Optimal Feeding Position for Stream Fishes in Relation to

Optimal Feeding Position for Stream Fishes in Relation to

HUMANS
and NATURE,
No.l, 63-81,March 1992
Optimal
Feeding
in
Position
Relation
for
Stream
to Invertebrate
Tetsuo
Fishes
Drift1'
Furukawa-Tanaka
Division of Ecology, Natural History Museum
Office,Nakayamate-dori
Projects,Administration
6-1-1,Chuo-ku, Kobe, 650 Japan
Abstract
The verticaland horizontaldistributions
of invertebratedrift,and the horizontal distribution of macrobenthic animals, were studied in a mountain stream to define advantageous
feeding positions for visual feeding stream fishes during the summer
driftabundance
season. The diurnal
of benthic or fallen animals were respectivelylinearlyor exponentially
correlated with the current speed. The estimated total daytime amounts
of invertebrate
driftpassing downstream through the study pool reached about 72 g in wet weight, of
which fallen drift accounted for 73 %. It is therefore suggested that the swift surface
current layer is the most advantageous feeding positionfor visual feeding stream fishes.
Key words: Invertebrate drift,Salmonid food
aquarium
Introduction
Invertebrate drift has
nized as
stream
an
available
macrobenthic
phenomenon
has been
reported
not an
of macrobenthic
recognized
for
Tanida
that
accidental
and
posi-
animal,
it
individual
and the choice of feeding
fish, the latter of which
realized mostly
is necessary
Furukawa-Tanaka,
accurately
estimate
fishes, and
invertebrate drift in the water
1985).
studies have revealed that stream
resident salmonid
to
1984; Furuet al., 1989), it
micro-distribution
1968, 1969; Waters,
fishes exhibit
territoriality, both
a
social
in
the
1) A part of doctoral thesis of Kyoto University.
and composition
animals
column
to clarify
and
macrobenthic
in a stream,
dominant
animals
salmonid
and
on the substratum.
The present study was designed
drift in a stream
the
of both
the microdistribution
Fausch,
to be
by social status (Jenkins,
of invertebrate drift
macrobenthic
habits
position by the
attention to the relationship
et al., 1970; Griffith, 1974;
1985;
is considered
1985; Tanida
b, 1970, 1973; Chaston,
1969;
feeding
1983; Bachman,
to the
in
et al., 1989). To elucidate the rela-
tionship between
kawa-Tanaka,
availability of drift as food (Elliott, 1967a,
hierarchy or
Jenkins,
1969; Fausch,
and the diet of stream
Further
1956;
subse-
have
Yamagishi,
1984; Furukawa-Tanaka,
workers
the components
1969; Jenkins,
nature (Newman,
Bachman,
1958;
1983; Ishigaki, 1984) and
to as part of inverte-
brate drift. Several
quently paid
recog-
fishes (Needham,
Mtiller (1954)
drift was
been
resource
but rather a normal
tive behaviour
between
long
food
resident salmonid
1928). Since
(Kalleberg,
1962; Fausch,
pool
composition
occupied
fish
by
(Jenkins,
1983; Furukawa-Tanaka,
and
of
of
large,
1969;
1985).
2 days of measurement
Materials and Methods
The
study was performed
at Sawamata
altitude) of
mountain
System,
in August
1978
(34 °13'N, 135 °55'E, 880 m
the
Jindoji-dani,
stream
flowing
of
the
from
a
typical
Kumano
River
the southern
slope of
(August 6 to 7).
At the Sawamata
station,resident redspot masu salmon, Salmo (Parasalmo)
masou
macrostomus,
Glnther,
predomi-
nated, followed by resident Japanese charr,
Salvelinus leucomaenis (Pallas).
