991), is a commercially important fish in
Monterey Bay, and was the only flatfish found in fecal samples throughout most
of the year. Harvey (1987) reported harbor seals in Oregon ate Pacific sanddab
with a mean estimated length of 15.4 em, which is smaller than the estimated
length reported for this study ("X
=20.4 em). Pacific sanddab had an IRI which
ranked from third to seventh in overall importance in the diet of harbor seals near
Monterey. Brown and Mate (1983) stated Pacific sanddab ranked fourth in
frequency of occurrence as a prey item for harbor seals in Oregon. Jeffries (1984)
also reported Pacific sanddab as an important prey item in Washington.
Speckled sanddab, which had a mean estimated standard length of 12.1 em,
was the smallest flatfish eaten by harbor seals in Monterey Bay. Harvey (1987)
reported similar results with harbor seals in Oregon, eating speckled sanddab with
a mean length of 7.9 em. Speckled sanddab, which was found in very low
38
numbers, was retrieved from fecal samples during winter and spring, which
corresponds to their spawning season.
Harvey et al. (in press) reported Citharichthys sp. in fecal samples and
sediment at harbor seal haul-out sites in Elkhorn Slough, California. Citharichthys
sp. was prevalent in most trawls conducted in Monterey Bay (Cailliet et al. 1979).
Greatest mean IRI values of English sole corresponded with the peak of
spawning as they move into waters between 50 m and 70 m depth (Emmett et al.
1991). Absence of English sole in fecal samples during winter may be related to
this fish species emigration to deeper waters (Emmett et al. 1991). Harbor seals ate
English sole with an estimated mean standard length of 23.8 em, similar to the
12.0 em to 32.0 em eaten by harbor seals in Oregon (Brown and Mate 1983).
Harvey (1987) reported harbor seals off Oregon ate primarily adult and juvenile
English sole averaging 11.4 em standard length and a range of 1.9 em to 33.5 em.
Other flatfish species such as starry flounder, slender sole, Petrale sole, Rex
sole, and Dover sole,which were identified from fecal samples in this study, also
were collected by Harvey et al. (in press) in Elkhorn Slough. None of these
flatfish reported were collected in trawls conducted in Monterey Bay (Cailliet et
al. 1979).
Hagfish teeth were recovered in fecal samples collected near Monterey
Bay throughout the year, although numbers were low in all seasons except
autumn. Hagfish is common in Monterey Bay, usually between 80 m and 400 m
depth. Although reported as a harbor seal prey in Washington, Oregon, and
Alaska (Pitcher 1977, Brown and Mate 1983, Jeffries 1984), this is the first study
to report hagfish as a prey item in California. Hagfish was not reported in trawls
conducted in Monterey Bay (Cailliet et al. 1979, Harvey et al. in press); however,
39
in Oregon, hagfish ranked in the top 10 prey items based on frequency of
occurrence (Brown and Mate 1983).
Pacific hake otoliths were recovered in harbor seal fecal samples near
Monterey Bay in each season except winter. Pacific hake is a pelagic schooling
fish that feeds nocturnally (Fitch and Lavenberg 1970, Matarese et al. 1989).
Reported as an important species for pinnipeds along the west coast of the U.S.
(Everitt and Gearin 1981, Antonelis et al. 1984), its importance for harbor seals
near Monterey Bay during summer was not previously known. Harvey et al. (in
press), however, found Pacific hake were eaten by harbor seals using Elkhorn
Slough. Absence of Pacific hake in winter fecal samples coincided with the peak
spawning period, in which adults move offshore (up to 400 km, Love 1991).
During trawls conducted during the 1970's in Monterey Bay, Pacific hake
occurred in shallow and deep-water trawls during summer, and was absent during
winter (Cailliet et al. 1979). Harbor seals near Monterey Bay ate Pacific hake with
an estimated mean standard length of 36.2 em. Greater estimated standard lengths
for Pacific hake reported for this study may be due to measuring only the otoliths
that remained intact.
Anchovy is a commercially important fish eaten by harbor seals in
Monterey Bay in all seasons except spring, with an IRI peak during summer.
Between March and June (spring and summer), anchovy move north forming
large schools (Fitch and Lavenberg 1970). Harbor seals ate anchovy with an
estimated mean standard length of 14.1 em, which is greater than the mean length
eaten by California sea lions (9.5 em, Antonelis et al. 1984) and the 11.5 em eaten
by harbor seals in the Columbia River, Oregon (Harvey 1987). Many researchers
have reported anchovy as an important prey item, whereas, stating dissolution of
its small otoliths may cause underrepresentation in the diet. This may explain
40
fewer numbers of anchovy otoliths found in harbor seal fecal samples near
Monterey Bay. Fewer anchovies in fecal samples of harbor seals may be because
of El Nino conditions. Anchovy landings in Monterey Bay in 1992, were less
than one-quarter of normal levels, equaling a 10-year low (Bob Leos pers. comm.).
Trawls conducted in Monterey Bay during non-El Nino periods (Cailliet et al.
1979) indicated anchovy were abundant in trawls during all seasons sampled.
Market squid also was an important prey item throughout the year, but
ranked sixth of 16 prey items during its spawning season. Harbor seals near
Monterey Bay fed on market squid with a mean DML slightly less than the range
of 12.0 em to 19.0 em reported by Morris et al. (1980). Many researchers have
found market squid in the diet of harbor seals (Scheffer and Perry 1931, Kenyon
1965, Pitcher 1977, Pitcher 1980, Selzer et al. 1986, Pierce et al. 1990, Harvey et
al. in press). Morejohn et al. (1978) found no evidence of harbor seal predation
on market squid in Monterey Bay, however, several other vertebrate predators of
market squid were identified. Recent data, however, from Elkhorn Slough
confirms the importance of market squid as an important prey item for harbor seals
inhabiting Monterey Bay (Oxman 1995). Although commercial catches of market
squid in Monterey Bay during 1992 (approx. 6 million kg, Bob Leos pers. comm.)
were one-half of a million kilograms less than the previous year, availability for
harbor seals apparently was not affected.
Octopus appeared as an important prey item for harbor seals in Monterey
Bay, especially during autumn and winter. This predation coincides with the
deep-water mating of octopus in Monterey Bay (Morris et al. 1980). Although
octopus is a common year-round resident of Monterey Bay, migration to inshore
spawning grounds during spring, which would seem to make them more
susceptible to predation, was reflected in a reduced IRI value during spring and
41
summer. The mean estimated dorsal mantle length (DML) of octopus obtained
from fecal samples near Monterey Bay was within the 5 to 10 em DML for
octopus (Morris et al. 1980). Although seasonal prey length for octopus was not
significantly different, the decrease in mean length during autumn ("X = 4.69 em,
.
range= 3.4- 7.4 em) may indicate feeding primarily on subadults during peak
spawning periods. Harvey et al. (in press) stated few (n = 1) octopus were
retrieved from harbor seal fecal samples collected from Elkhorn Slough between
1975 and 1977. Perhaps there were more octopus or fewer fishes in Monterey
Bay during this study. Although octopus is not uncommon in harbor seal diets
and appears to be an important food throughout the eastern North Pacific
(Scheffer and Perry 1931, Spalding 1964, Kenyon 1965, Bishop 1967, Pitcher
1980), its high ranking as a prey item was unknown in the Monterey Bay area.