A small
Mt. Sanjo-gatake
(1719 m)
in the centre of
number of the cyprinid,Phoxinus oxycephalies,Sauvage
et Dabry, was also present.
the Kii Peninsula
(Fig. 1). This is a third
Bottom and surface iso-current speed lines
order stream,
maximum
formed
3-8 m
depth.
by
in width and
The
bedrock,
stream
large
1-2 m
in
bed
was
boulders
and
gravels. Terrestrial vegetation consisted of
successional bushes
after clear cutting for
based on 1 metre grid intervalmeasurement
over the study area, which was composed of
a pool area (point 11-14 in Fig. 2) and a
downstream
C-riffle area (point 1-10, for
definitionof C-rifflesee Furukawa-Tanaka,
logging, but a littlevirgin forest remained
1985), were drawn based on measurements
near both sides of the stream.
made using an SPC-5 compact current
speed meter (Sanko Precision Machinery;
upstream
of Sawamata
virgin forest. The
Sawamata
water
was
The
portion
covered
temperature
varied within a narrow
with
at
range of
15.5°C(2:00-6:00)-16.5°C(12:00-18:00) during
Fig. 1. Map
of study area. The Kii Peninsula
fan diametre 20 mm, Tokyo, Japan).
Macrobenthic animals were sampled at
the pool area, C-rifflearea, and adjacent
is located in the centre of Honshu
Island.
Fig. 2. The isopleths of the bottom
the surface current speed and water
area) and
meter
boulder
current speed (cm*s-1)
(illustrated area) follow Hynes
grid measurement
of the water (cm),
(1970). Each
respectively. The minus
adjacent R-riffle (15-20). The
diameter
of bottom
the figure.
stones ( 0max
(upper), and
(dotted area), pebble (blank
isopleth is drawn
based
on every one
figure indicate bottom
lines in the lower figure indicate surface current speed and
value of the current speed in the both sides of transect
line of the pool indicates counter current. The macrobenthos
from
the outline of substratum
(dotted 67 points). Solid and chain lines in the upper
current speed (cm ・ s 1) and solid and broken
depth
and
depth (lower). The terms, sand-pebble
water depths
were
sampled
from
of these R-riffle areas were
in cm) and bottom
current speed is shown
1-14 in the figure and
20-30 cm. The maximum
in the upper left corner of
upstream
R-riffle area (point 15-20, for the
preserved immediately in 3% formalin solu-
definition of R-riffle see Furukawa-Tanaka,
tion and returned to the laboratory where
1985) by a Surber
they were sieved by a 1 mm-mesh
cm, GG
net sampler
40 with 475 fi m mesh)
to 10. Drifting
animals
bag-like nets which
1 m
on August
were
were
coloured polyester gauze
mouth,
(25 cmX25
made
(20 cm
a transect running
Current
X 20 cm
identifiedto species,following Tsuda (ed.)
in
Twelve
(1970) or Kawai (ed.) (1985).
across the centre of the
6 to 7. Sur-
face nets (A-l, B-l, C-l, D-l and
stream
specimens were examined under a dissecting binoculer microscope and, if possible,
horizontally on
pool (Figs. 2 and 3) on August
set with
sorted to order by naked eye. After sorting,
of black-
long, 100 fi m mesh).
nets were set vertically and
9
collected by
sieve and
E-l) were
Results
the
Amount and distributionof macrobenthic
animals
surface to collect the surface drift.
Surface and bottom current speeds, the
half of the net-mouth
above
speed at each net-mouth
was meas-
substratum topography, and the depth of
ured just after the first setting of drift nets
water are shown in Fig. 2. The high current
at 16:00 on August
speed area was restrictedto the head of the
pool and eddies were present at both sides
discharge seemed
sampling
6. Normal
and constant
to persist throughout
the
period.
Macrobenthos
of the pool. The differencebetween surface
and
drift samples
were
and bottom current speed was largest at the
Fig. 3. The position of drift nets set in the centre of the study pool. The isopleths of current speed (cm ・
s"') are drawn
based
on 47 point measurements
represent the counter current against the main
between
these currents.
(dotted points). Dotted areas in both sides of the figure
current and the chain lines indicate supposed
border lines
deepest point of the pool area, whereas it
was small in the shallow area at both sides
of the pool and in the shallow C-rifflearea
below the pool.