Differences in prey diversity between trawls conducted in Monterey Bay
(Cailliet et al. 1979) and food habit analyses revealed species such as herring,
anchovy, English sole, Pacific butterfish, medusafish, and lincod missing or
reduced as a potential prey item for harbor seals in Monterey Bay. Because the
harbor seal diet is similar to prey diversity, differences found between trawl data
and harbor seal food habit analyses may be an early indication of health problems
or anomalies in the bay.
Assessments of the quantitative relationship between harbor seals and
commercial fisheries in Monterey Bay are just beginning to reveal important
information. Conservative estimates of biomass of prey consumed by harbor seal
in Monterey Bay (Hanan et al. 1993), indicated approximately 215,000 kg of
rockfishes, market squid, cusk-eel, anchovy, lingcod, Dover sole, English sole, and
Rex sole were consumed. This is equivalent to 2.3% of landings for similar species
in Monterey Bay (Spratt and Leos 1993). In Monterey Bay commercial fisheries,
42
rockfishes, market squid, anchovy, and Dover sole accounted for 90% of all
landings, and 53% of total revenues in 1992. During 1992, harbor seals ate fish
that equaled about 4.4% of the commercial landings. Harbor seals ate an
equivalent of 11.3% of English sole landings and 3.2% of Rex sole. These data
are much less than results of Harvey (1987), who reported consumption estimates
of 60.3% and 63.1% of landings for English sole and Rex sole off Oregon. Harbor
seals in Monterey Bay ate large amounts of cusk-eel (equivalent to 143% of
landings). Although cusk-eel was an important fishery in the late 1970's and early
1980's, it is now considered a minimal fishery (Bob Leos, pers. comm.). Harbor
seals also competed with the Citharichthys sp. fishery (equivalent to 86% of
landings).
Estimates of commercially important fish species consumed by harbor seals
in Monterey Bay were approximately 2% of total bodyweight per day per seal. It
would appear harbor seals in Monterey Bay do not compete extensively with
commercial fisheries. It is difficult, however, to assess competition between harbor
seals and fisheries because of year to year variability in predation and fish
recruitment.
Based on dive and movements data, it appeared harbor seals fed in
Monterey Bay exclusively, feeding on fish with an estimated standard length of
18 em. Data concerning harbor seal prey consumption are few, although
Antonelis et al. (1984) reported California sea lions in southern California foraged
on similar prey items with similar size ranges. Hawes (1983), studying California
sea lions at San Nicolas Island, California, identified market squid, anchovy, and
rockfishes among the top prey items. In Monterey Bay, there appears to be much
overlap in the diets of harbor seals and California sea lions. In non-El Nino
periods, there are 1000 to 2000 California sea lions in Monterey Bay between
43
May and November. During the El Nino of 1992, numbers of California sea lions
increased dramatically to approximately 2500 in Monterey Bay because of their
exodus from southern waters (P. Browne, pers. comm.).
Harbor seals near Monterey Bay feed on spatially and temporally
abundant prey, with some prey items more important seasonally. Based on
estimated prey sizes and movements of radio-tagged harbor seals in Monterey
Bay, it appears harbor seals feed primarily on nocturnally active nearshore
juvenile fishes and adult-sized benthic and epibenthic species. Harbor seals
. generally consumed adult sized market squid and octopus throughout the year,
with greatest predation during winter and spring. Winter directly precedes the
lactation period of harbor seals near Monterey Bay.
Prey consumed before lactation must provide enough energy reserves to
cope with the stresses of an intense lactation while fasting. When compared with
fishes consumed by harbor seals, octopus, the dominant prey item found in harbor
seal fecal samples before pupping, may have provided insufficient energy reserves
to cope with fasting during lactation. Fat content for seasonally important fishes
consumed by harbor seals in Monterey Bay ranged from 0.4% by weight for
white croaker to 10.7% by weight for anchovy (Sidwell1981). Flatfishes and
rockfishes, which composed nearly 50% of all fishes consumed by harbor seals,
averaged 2.0% fat content by weight (Sidwe111981). The caloric value per 100
grams of fish ranged from 76 cal. for rockfish to 152 cal. for anchovies (Sidwell
1981). Octopus yielded 0.6% fat content and approximately 70 cal/100 g
consumed (Sidwe111981).
Counts of mother-pup pairs on the pupping grounds, suckling durations of
mother-pup pairs throughout the diel cycle, along with movements of radio-
44
tagged harbor seals and seasonal prey consumption may give insights to whether
cows feed during the lactation period.
Many researchers have counted harbor seals during pupping (Allen et al.
1980, Renouf et al. 1983, Allen et al. 1985), reporting midday peaks in numbers
ashore (Boulva and McLaren 1979, Thompson et al. 1989). Allen et al. (1980)
reported maximum numbers of harbor seals on a pupping site in Bolinas Lagoon,
California during low tide after 1400 h. Wilson (1974) also reported time of daytide interaction affected numbers of harbor seals ashore.
Although confounding variables did not seem to affect overall trends in
haul-out abundance, an increase in abandoned pups at S. Fanshell Beach in 1992
may have resulted from factors that separate mother-pup pairs, such as frequent
high swells associated with El Nino conditions. An opaque fence separated the
pupping site from an adjacent road, reducing most disturbances. Human
disturbance (tourist buses), however, did create mass exodus on two occasions
(27 April and 29April). After disturbances, mother-pup pairs at S. Fanshell Beach
came ashore again within 2 to 3 h. Allen et al. (1980) stated mother-pup pairs
spend less time in the water after disturbances than solitary harbor seals.
Excessive disturbances, whether natural or human, may increase pup mortality
and mother-pup separation (Kenyon 1972, Boness et al. 1992).
As lactation progressed for harbor seals near Monterey Bay, nocturnal
haul-out percentages remained lower than diurnal percentages. This may indicate
greater waterborne activity during darkness. Previous researchers have reported
increased activity of radio-tagged harbor seals at night (Allen et al. 1987, Yochem
et al. 1987, Boness pers. comm., Oxman 1995). Few direct nocturnal observations
have been attempted because of the inherent difficulties of visual observations.
Fogden (1971), however, used infrared lamps to periodically observe grey seal
45
(Halichoerus gcypus) behavior. He concluded that grey seals were as active
nocturnally as diurnally, and females always outnumbered pups at the haul-out
site, except during darkness.