The substratum types in the study area
was seemed to be roughly correlated with
the surface current speed, but not with the
bottom current speed. Substrata under the
surface main current course were composed
of large boulders and pebbles, while those
of the shallow areas at both sides of the pool
were sand and pebbles. The terms used here
to describe particlesfollow Hynes (1970).
In the current speed range of 0 to 50 cm ・
s"1, the density (significantat 1 % level),
biomass, and number
of species (significant
at 0.1 % level)of macrobenthic animals at
the sampling
points generally showed
an
increase with the bottom current speed, but
they tended to decrease with current speeds
higher than 50 cm ・ s 1,they were especially
low at the points of large rock surface (Fig.
4). The relationship among
these three
variables and the substratum was unclear.
Differences in the density or biomass
macrobenthos
between
substrata
of
were
smaller than results represented by Tsuda
(1959), Mizuno et al. (1966) or those by
Furukawa-Tanaka
(1985) who studied other
Japanese mountain streams.
Epeorus uenoi , Matsumura,
Pseudo-
cloeon sp. and Simuliidae were restrictedto
the swift current R-rifflearea. Baetis spp.
were abundant in the swift current R-riffle
area, but a small number
of them
were
Fig. 4. Relationships of the density, biomass
found on all substrata with any current
(wet weight in mg)
speed (Table 1). Net-spinning Trichoptera
larvae of Stenopsychidae and Hydropsychi-
collectedmacrobenthic animals (625 cm2) to
bottom current speed at each sampling point.
dae are usually abundant and have a large
biomass in other Japanese
mountain
diameter of stones in the substratum sampled
streams (Tsuda, 1959), but they were scarce
(dotted open circle:>50 cm; solid square: 50-20
in thisstudy area.
cm; solid triangle:19-5 cm; open circle:<5 cm).
For the numerically dominant
5 taxa,
and number
of species of
Marks in the figure indicate the maximum
Table 1. The density (in 625 cm2) and biomass (mg wet weight per 625 cm2 in parentheses) of macrobenthic animals
in the study pool and adjacent upstream R-riffleat Swamata
of the Jindoji-dani Stream in August 1978.The micro-
location of each sampling point is shown in Fig. 2. Asterisks (#) indicates the availablebenthic food animals, which
are tentativelydefined as more than 1 % mean
wet weight contributionin the stomach contents of either residentsalm-
on or charr inhabiting thisstream (Furukawa-Tanaka,
substrata sampled.
unpublished). The current speed was measured just above the
Baetis spp.,
(Imanishi),
Ephemerella
cryptomeria
Rhithrogena
kibunensis
(Imanishi)
perid species, which
sp., Ecdyonurus
and
various Chloro-
were apparently
distributed over the pool and
C-riffle areas (sampling
was
2). The
found
forms
between
of
more
degree
the same
(0.42)
crawling
nymphs,
and Rhithrogena
sp. The
abundant
species
of Morisita's Ci,
smallest
Ephemeroptera
kibunensis
was
between
calculated (a matrix
1971, Table
evenly
the adjacent
points, 1-14), the
degree of habitat overlap
was
The numerically dominant components of
at medium
lifeE.
former
to low cur-
rent area, while the latter was
was Baetis spp., Pseudocloeon sp., Ecdyonurus kibunensis , Rhithrogena sp.,Epeorus
latifolium (Ueno),
Ephemerella
crypto-
meria, Chloroperlidae, Chironomidae
Simuliidae (Table 3). Among
and
these com-
ponents, Baetis spp. predominated throughout the day, followed by Pseudocloeon sp.
These nine components occuppied 85 % of the
total daytime catch, 92 % of the total nightfall catch, and 93 % of the total nighttime
catch of benthic driftin number.
abun-
The similarity index of the faunal compo-
(Table
1). The
sition (CA of Morisita, 1959) between the
shows
a
high
total daytime and night (including the night
degree of overlap with that of Baetis spp.,
fall)catches, based on these major 9 taxa
dant at high
current
area
habitat of E. cryptomeria
Rhithrogena
sp. and
5 taxa
Eriocera
majority
and
more
benthic drift (consisted of macrobenthos)
Chloroperlidae.
spp.
of macrobenthos
These
comprised
the
mentioned was 0.97.The
similarity indices
between the faunal composition of the total
in number
(mean,
catches of 12 nets collectionbased on these
92 %; range, 82-97 %) and in biomass
(mean,
78 %; range, 27-99 %) in both the pool and C-
major 9 benthic taxa were close to 1 (Table
4). As a whole, the composition of benthic
riffle areas.
driftwas quite similar,not only between the
day and night, but also between the nets.