Increases in numbers of solitary harbor seal pups at night and a concurrent
decrease in numbers of females may indicate females entered the water without
their pups, presumably to feed, between 2100 hand 2400 h. There were only
scattered reports of feeding by lactating phocids (Stewart and Murie 1986,
Oftedal et al. 1987, Boness et al. in press). Phocids rarely feed during their
abbreviated lactation period (Riedman 1990). This is opposite to otariid behavior
of solitary feeding trips and a return to feed their young (Bonner 1984, Oftedal et
al. 1987).
Recent studies have contradicted the accepted belief of maternal fasting
during lactation. Watts (1992) observed most harbor seals were in the water
during night time, and concluded they were foraging. Boness et al. (in press),
using time-depth recorders, found harbor seals foraged more frequently during
daylight than night. He concluded mother harbor seals on Sable Island had a
foraging cycle during lactation similar to otariids, by mid-lactation, female harbor
seals began diving an average 12 m to 40 m depth and increased diving bouts as
lactation progressed. For harbor seals at S. Fanshell Beach, foraging trip duration,
although impossible to accurately quantify, was less than 1 day. Boness et al. (in
press) also reported harbor seals near Sable Island had foraging trips less than 1
day duration. Bowen et al. (1992), found the average female harbor seal lost 80%
of stored fat during the first 19 days of lactation, and could not continue the same
net rate of body fat loss for the remainder of lactation. Thus, feeding would
appear necessary during lactation.
46
Previous studies indicated suckling session duration and frequency of
suckling can be useful in estimating energy transfer in pinnipeds (Riedman and
Ortiz 1979). Milk intake is a good index of energetic aspects of parental
investment (Ortiz et al. 1984). Although weighing mother and pup from
parturition until weaning would appear "the best measure of mass transfer of
energy (Kovacs et al. 1991), this method is not possible for less approachable
phocids or if separation was frequent (Riedman and LeBoeuf 1982). A possible
alternative is a quantitative assessment of suckling duration and frequency. It is
well documented that changes in suckling patterns throughout lactation differ
among pinniped species (Oftedal et al. 1987). Previous research indicated
duration of suckling session and suckling frequency increased with pup age for
the northern (Mirounga angustirostris) and southern (Mirounga leonina) elephant
seals (Bryden 1968, LeBoeuf et al. 1972). Fogden (1968) and Davies (1949)
reported that duration of grey seal suckling session increased, whereas, frequency
remained constant throughout lactation. A decrease in duration of suckling
session (daily duration) with age in the Weddell seal (Leptonychotes weddelli)
resulted from decreased suckling frequency and duration (Tedman and Bryden
1979). Stewart (1983) stated harp seals (Phoca groenlandica) increased duration
of suckling session, whereas, suckling frequency decreased with pup age. Kovacs
(1987), however, reported duration of suckling session in harp seals was not
correlated with pup age.
Mean diel suckling durations for pups at S. Fanshell Beach differ from
previous reports. Mean duration of suckling session for harbor seal pups at
Fanshell Beach was greater than the 1.2 min. (n =6, SE =0.73 min.) reported by
Newby (1973) and less than the 6.6 min. (n =23, SD =3.4 min.) reported by
Knudtson (1975). It was not mentioned whether these data were representative
47
of the diel cycle or the entire lactation period. Differences among reported values
also may be an artifact of discrepancies in sample size.
Total time spent suckling per diel cycle for harbor seals at S. Fanshell
Beach was lower than previous reports for pinnipeds. Oftedal et al. (1987) used
duration of suckling sessions (daily duration) as an index of time spent suckling
per day. Harbor seals near Monterey Bay suckled for l.Ohf24h during daytime,
0.46h/24h during nighttime, and a combined suckling time of 0.74h/24h. One
explanation for the discrepancies in suckling durations is the omission of data
regarding nocturnal suckling. During this study, diurnal duration of suckling
sessions was 117% greater than nocturnal suckling duration values, which may
prove important for energetic studies relying on quantitative suckling data.
Mean suckling duration for harbor seal pups at S. Fanshell Beach
increased significantly throughout the course of the lactation period. Although
previous studies have indicated energy demands in phocid pups increase
throughout lactation (Lavigne et al. 1982, Stewart and Lavigne 1984, Kovacs
and Lavigne 1986, Lawson and Renouf 1987), no study to date has linked the
food habits of harbor seal cows before pupping with an increase in suckling
duration of pups. If harbor seal cows are not storing substantial energy reserves
feeding primarily on octopus before pupping, foraging during the lactation period
will ensue. Suckling durations of pups may increase throughout the lactation
period in order to compensate for the absence of the foraging cow.
Foraging during lactation because energy intake before lactation may be a
regional phenomenon. The abundance of prey taxa in nearshore waters may
negate the need for fasting during the lactation period for harbor seals in
Monterey Bay.
48
Many parameters including energy content in milk, duration and
frequency of suckling, sex of offpring, and lactation period, are needed to
accurately assess maternal investment in phocids. This study may offer another
parameter indicating differences in adaptive responses.
This was a correlative study based on observations made under
uncontrolled conditions, and comments and conclusions are speculative. Because
of the inability to individually recognize mother-pup pairs, potential problems
with independence, and suckling and frequency estimations, may have been
created.
Summey
Food habits and movements and habits of prey items of harbor seals near
Monterey Bay, indicate nocturnal foraging. Twenty-nine prey items identified
from fecal samples collected over a 12-month period revealed adult-sized market
squid, octopus, and benthic fishes as important prey items in all seasons except
summer. Juvenile-sized rockfishes became the most important prey item during
summer, which coincided with spawning behavior of this species. Previously
unreported, this study revealed the importance of hagfish and market squid in the
diet of harbor seals in Monterey Bay. There was no significant difference in
calculated prey array indices throughout the year. Differences between
previously reported winter trawl data and fecal samples from harbor seals
collected for this study may reflect changes in prey diversity in Monterey Bay.
Harbor seals in Monterey Bay do not appear to compete extensively with
commercial fisheries. Morejohn et al. (1978) reported the California sea lion,
harbor porpoise (Phocoena phocoena), and Dall's porpoise (Phocoenoides Qillill
as a heavy consumer of L. opalescens in Monterey Bay, whereas harbor seals
were not considered in this food web. From data in this study, it appears the
49
harbor seal are largely dependent on cephalopods and must be included in the
food web of market squid consumers in Monterey Bay. Octopus dominated the
diet before pupping for harbor seals in Monterey Bay. Based on percent fat and
caloric value, octopus may not have provided adequate energy reserves to cows
before pupping. Low energy reserves may have influenced pup suckling
durations throughout lactation.
Between the second and third week of lactation, during harbor seal
pupping at S. Fanshell Beach, numbers of solitary pups increased duing nocturnal
hours. This increase was due to females entering the water without their pup to
presumably forage. The belief mother and pup harbor seals need to remain in
close proximity throughout an intense lactation period may be an artifact of
observing harbor seal mother-pup behavior solely during daylight hours. There
was no significant difference between diurnal and nocturnal hourly suckling
session durations. Hourly "proportion of animals suckling" was significantly
greater during the diurnal periods, which may prove important in energetic
studies. The total suckling time was 117% greater during diurnal than nocturnal
periods, whereas combined duration was lower than previous reports for harbor
seals. In future studies, more emphasis should be placed on methods to identify
mother-pup pairs during nocturnal observations.
This study has raised questions concerning the possible link between
seasonal food habits, movements, and mother-pup suckling throughout lactation.
Future studies should address the problem of identifying and aging individual
pups during lactation. A larger sample size of harbor seal mother-pups over
consecutive seasons is needed to answer questions of suckling duration over the
diel cycle. A larger sample size of radio-tagged harbor seals is needed to
adequately assess movements of harbor seals in Monterey Bay.
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APPENDICES
APPENDIX A
TABLES
64
Table 1. Monthly mean dive information including number of dives per tracking (#of
Dives), mean dive and surface interval (SI) duration, percentage of time spent hauled-out
and diving, and percentage time spent diving at night for individual harbor seals tagged
near Monterey Bay in 1992. Standard error in parentheses. ES = Elkhorn Slough
660-0ct
41
Mn Dive Mn SI,
Duration, min(SE)
min (SE)
1.04 (0.6)
5.2 (2.5)
660-Nov
183
3.3 (1.9)
0.63 (0.5)
660-Dec
182
2.3 (1.7)
0.55(0.18) 36.(7.2ES)
660-Jan
147
4.0. (3.4)
0.7 (0.3)
58
42
93
660-Feb
132
3.6 (1.9)
0.6 (0.18)
57
43
62
680-Feb
54
10.0 (2.8)
0.77(0.17)
61
39
82
680-Mar
59
7.6 (3.9)
0.77 (0.3)
75
25
67
680-Mar
35
10.3 (3.1)
0.83(0.14)
50
50
100
680-Apr
89
3.3 (3.6)
0.46 (0.3)
53
47
78
680-Apr
40
7.9 (4.1)
0.68(0.15)
36
64
100
680-May
74
7.4 (3.1)
0.77 0.16)
59
41
85
800-0ct
141
2.7 (2.0)
0.77(0.22)
48
52
85
800-Nov
117
3.9 (1.7)
0.66(0.28)
61
39
87.5
800-Dec
217
2.4 (1.6)
0.7 (0.55)
53 (4.2ES)
46.5
95
800-Jan
117
4.1 (2.3)
0.67 (0.3)
39
61
95.1
800-Feb
110
3.1 (2.3)
0.7 (0.37)
36.1
63.9
87.5
951-Feb
54
9.1 (2.7)
0.75 (1.9)
87.5
12.5
57
951-Mar
27
7.8 (3.9)
0.75(0.15)
56.6
43.2
88
Seal # Month
#of
Dives
%Time
Spent
%Time
Diving at
42
%Time
Spent
Hauled
58
50
50
73
63.6
61
Divin~
Ni~ht
98.6
65
Table 2. Mean dive and haul-out information for harbor seals including mean hours hauledout per harbor seal, mean individual haul-out time, total hours spent diving, and hours
spent diving during daytime and nighttime near Monterey Bay in 1992. Standard deviation
in parentheses.
800
Mn hours
Hauled out
(SD)
12.9 (3.02)
Mn Single .
Haul-out
in hrs (SD)
6.65 (3.06)
Mn Hrs
Diving
Total (SD)
11.1 (2.83)
Mn Hours
Diving at
Night (SD)
10.8 (0.51)
Mn Hours
Diving Per
Dar
2.26(0.17)
660
11.4 (3.06)
9.47 (3.56)
11.7 (3.17)
9.30 (1.87)
2.38(0.5)
680
10.64 (2.2)
6.56 (3.83)
13.36(2.45)
10.2 (1.41)
3.12(1.6)
951
6.70 (2.75)
6.70 (2.75)
17.3 (3.64)
8.70 (1.86)
8.6(2.1)
SEAL#
66
Table 3. Prey species in 65 harbor seal scats (in decreasing order) collected during
summer, 1991-1992. Mean Index of Relative Importance was calculated by sum of mean
percent number (%N) and mean percent mass (%M) times percent frequency of occurrence
(%FO). n is the greatest number of left or right otoliths or upper or lower beaks. Standard
deviation in parentheses.
SPECIES
(Common Name)
Sebastes sp.
(Rockfish)
Merluccius productus
(Pacific Hake)
Porichthys notatus
(Plainfm Midshipman)
Chilara taylori
(Spotted Cusk-eel)
Octopus sp.
(Octopus)
Loligo opalescens
(Market Sqnid)
Engraulis mordax
(Northern Anchovy)
Genyonemus lineatus
(White Croaker)
n
MN %N
(SD)
100 18.0 (4.12)
MN%M
(SD)
16.3 (4.9)
35.5
MN IRI
(SD)
1218 (357 .2)
24
13.7 (4.7)
16.3 (5.1)
24.2
725.5 (235)
57
13.1 (4.7)
12.5 (4.6)
22.6
579 (211)
152 11.9 (3.9)
7.9 (3.6)
24.2
479 (181)
30
8.6 (3.4)
8.9 (3.7)
24.2
423.5 (171)
58
10.0 (4.3)
10.2 (4.2)
16.1
326.5 (136)
14.5
160.5 (80.3)
%FO
51
6.95 (3.1)
4.1 (2.4)
47
4.7 (2.8)
5.7 (3.0)
Pl!:l!.J!:QO~~ vernly~
11
1.8 (1.0)
3.1 (1.7)
11.3
56.0 (30)
(English Sole)
Citharichthys sordidus
(Pacific Sandab)
28
2.4 (1.5)
3.1 (2.0)
9.7
54.0 (34)
MicrQ~lQID!.J~ pacificy~
7
1.4 (1.0)
2.6 (1.9)
6.45
25.6 (18.6)
3
2.7 (2.3)
2.5 (2.2)
3.23
16.8 (14.4)
5
1.3 (0.6)
0.3 (1.6)
3.23
5.0 (7)
3
0.4 (0.3)
0.5 (0.5)
4.84
4.6 (3.9)
1
0.5 (0.4)
1.1 (0.9)
1.6
2.5 (2.1)
1
0.3 (0.3)
0.05 (0.06)
1.6
0.5 (0.6)
(Dover sole)
Clupea pallasi
(Pacific Herring)
Trachyrus symmetricus
(Jack Mackerel)
Eop~J1il jQrdani
(Petrale Sole)
Eptatrerns sp.