Diel change
and composition
of benthic and
fallen drift
A marked
difference was
evident in
several taxa between the total number
In this study, three time periods, daytime
(4:40-5:10, 6:00-6:30, 8:00-8:30, 10:00-10:30,
of
fallen drift (consisting of fallen terrestrial
and fallen emerged
aquatic
animals)
12:00-12:30, 16:00-16:30, 18:00-18:30), night-
collectedin the three time periods (Table 3).
fall (20:00-20:30) and nighttime
Imagines or subimagines of Ephemeroptera, adult Diptera, Hymenoptera (includ-
0:00-0:30, 2:00-2:30) were
(22:00-22:30,
adopted
following analyses.
sample from many
the
ing Formicidae) and Odonata
Table 2. Degree of spatialoverlap among
same
in
were abun-
5 taxa of macrobenthos, which were collectedtogether in the
sampling points (see Table 1), by the indicesof Morisita's (1971).
Table
3. The total catches of benthic and fallen drifts sampled
by 12 nets, (daytime;
4:40-5:10, 6:00-6:30, 8:00-
8:30, 10:00-10:30, 12:00-12:30, 16:00-16:30, 18:00-18:30; nightfall; 20:00-20:30; nighttime;
2:00-2:30; n: number
of individuals caught, w: wet weight in mg)
22:00-22:30, 0:00-0:30,
Table 4. A matrix of similarity of faunal composition represented by CA (Morisita, 1959) among
12 nets
set verticallyand horizontally.As daytime and nighttime faunal composition were quite similar (see
text),11 time samples of each net were combined. Calculation was based on 9 major benthic taxa, Baetis
spp., Pseudocloeon sp., Epeorus
latifolium,Ecdyonurus
kibunensis, Rhithrogena
sp., Ephemerella
cryptomeria, Chloroperlidae,Chironomidae and Simuliidae,in the driftcollection.
dant in the daytime, whereas
were abundant
Coleoptera
at night. In the nightfall,
adults of Trichoptera and Coleoptera predominated in the fallen drift,especially in
wet weight
than that of the benthic compo-
nent (Table
3). During
benthic drift and
the nightfall, the
fallen drift fractions were
equal in weight.
terms of weight.
A diel periodical pattern of benthic drift
was especiallyevident in samples from the
nets in the swift current areas (C-l, C-2, C-
Microdistribution
of drift abundance
and
drift density
Drift abundance
was
firstly by
3 and B-l), but the periodicitywas unclear
Wankowski
at the other 8 nets (Fig. 5). In the fallen
drift collections,no such periodical pattern
value for feeding
was evident at any nets.
In the surface net collections,the fallen
to refer
drift component
metre) at right angle to the current direc-
was more
abundant
than
benthic drift in the daytime, whereas
at
fish. Here
passing
and Thorpe
proposed
the term
to
a
(1979) to measure
position of a
"the
drift abundance
amount
vertical unit
of
area
a
salmonid
is used
organisms
(a
square
tion per unit time (a second)", which
is the
night (including nightfall),
the benthic drift
slightly modified unit from that of Dill et al.
component
(1981). Fig. 6 shows
was more
abundant
in all net
collections.The benthic drift component
in
zontal distribution
the total nighttime catch exceeded the fallen
driftboth numerically (10 times) and in
drift abundance
biomass (2 times), whereas the fallen drift
in wet weight.
component
in the total daytime catch was
twice as numerous
and 11 times larger in
and
the vertical and
of the mean
hori-
daytime
of the mean
daytime
drift density (Elliott, 1967a) in number
It is clear that the drift abundance
fallen and benthic drift both in number
and
of the
and
Fig. 5. Diel change
30 minutes
of invertebrate drift sampled
of every 2 hours on August
Figures
in parentheses
mersed
into a half depth of the net mouth
show
the mean
by the 12 nets (see Fig. 2). Number
6 to 7, 1978 at Sawamata.
current speed of five measurements.