(Hagfish)
Spirinchus starksi
(Ni~ht Smelt)
6.45
66.8 (37.3)
67
Table 4. Prey species in 67 harbor seal scats (in decreasing order) collected during autumn,
1991-1992. Mean Index of Relative Importance was calculated by sum of mean percent
number (%N) and mean percent mass (%M) times percent frequency of occurrence (%FO).
n is the greatest number of left or right otoliths or upper or lower beaks. Standard deviation
in 12arentheses.
SPECIES
(Common Name)
OctQpus sp.
(Octopus)
l&!igQ Qlli!l~~~~n~
(Market Squid)
Sehastes sp.
(Rockfish)
.c!Jililra JiU1Q!:i
(Spotted Cusk-eel)
Elltalrl:~IS sp.
(Hagfish)
Geoynnemus linean1s
(White Croaker)
Ci!llad~btll:.s sordidus
(Pacific San dab)
PQriQht!n:s lilllllll&
(Piainfm Midshipman)
Flatfish (un)
Mi~smmus uadfi~ys
(Dover Sole)
Merlnccius pmductus
(Pacific Hake)
Qphjodon elongaOJs
(Lincod)
Pleumnectes .Y5lliJJ..us
(English Sole)
!&1.1Wk!lllmi armatus
305
MN %N
(SD)
29.1 (4.3)
MN %M
(SD)
24.8 (4.2)
50.74
MN IRI
iS D)
2735 (431.9)
145
19.8 (4.04)
15.7 (3.9)
31.3
1111 (249.6)
n
%FO
19
7.1 (2.3)
8.2 (2.6)
28.3
433 (139.4)
44
7.3 (2.0)
4.6 (1.6)
34.3
408 (124.5)
35
7.7 (2.4)
10.0 (2.9)
22.4
396 (119.7)
34
7.8 (2.8)
10.2 (3.2)
16.4
294 (99)
36
6.3 (2.15)
6.6 (2.3)
16.4
212 (74)
11
3.1 (1.0)
4.2 (13)
19.4
141 (272.3)
8
6
1.54 (1.5)
1.35 (0.7)
1.5 (1.5)
1.38 (0.7)
11.9
8.9
37 (36.1)
24 (12)
7
1.02 (0.6)
1.77 (0.9)
7.4
21 (11)
3
1.05 (0.8)
3.12 (1.9)
4.5
19 (12)
4
1.6 (1.5)
1.6 (1.5)
4.5
14 (13.5)
3
0.63 (0.4)
1.3 (0.8)
6.0
12 (7.5)
4
0.9 (0.6)
0.6 (0.3)
4.5
7 (4.1)
2
0.5 (0.4)
1.6 (1.2)
2.98
6 (4.5)
5
0.22 (0.22)
0.7 (0.6)
2.98
3 (2.6)
2
0.53 (0.3)
0.33 (0.32)
2.98
3 (1.9)
3
0.34 (0.33)
0.18 (0.17)
1.5
1 (0.7)
8
0.12 (0.12)
0.05 (0.05)
2.98
1 (0.5)
1
0.13 (0.12)
0.05 (0.05)
1.5
0.27 (0.26)
2
0.05 (0.04)
0.03 (0.11)
2.98
0.25 (0.44)
1
0.03 (0.03)
0.08 (0.07)
1.5
0.16 (0.15)
(Staghorn Sculpin)
fu!M zachirus
(Rex Sole)
Anoolopmna li:i!!.!lml
(Sablefish)
EopsettajQnjm)j.
(Petrale Sole)
Atherinopsis .affinis.
(Jacksmelt)
L:.opsetta l:XiJili
(Slender Sole)
At!l~rinQllS llffiiJlli
(Topsmelt)
Engrnglis l.!!!ll1!l!l!;
(Northern Anchovy)
CymatQgasler aggregata
(Shiner Surfperch)
Elati~blb:<s stellaJ.JJs
(StarrY Flounder)
68
Table 5. Prey species in 43 harbor seal scats (in decreasing order) collected during winter,
in parentheses.
333
MN %N
(SD)
47.5 (5.2)
MN%M
(SD)
50.7 (5.33)
79.1
MN IRI
(SD)
7767 (833)
287
42.8 (5.3)
37.4 (5.1)
67.4
5405 (679)
22
3.5 (1.2)
2.2 (0.9)
16.3
93 (33)
4
1.1 (0.4)
1.3 (0.6)
11.6
28 (12)
8
1.8 (0.9)
0.14 (0.5)
9.3
27.5 (13)
3
0.5 (0.2)
1.4 (0.75)
9.3
17.7 (9)
3
0.7 (0.3)
1.0 (0.5)
9.3
16 (8)
4
0.3 (0.2)
1.2 (0.7)
6.98
10.5 (6)
2
0.8 (0.1)
1.14 (0.06)
2.32
4.5 (0.4)
1
0.3 (0.2)
1.0 (0.8)
2.32
2.9 (2.4)
5
0.5 (0.15)
0.6 (0.2)
2.32
2.5 (0.4)
Eptatretus sp.
(Hagfish)
1
0.15 (0.13)
0.9 (0.8)
2.32
2.5 (2)
~zachirus
1
0.3 (0.2)
0.4 (0.3)
2.32
1.7 (1.3)
1
0.2 (0.2)
0.3 (0.2)
2.32
1.2 (1.0)
1
0.15 (0.13)
0.08 (0.06)
2.32
0.5 (0.4)
SPECIES
(Common Name)
Octopus sp.
(Octopus)
Loligo sp.
(Market Squid)
Sebastes sp.
(Rockfish)
Porichthys notatus
Chilara taylori
(Spotted Cusk-eel)
Citharichthy~ ~QrQiQ!l~
(Pacific Sandab)
GenyQnem!JS linea(Us
(White Croaker)
LeptocQttus annatus
(Staghorn Sculpin)
Flatfish (un)
Merl!JCCilJS produc(Us
(Pacific Hake)
Unid fish
(Rex Sole)
Citharichthys ~tigrm.illu~
(Speckled Sandab)
En gnm lis m Qrgax
(Northern Ancho~)
n
%FO
69
Table 6. Prey species in 44 harbor seal scats (in decreasing order) collected during spring,
in parentheses.
MN %N
MN%M
(SD)
(SD)
55.03 (6.76) 48.8 (6.6)
65.9
MN %IRI
(SD)
6843 (897 .8)
31
17.08 (4.33)
45.5
1582 (397.1)
Citharichth~~ sQrgigy~
21
5.54 (2.15)
8.05 (2.9)
18.2
247 (92.4)
(Pacific Sandab)
Plegmnectes yetulus
(English Sole)
14
4.27 (2.36)
5.0 (2.7)
15.9
148 (80)
PQrichth~s nQtB!l!~
13
4.6 (3.02)
5.0 (2.9)
6.8
65 (40.2)
4
2.7 (2.44)
3.7 (2.3)
9.1
58 (44)
8
2.9 (1.88)
2.1 (1.4)
6.8
34 (22.1)
5
3.25 (2.45)
3.2 (2.4)
4.5
29 (22)
Eptatretus sp.
(Hagfish)
GenyQnemys lineatus
(White Cmaker)
3
0.9 (0.7)
1.8 (1.7)
4.5
12 (10.7)
2
0.6 (0.6)
1.3 (1.3)
4.5
8 (8.4)
Citharichth~~ stigmaeu~
2
0.8 (0.6)
0.8 (0.6)
4.5
7 (5.4)
(Speckled Sandab)
Cm~atoga!.!lllr <l!:li:l£li:<ltl!
(Shiner SurfEerch)
1
0.2 (0.2)
0.05 (0.05)
4.5
1 (1)
SPECIES
(Common Name)
Loligo sp.
(Market Squid)
Octopus sp.
(Octopus)
Sebastes sp.
(Rockfish)
Chilara lllili!ri
(Spotted Cusk-eel)
Flatfish (un)
n
187
17.7 (4.4)
%FO
70
Table 7. Seasonal mean lengths of important prey items (em) found in harbor seal fecal
samples near Monterey in 1991-1992. Standard error in parenthesis. Note: Lengths sharing
common underlines (solid or dashed) indicate no statistical difference.
SPECIES
(Common Name)
N
Loligo opalescens
(Market Squid)
Oc!Qpys sp.
(Octopus)
Sellll~~s sp.
(Rockfish)
Citharichthys sodidus
(Pacific Sandab)
Chilam taylori
(Spotted Cusk-eel)
Porichthys notatus
(Plainfm MidshiEman)
677
Winter
Lengths
(SE)
ll.6 (Q.8)
Summer
Lengths
(SE)
lQ,S (1,5)
Autumn
Spring
Lengths
Lengths
(SE)
(SE)
ll,Q (Q.96) ]].3 (0.8)
4,68 (Q.4)
699
:i l:i (Q.4)
:i.42 (Q,l)
145
1.41(Q.8)
1.2Q (Q.5) 14.5 (1.9)
18.5 (1.5)
12,2 (1,l:i) 12.4 (Q,9)
21.6 (0.6)
16,9 (1,2)
23.9 (0.7)
17.0 (1.1)
!2.9 (1.4)
88
no data
212
20.5 (0.9)
16.3 (Q.3)
85
2!.l (Q.3)
21.4 (Q.;j)
:i.4:i (Q,l)
-------------- -------------
71
Table 8. Percent similarity indices based on prey items found in harbor
seal fecal samples among seasons. Fecal samples were collected near Monterey
from May 1991 to June 1992. Significance based on 75%.
SEASONS
SUMMER
AUTUMN
AUTUMN
39.0
SPRING
31.0
46.0
WINTER
22.0
72.0
SPRING
58.0
72
Table 9. Estimated biomass of commercially important fishes and cephalopod species eaten
by harbor seals in Monterey Bay between 1992 and 1993 compared to total catch of these
species in commercial fisheries (Cal Fish and Game) .
SPECIES
(Common Name)
Errex zachirus
(Rex Sole)
Engraulis mordax
(Anchovy)
MicrQS(Qmus !lecificgs
(Dover Sole)
Pleuwnectes yetulus
(English Sole)
OphiQdon elongatus
(Lingcod)
LoligQ opalescens
(Market Squid)
Sebastes sp.
(Rockfish)
Sand dabs
GenyQnemus line;rtus
(White Cwaker)
TOTAL gSCATS
• ESTIMATED
PERCENT
ANNUAL
TOTAL BAY
BAY
BIOMASS
CATCH (KG)
CATCH
EATEN
1,209.8
2,434
75,364
3.23
1,506.6
3,031
608,814
0.5
3,811.8
7,669
645,588
1.20
6,502.9
13,083
115,770
11.3
9,152.9
18,415
123,034
14.9
14,100.9
28,371
6,116,742
0.46
15,854.3
31,899
1,418,296
2.25
20,587.2
34,326.1
41,421
69,064
48,124
48,124
86.1
143
APPENDIX B
FIGURES
37°N ,------------.---------------------r--~~------,
Santa
r
Monterey
Bay
36"40'
Monterey
121°50'
Figure 1. A map of Monterey Bay showing five locations of study and
haul-out sites: Cypress point, S. Fanshell Beach, Hopkins Marine
Station, Seal Rock, Elkhorn Slough, and Davenport.
74
UJ
75
1-
m s4o
1-
5 290
....:..
~ 240
:c
5 190
~
<(
~
~
140
9oJ==c~==~~=:c~c=:c==c~==c=~~
180
(f)
~ 160
(i) 140
1-
FANSHELL MNS
5• 120
~
HOPKINSMN
<(
-
SEAL ROCK MN
~
CYPPT.MN
5 100
:c 80
z
0
~
<(
~
60
40
z<( 20
Figure 2. Mean monthly abundances of a) harbor seals at
offshore haul-out sites near Monterey Bay in 1991 -1992.
Dark bars indicate mean, boxes indicate one standard
deviation, vertical lines indicate range; b) Individual locations
between Seal Rock and HMLR. Vertical lines indicate one
standard deviation.
37°N r-----------.-------------------.-~==~----.
r
Monterey
Bay
121"50'
Figure 3. Movements of harbor seal #660 in Monterey Bay from
October 1992 through February 1993. Lines represent 24-hr trackings.
All animals tracked returned to Seal Rock within 24 hrs.
76
77
37°N
r··
Monterey
Bay
- - - - December
Depth in Meters
121 °50'
October 1992 through February 1993. Lines represent 24-hr trackings.
78
37°N
Santa
r
Monterey
Bay
,., .
"l··
50m
-----
~-.·
.
• ·.r.
-:.:~~:.
.......... .
:;a..
:,··.:.-,
36°50'
36"40'
121 °50'
February 1992 through May 1992. Lines represent 24-hr trackings.
79
r
Monterey
Bay
---February
-March
121°50'
February 1992 through March 1992. Lines represent 24-hr trackings.
Mean Dive Duration (sec)
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83
0
Q)
(/)
c::
0
+::::
ttl
,_
700
600
500
400
300
200
100
0
lost
Feb
0
0
0
0
0
0
0
0
0
<t
';"
0
0
0
0
~
~
co
0
0
0
0
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0
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9"' ' '
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0
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0
0
0
0
0
0 0
....0 0>0
"' "' .... 0> c;; "' 0 "'0 "'0
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::::l
0
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~
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Time of Day "'
~
~
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0
.:::
~
0
c::
ttl
Q)
:::2:
700
600
500
400
300
200
100
0
Mar
0
0
"'9
~
0
....