(area, 20 cm
of drift animals
The solid and open
The
is represented
by total catches for
circles are benthic and fallen drifts, respectively.
surface nets (A-l, B-l, C-l, D-l
X 10 cm) and the other nets were immersed
and
entirely (area, 20 cmX20
E-l) were
cm).
im-
Fig. 6. Vertical and horizontal distributionof the mean daytime driftabundance in number
(m
2 's-1)
and in wet weight (mg* m 2・ s 1),and of the mean daytime driftdensity (m :!)and of the mean daytime
drift biomass (mg'm-3). Solid upper half circlesand open lower half circlesindicatethe fallenanimals
and the benthic animals, respectively.
The current speed is shown in Fig. 3.
in weight was
extremely
layer of
main
decreased
the
large in the surface
current
vertically and
course,
horizontally
and
or biomass,
with
vertical distribution
distance. In net C-l, the drift abundance
fallen animals
was
5 and
of
52 times as large
as that of benthic animals
in number
and
weight, respectively.
was
also
high in the surface nets, especially in net Ca clear trend
which
seemed
to show
within
the
an even
water
column.
Linear correlations were
relationship between
dance
Fallen drift density or biomass
1 and then B-l or D-l. Such
was not observed in the benthic drift density
and
current
correlations were
drift abundance
observed
benthic
speed, and
observed
in the
drift abunexponential
between
fallen
and current speed (Fig. 7).
Fig. 7. Relationships between
the drift abundance
of the fallen component
fitted separately to the liner and exponential curve regression, respectively. Solid circles, open
were
solid triangles indicate the drift abundance
(asterisk means
(D.A.)
and the current speed. The drift abundance
at night, in the daytime, and the nightfall peak.
significant level:*** 0.1 %, ** 1 %, * 5 %)
of the benthic component
and that
circles and
distributed in the water column, but was
Discussion
Importance
resource
of daytime
fallen drift as a food
To estimate the total amount
Furukawa-Tanaka
(1985)
charr are unable
and noted
showed
passing downstream
are unable
current
through
of drift
the study
pool, the following three assumptions were
fishes also
used: (1) the drift abundance changed with
visual feeders. So it is
fishes inhabit-
current speed in accordance with the regression formulae given in Fig. 7, (2) the area
to feed on the
of each current speed range was calculated
that the salmonid
ing this stream
that
to feed at night
that other salmonid
to be daytime
considered
abundant in the main
course near the surface (Fig. 6).
of fishes
Japanese
seem
much more
bulk of drift at night. The total weight of the
based on the iso-current-speed figure of
fallen drift collection exceeded
Fig. 3, (3) the median of each current speed
benthic drift by
(Table
more
11 times
in the daytime
3). Fallen animals
abundant
salmonids
in stomach
and
source for daytime
and
Sakai,
et
time, the nightfall,and the nighttime were
important
71.8,20.4 and 173.lg, respectively. The
estimated total amount of daytime fallen
food re-
visual drift feeding fishseason. And,
important
was
resource
Table 5. The amounts
s. The totalamounts of driftduring the day-
therefore, that
es, at least in the summer
food
the study pool during the three time period-
Macdonald,
1985; Tanida
al., 1989). It is concluded,
fallen drift is the most
Table 5 shows the estimated amounts of
drift in each current range passing through
contents of lotic
1980; Mason
1982; Furukawa-Tanaka,
range was tentativelyused.
are usually much
(Hunt, 1975; Nagoshi
1980; Smith,
that of the
not
drift was about three times that of benthic
this
drift.
evenly
of driftpassed downstream
Since there is no report concerning daily
through a transect plane of the study pool during the
daytime, nightfalland nighttime in August 6 to 7, 1978. The detailcalculationprocedures are given in
the text.The nighttime period excludes the nightfall.
(B: benthic drift,F: fallendrift,+: less than 0.1 mg.)
food requirements
masu
salmon
of
resident
or Japanese
red-spot
charr in natural
conditions, the daily ration of the brown
trout, Salmo
body
trutta, which comprises
wet weight
1975) was
10 % of
of a 10-50 g fish (Elliott,
adopted
to enable a rough estima-
tion of the food level for the salmon
and
of benthic
favourable
Therefore, the
tum
where
where
fall period, the amount
drift can
of daytime
fishes.
optimal
visual
foraging position
drift feeding
fishes
should not be the layer close to the substra-
and charr are considered
the night
food for stream
for daytime
the upper
not to utilize noc-
(Furukawa-Tanaka,
1985), indicating the fallen drift is the more
charr inhabiting this pool. Since the salmon
turnal drift, including that during
animals,
the benthic drift originates, but
surface
abundant,
layer of swift current,
dense
and
choice
food
occurs.
Everest and
Chapman
(1972) stated that
theoretically sustain a total of 700 g of these
fishes consistently face toward
fishes. This figure corresponds
current velocity which is adjacent to an area
well with the
total wet weight (400-500g) of fishes inhabit-
of swift current with abundant
ing this pool (Furukawa-Tanaka,
imize
unpub-
lished data).
their net
maintain
energy
a moderate
food to max-
gain,
position in a
because
to
swift current
re-
quires a large energy expenditure. But
Feeding
position to be defended
Stream
ly show
resident salmonid
speculation was
fishes frequent-
intra- and inter-specific aggressive
behaviour
between
nature and
under
('Thymallus:
inhabitants,
in
experimental
conditions
and Smith,
1979; Salve-
Krat
linus: Newman,
both
1956; Noakes,
Kalleberg, 1958; Yamagishi,
1980; Salmo:
1962; Jenkins,
1969; Oncorhynchus:
Chapman,
et al.y 1972). Fausch
and White (1981) specu-
lated that the main
function of such aggres-
sive behaviour
of brook
fontinalis (Mitchill),
ter. Other
authors
1962; Stein
trout, Salvelinus
is to defend the shel-
believe that the domi-
nance
relationship of these salmonid
which
is established as a result of aggres-
sive encounters, plays an important
defending
a
more
position, but
advantageous
they have
fishes,
role in
feeding
given no data
for
distribution of available food.
As shown
in this paper, the fallen drift is
diurnally more
abundant
and this important
abundant
main
and
than benthic drift,
food source is extremely
dense
at the surface of the
current course. In addition, the mean
weight of fallen animals
is higher than that
data and
they
importance
were
not conscious
of the
of the fallen drift. To minimize
a large swimming
marked
this
based on few benthic drift
energy
vertical and
expenditure,
the
horizontal changes
of
current velocity in the pool area constitute a
very important
structural character
of the
lotic habitat for fishes.
On
the other hand,
and C-riffle areas
change
of current
the shallow
showed
only
R-riffle
a
velocity within
small
a
short
distance and in addition, the R-riffle area is
usually partitioned
boulders, cobbles
whereas
pool
space without
into
and
areas
small
spaces
turbulent
usually
by
water,
have
a
large
visual obstacles. Therefore,
the feeding range within a pool area may
be
larger than that in an R-riffle area.
As
it may
be considered
that the R-riffle
area is not so effective feeding place for sal
monid
fishes, the drifting animals
riffle are presumably
pressure from
under
low predation
these fishes. The
the R-riffle area which
ing animals
flows
water from
contain more
into the
following pool, where
in the R-
head
driftof the
the fishes can
keep
their position in a moderate
current
speed
References
near bottom.
Furukawa-Tanaka
large Japanese
maintained
(1985) showed
charr
with
that a
a full stomach
its position at the head
of the
pool, while a smaller fish with low stomach
content maintained
its position just below
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great thanks to
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Dr.
University of
Yasuhiro
Animal
me
Kazumi
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A
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me
my
his
Contribution
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Mr.