0
0
0
0
~
'
lost
0
0
0
0
~
"''
0
0
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~
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'
'
0
0 0 0
m
0
0
0
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~
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~
0
0
0
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0
0
gj
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0
0'
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0
0
0
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9
0 0 0
0
0 "'
"'0
<t
~
0
c;; "'
Time of Day "'
0
0
\
"'9 "9
0
0
co
Figure 10. Mean duration of dives each hour for
harbor seal #951 (n=81) during monthly 24-hr
trackings for February and March 1992. Spaces
indicate haul-out unless indicated lost. Vertical
lines indicate standard error.
84
OTHER
HOPKINS
SEAL ROCK
15
~
Ql
.0
E
1
::l
z
w ~ Ia: a: a:
>>en a:
<(
a:
w w >w
z :::>
a:
w
:::>
C!l <( <(
J:
(.)
...1
a:
C!l C!l C!l
a: c..
::2 :::>
-, -, 0 ::2
0 ::2 ::2 :::> :::> <( <(
:::> w I- w w z a: ::2
<( I- (.)
(.) <( C!l
w
c.. 0 >
0 w -, u..
w
z Cl
en
Figure 11. Location and quantities of fecal samples
collected from harbor seal haul-out sites near
Monterey in 1991-1992 (n=222)
85
0
5
10
15
20
25
30
35
Cumulative Number of Fecal Samples
Figure 12. Cumulative species curve representing number of
harbor seal fecal samples collected during autumn 1991 near
Monterey Bay. Other seasons fell below sample size needed for
autumn.
40
86
3
3
en 2.5
en 2.5
LU
0
0
~
z<(
LU
LU
2
2
0
0
1.5
1
~
1.5
z<(
1
LU
:2 0.5
:2 0.5
0
en I:
...,
0
:2 a:
...,
en I:
Cil
0
0
;ft.
SUMMER
AUTUMN
3
3
en 2.5
en 2.5
LU
LU
0
~
z<(
LU
2
Q
0
1.5
~
z<(
1
LU
:2 0.5
0
Cil
0
0
;ft.
Q
:2 a:
2
1.5
1
:2 0.5
0
en I:
...,
0
:2
0
0
~
0
WINTER
a:
Cil
en I:
...,
:2 a:
Cil
0
0
;ft.
SPRING
Figure 13. Seasonal prey array indices calculated from harbor
seal fecal samples collected near Monterey Bay in 1991-1992; S
=Number of prey, H' =Diversity, J = Eveness, R =
Specialization, B =Niche breadth. Error bars denote± one
standard error.
87
40,-------------~
(;' 30
c:
g: 20
C"
WINTER
WINTER
~ 10
lL
o--1-rM.,...,_.,-f'M..,..,..,.....,..,..,...,..I
OVOOC\JtOOVOOC\ItD
OVOOC\ICOOVOOC\ICOO
..--r-C\IC\JC\IMMV
Sebastes sp. Length (em)
Chilara taylori Length (em)
..... ..-C\IC\IC\1(1')(1')
40,--------------,
g3o
g:
20
SPRING
SPRING
U::if 10
0 -!-rTTT"f"rl.,..,..,..,.,..,'"T"M..,...,..I
OVCOC\ICOOVOOC\ICO
OVCOC\ICOOVOOC\JCOO
..-..-C\IC\IC\IC')('I)
..-.--C\IC\IC\1('1)(')"¢
40,-------------~
g3o
g:
20
SUMMER
C"
~ 10
lL
O'V CO C\1 co 0 V OJ C\1 (0
'
..-,-C\IC\IC\IMM
OVCOC\ICOOVCOC\1(00
40,-------------~
AUTUMN
OVCOC\ICOOVOOC\!CO
..-..-C\lC\IC\IMM
..-..-C\IC\IC\IMMV
40,--------------,
AUTUMN
OVQ)C\I<OOVCDC\l(OO
..- ..... C\IC\IC\IMMV
Figure 14. Length frequency histograms for Sebastes sp. and
Chilara taylori found in harbor seal fecal samples near Monterey
Bay in 1991 - 1992.
88
150~---------------,
iS
AUTUMN
ljj 100-
l
Ql
g.
:::J
J:
50-:
0
g
+,.....,
11-"l""r
II
I
I
I
100,----------------,
80
60
AUTUMN
40
20
o;,.,,TT~~~,
I
020406080
Octopus DML (mm)
,., 150,----------------,
gQl
l
WINTER
100
::J
Loligo DML (mm)
100,-----------~---,
iS
ljj
80
60
U::
20
WINTER
g 40
50
0 4--T""T".,.-,
o,_,.,TT~~~~
020406080
00000000
C\l~tO
COOC\1""'"
Octopus DML (mm)
Loligo DML (mm)
,., 150 . . . . , - - - - - - - - - ,
~
l
SUMMER
100-:
100,----------------,
iS 80
SUMMER
ljj 60
40
20
g
::J
u:
50-
o;,.,,TT~~~,
,,r-,...,
0 +"T'""'1
,,r-"T'""'1 ,,rill,-,
,,r-"T'""'1
,,r-1
0 '20 40 60 80 100
Loligo DML (mm)
Octopus DML (mm)
150~---------------,
iS
ljj 100-
l
g
Ql
SPRING
::J
l
:::J
50-:
o;-.,-,~·-~r;-,_,
I
I
I
I
I
0
20
40
60
80 100
Octopus DML (mm)
100.,----------,
80SPRING
6040200
I I I I I I I I I I I
0
0
C\J
0
v
0
0
co co
0
0
lrJ/'\
0
C\1
I
0
'¢
Loligo DML (mm)
Figure 15. Length frequency histograms for Octopus sp. and
Loligo opalescens found in fecal samples collected near Monterey
Bay in 1991 - 1992.
89
10
(;' 8
"Ql 6
I:T 4
Ql
ll: 2
0
20.-------------~
SUMMER
(;' 15
SUMMER
"
"'
~ 10
I:T
~
5
IJ..
0 4 8 1216 20 24 28 32
0 4 8 1216 2024283236
Cithariehthys sordidus Length (em)
>.
""
"'~
Ql
10
8
6
20.-------------~
AUTUMN
l
2
IJ..
(;' 15
"~
4
I:T
Poriehthys notatus Length (em)
10
5
0 --J-,..,..,..,-IIr-~Wir-i"r-,....-,-.,.1
0 4 8 1216 20 24 28 32
0 4 8 12162024283236
>.
""Ql
"'
I:T
Ql
~
IJ..
10
8
6
4
AUTUMN
20-r--------------~
SPRING
(;' 15
"~
l
2
0
SPRING
10
5
0 --h-TT"T..,..,-~I,B,.-/~,..,.,-.,.1
0 4 8 121620242832
0 4 8 12162024283236
20.-------------~
(;' 15
"~ 10
l
WINTER
5
0 4 8 12162024283236
Figure 16. Length frequency histograms for Citharichthys sordidus
and Porichtbys notatus found in harbor seal fecal samples collected
near Monterey Bay in 1991- 1992.