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Acknowledgments
Professor Hiroya
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biomass
Appendix
Drift density and drift biomass
Vertical and horizontal change
of the benthic drift,but those for fallen
drift were as follows.
Daytime:
of the drift
F.D.D.=-0.0653+0.0109V
F.D.B.=-0.6500+0.0937V
density are calculated for the benthic and fallen
drifts of the C net group (Appendix table 1) and
Nightfall: F.D.D.=
F.D.B.=
for the surface nets (Appendix table 2). The
drift densities of major four taxa, Baetis spp.,
Nighttime:F.D.D.=-0.0479+0.0090V
(r=0.89)
F.D.B. = 0.8346+0.0030V (r=0.03)
Ecdyonurus kibunensis, Rhithrogena
sp. and
Pseudocloeon sp. which comprised 80 % of the
where
benthic drift,and of others were represented by
small values by day, increased considerably at
night.
The drift densities of Baetis spp. and E. kibunensis were highest in C-3 net both in the nighttime and at nightfall, but showed no definite
vertical change in the daytime. In contrast with
these three taxa, Pseudocloeon sp. did not show
any verticaltrend even in the nighttime density
and seemed to be evenly distributed throughout
the day. The drift density of fallen adult Diptera
was highest in net C-l and showed no noticeable
0.0344+0.0141V
0.2032+0.1002V
(r=0.95)
(r=0.89)
(r=0.58)
(r=0.51)
F.D.D.: Fallen Drift Density (n*m 3)
F.D.B.: Fallen Drift Biomass(mg ・ m 3)
V : Mean
Current Speed (cm・ s"1)
Drift density in relation to distributional pattern
of macrobenthos
The drift density of Baetis spp. was
high in
the near-bottom layer of slow current and low
in the upper surface layer of swift current in the
pool area (Appendix table 1). E. kibunensis and
Rhithrogena sp. also showed the same tendency.
In the macrobenthos
distribution (see Table
1), the highest density of Baetis spp. was observ-
diel change. In the daytime, the density of adult
Diptera in net C-l was 9 times greater than that
ed in the adjacent upstream R-riffleof the study
pool, though a considerable number of Baetis
spp. were also sampled from all other substrata
of Baetis spp. of the benthic component.
In the horizontal axis, the maximum
densities
of the study area. Rhithrogena sp. and E.
kibunensis shbwed high densities in the macro-
of Baetis spp. and fallen adult Diptera in the
daytime were observed in net C-l, with an
benthos in the medium
current speed (Table 1),
and these taxa also showed wide distributions
over the study pool and C-riffle areas. In contrast, Pseudocloeon sp., for which the drift
exceptionally high density of the former taxon
in net A-l. This exception was the result of few
individuals and very low current speed.
Relations of the drift densities and the drift
density was evenly distributed vertically and
horizontally (Appendix tables 1 & 2), show a
biomass to the current speed in the three time
periods (same as Tables 3 and 5, and Fig. 5)
are calculated based on all collectionsby the 12
restricted habitat of large rock surfaces with
high current velocity.It seems that the animals
which were distributed over a wider range of
nets (a large Odonata caught in B-l net is omitted because of few collections).No correlations
habitat, showed high drift density in the nearbottom layer of slow current under the main
current course in the pool.
were obtained
either for the density or the
Appendix
table 1. Verticalchange of the mean driftdensity and the mean
driftbiomass (wet weight in mg) with standard deviationof C
net group. The day and the night values are means of 7 and 3 sampling times as in Table 3, respectively.
The mean current speed and its
standard deviationof C-l, C-2 and C-3 net were 71.9+ 2.9,37.5 + 1.7and 14.0± 1.6 cm* s_1,respectively.
Appendix
table 2. Horizontal change
deviation of surface nets. Each
and
its standard
6.1 ±0.5 (cm
of the mean
net is arranged
drift density and the mean
drift biomass
in order of current speed, and
the mean
with standard
current speed
deviation of C-l, B-l, D-l, E-l and A-l were 71.9±2.9, 25.2±3.3, 16.8±0.8, 9.2±0.7 and
* s 1),respectively.