/\:) "" ..p. 07 OJ
0000000
(") 'TJ
0I=(JQ
-·
~
~ (1l
(1l
\
(1l
0..-.l
::s
(1l
e;
Cl:l
:S::i3"'
0 ::s
L.annatus
0. elongatus --1----1...
T. symmetricus
S. shirks/
a~=..
(ll'O
(ti (1l
'< (=l
t;OC1l
"' ::s
'<
.....
::r ~
-c:r'
\O(ll
\0~
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'""
-'0
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t:S«
-·
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Cti'
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"'
~
-
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( 1l
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a.
Q
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(1l
!:..
"'
0
06
~
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(1l
/\} "" .A 07 OJ
000000
'\
91
00000000
C:O/'-CDI.OVcr:;JCV...-0
000000000
COf'..CDI.OVcr:;JCV..-
92
9o
9o
so
so
7o
7o
......_, .....,...60
6o
so
......_ I /
......._,...,.,...so
~f"H...L
40
1.....+- 40
:soGf+..l.l L-h 30
ao
<o
......._,...,.,...<o
>V<.XI-JL I
......._, ....,..10
1o
"""'-' •..-.... 0
0 ..,__, ....
-"
·~
"CI
"S
"'
~
J
Fiqure 19. Summer frequency of occurrence (%FO) of prey found in harbor
seal fecal samples near Monterey Bay in 1991-1992 versus summer shallow
and deep trawls conducted in Monterey Bay during the mid 1970's (Cailliet
et al. 1979).
1
0.9
93
o.a
0.7
0.6
0.5
0.4
0.3
0.2
0.1
April
12
04--.--.--r-.--.-~
1
1200-
0.9
o.a
l\prll
l\prll 22
0.7
:::J
0
.,•
"5
:I:
c:
0
en
iii
Q)
en....
0
.,
:I:
.c
....
s0
.,..
..:::..
-...
c:
Q)
(.)
Q)
a.
0. 6
o.s
0.4
0.3
0.2
0.1
HH
0~--~~---r--~~r--,
1
0.9
1
0.9
o.a
o.a
0.7
0.7
0.6
0. 6
o.s
o.s
o.4
1200-
l\pril 29
HL
0.4
0.3
0.2
0.1
o.1
o4---r-~--~~~~~
04--,--.-~~~~~
0.3~~~~
0.2
1
0.9
0.8
1
0.9
o.a
0.7
0.6
0.7
0.6
o.s
o.s
0.4
0.3
0.2
0.1
HHo.2
0.1
0.4
0.3
0,__,--.--.--.--r-;
0000
1200
04---r--r--~-.---r~
24000000
1200
2400
Hour of Day
Figure 20. Percent of maximum of harbor seals hauled-out at S.
Fanshell Beach, Monterey Bay California during the 1992
pupping season. Missing data indicated by discontinued line.
Note: LH =low-high tide, HH =High-high tide, LL =low-low
tide, HL = high-low tide. Disturbances also indicated on 27
April and 29 April.
94
18
16
(/)
14
(ij
E 12
·c
<(
....0 10
....
Q)
.c
E 8
::J
z
c
til
6
Q)
:.2: 4
2
0
0400
0800
1200
1600
2000
2400
TIME OF DAY
Figure 21. Mean number of lone harbor seal pups (closed
circles) and mother- pup pairs (open circles) at S.Fanshell
Beach, Monterey Bay during the 1992 pupping season.
Vertical lines represent one standard deviation.
95
2
i:/5
60
'5
50
0
..!.
:::J
I
co
c
0
UJ
al
(/)
~
Q)
.0
30
20
E
:::J
z
c
al
10
~
4-9
4-12
4-15
4-22
4-27
4-29
5-6
5-14
Date
Figure 22. Mean abundance of harbor seal adults (open bars)
and pups (closed bars) at S. Fanshell Beach, Monterey,
California, during the 1992 pupping season. Vertical lines
represent one standard error.
96
500
450
u;-
-g 400
0
al
.e 350
N=630
§ 300
~
6 2so
Q)
.!:
::;;;
200
(.)
rJl
150
c
Sl 100
:2
50
0000
0600
1200
1800
2400
Time of Day
Figure 23. Mean hourly suckling duration of pups at S. Fanshell Beach,
Monterey, California, during the 1992 pupping season. Vertical lines
indicate one standard error. Closed bars indicate nocturnal times. Numbers
for each hour represent number of suckling sessions observed.
97
1
~
c
0
900
800
(.)
Q)
.!!!. 700
c
0
600
::l
500
c
400
~
"C
Ol
32
g
300
(J)
c
ctl
Q)
::2 100
0
9Apr
12Apr
15Apr 22Apr
27Apr
29Apr
06May 14May
Date
Figure 24. Mean duration of diurnal (open bars) and nocturnal
(closed Bars) suckling sessions for each observation day
throughout lactation during the 1992 pupping season at S.
Fanshell Beach.Verticallines indicate one standard error. Samples
sizes are located under axis.
98
1.6
Ol
.!: 1.4
.)<:
u
c7.l 1.2
rn
1
E
~ 0.8
tii
0
c:
0.6
0
'E 0.4
0
c..
e o.2
0..
0
0400
08 0
1200
16 0
2000
2400
Time of Day
Figure 25. Mean proportion of harbor seal pups suckling
per hour at S. Fanshell Beach during the 1992 pupping
season. Lines indicate ±one standard error. Note: Dark
bars indicate nocturnal times.
99
1.2
1
Cl
.!:
-a 0.8
:::l
(/)
en
c.
:::l
a.. 0.6
0
c:
0
t
g_ 0.4
e
a..
0.2
0
171
2221 4232 7942 9237 9447 7025 123
9Apr 12Apr 15Apr 22Apr 27Apr 29Apr 06May 14May
DATE
Figure 26. Diurnal (open bars) and nocturnal (closed
bars) proportion of animals suckling per diel cycle
during the pupping season at S. Fanshell Beach,
Monterey. Vertical lines indicate± one standard
error. Sample size under each bar.
100
1.6
.~ 1.2
:52
u
~
1
c
"'
bj-0.8
"'E
i= 0.6
'iii
~ 0.4
0.2
09 Apr 12 Apr 15 Apr 22 Apr 27 Apr 29 Apr 06 Apr 14 Apr
Date
Figure 27. Diurnal (open bars) and nocturnal (closed
bars) total time spent suckling (number of suckling
sessions multiplied by proportion of animals suckling)
for harbor seal pups at S. Fanshell Beach, Monterey,
during the 1992 pupping season.

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