an introduction to the freshwater fish communities of corcovado

BRENESIA 21: 47-.66.1983
AN INTRODUCTION TO THE FRESHWATER FISH COMMUNITIES
OF CORCOVADO NATIONAL PARK, COSTA RICA
Kirk O. Winemiller
Department of Zoology, University of Texas, Austin, Texas, USA 78712
Key Word Index: Corcovado, Costa Rica, fish communities, freshwater ecosystems,
trophic relationships
ABSTRACT
The distributional pattern and ecologyof eachspeciesof the freshwater
fish communities surrounding the Sirena station of CorcovadoPark
were investigatedduring the early rainy seasonof 1982.Thirty species
were observedor collectedin freshwatersof the park, and an additional
10 specieswere restrictedto intertidal regions.Peripheraldivision and
invading marine fishes were major integral componentsof the freshwater ichthyofauna. These forms were particularly prevalent in the
rivers and lower stretchesof smallstreams.Streamwidth waspositively
correlated with speciesrichness.Most of the primary and secondary
division fishesappearedto be fairly restricted in their use of the range
of available food resources.The habitat affinities and approximate
abundancesof eachfish specieswas reported,basedon visualcensuses.
The route of dispersalinto the region by primary and secondarydivision fishes is discussed.Basedon comparisonswith earlier studies in
CostaRica and Panama,it is hypothesizedthat the marineelementsin
the park's freshwaterecosystemsdecline during the dry seasons.Food
webs for the QuebradaCamaronaland Rio Claro fish communities
were constructed.The major sourcesof primary productivity for the
communities are apparently allochthonous, indicating a dependency
of the present ichthyofauna on the tropical rainforest as a sourceof
energyand nutrients.
The freshwaterecosystems
of CorcovadoNationalParkin CostaRicaareunusualin
permitting the study of freshwatertropical fish communitiesin a regionof relatively low freshwaterfish speciesdiversity. Freshwaterfish communitiesin the neotropics are generallycharacterizedby an abundanceof primary freshwaterfish species
(i.e. membersof families with little or no toleranceto salineconditions) of the
47
subordersCharacoideiand Siluroidei (Lowe-McConnell,1975). By markedcontrast,
the freshwaterfish faunaof Corcovadocontainsonly three speciesof primary freshwater fishes (two char.:oids and one siluroid), and this has been attributed to
geographicalfactors inhibiting invasion by these forms onto the Osa Peninsula
(Constantzet al., 1981). Secondarydivision freshwater species(i.e., membersof
families with limited degreesof toleranceto marinesalinities)areunderrepresented
at Corcovadoas well, with only three reported (Constantzet aI., 1981) from the
more than 25 secondarydivision speciesknown from the Costa Rican mainland
(Bussing,1967). The remainderof the Corcovadofreshwaterfish fauna is comprised of peripheraldivision and marine specieswhich have invadedthe streams,
often many kilometers inland. I havechosento define peripheraldivision speciesas
membersof familieswhich areclearly marinein origin, yet they must residein fresh
or brackishcoastalwaters for variable periods of time during their life histories.
Peripheralfishes may require freshwaterecosystemsfor successfulfeedingduring
juvenile or adult life, or as an environmentfor reproduction and/or larvalfeeding.
Marinefisheswill be defined asspecieswhich may residein fresh or br.:kish waters,
often for extensiveperiods, yet they are fully capableof completing their entire
life cycles in the marineenvironment.Unlike peripheraldivision fishes,many populations of marine fishes resideexclusively in the marine environment,either offshore or in intertidal zones.The significanceof this new distinction will become
apparentin subsequentsectionsof this report.
Constantz et 8/. (1981) reported a total of 20 fishes from the freshwaters of Corcovado Park. This represents relatively low speciesrichness for neotropical freshwaters
in a region of this size. The freshwater ecosystemsof Corcovado Park provide good
opportunities for comparatively-based hypotheses concerning freshwater fish
dispersal abilities, historical aspects shaping local species diversity, and tropical
freshwater community dynamics. The present study was undertaken to: 1) provide
more detailed information on the relative abundances, distributions, and habitat
utilization of each freshwater fish species;2) determine the food habits of each fish
species; 3) obtain information on the trophic structure of the freshwater ecosystems; and 4) provide a preliminary record of intertidal and estuarine fish species.
Material and Methods
Field siteswere chosenalong sectionsof the R(o Claro, Rio Pavo,QuebradaCamaronal, Q. Danta, two small streamswhich emptied directly into the ocean,the
headwatersof a small, unnamedstream,and one rocky intertidal pool. Additional
fish collections and notes were made in intertidal regionsof bedrocksubstrate(A),
an oxbow lake near the mouth of the R(o Sirena (B), the Rio Sirenaestuary,and
sectionsof streamsbetweenthe selectedfield sites.Mapswereconstructedfor each
field site containing: Profiles of streamdepth, poll or riffle length, streamwidth,
substratecomposition, and the location of potential fish refugia (i.e., undercut
streambank, roots, logs,boulders,etc.). The length of eachfield site wasdelineated
by the natUralboundariesof the pool or riffle under investigation.Consequently,a
4A
pool field site in a river was often much longer than a.pool in a smallerstream.
Watercurrent velocity was determinedby tossinga35 mm film canister,half-filled
with water, into the region of greatestcurrent and timing its passagerate over a
designateddistance.At eachfield site, the following datawere recorded:1) pH; 2)
salinity (ppt); 3) temperature;4) maximum water current velocity; 5) the abundanceof eachfish speciespresent;and 6) the relativeratio of juvenilesto adultsof
each fish speciespresent. The presenceof epiphytic algae,zooplankton, crustaceans,insolation, and fruiting trees was also noted at eachsite. Eachfield site was
photographedto supplementthe mappedinformation in later analyses.
Fisheswere collected by dipnetting, seining,fish traps,angling,and by hand. Habitat utilization and estimatesof the relative abundancesof fishes were made by
direct observationfrom abovethe water'ssurfaceand snorkellingbelow the water's
surfaceduring normal streamdischarges.Specialattempts were made to trap or
dipnet rareor nocturnalfishesunder sectionsof erodedstreambank. Selectedfishes
were measuredfor standardlength and their stomachmorphology and contents
were examined.One to six specimensof eachspeciescollected were preservedin
10 percentformalin and later depositedin the TexasNatural History Collectionof
the TexasMemorialMuseum,Universityof Texas,Austin.
Descriptionsof Field Sites
Corcovado park is located on the extreme western edge of the Osa peninsula of
Costa Rica's Pacific coast. The park is approximately 41,800 ha and lies at approximately SO30' N latitude and 8'jO30' longitude. Corcovado Park contains extensive
areas of primary tropical rainforest plus patches of regenerating second growth,
including dense stands of He/iconia, Ca/athes, Trema, Piper, Cecropia and species
of other plant genera (Janzen, 1979). The park received 4701 mm of rainfall in
1980 and averaged 364.4 mm over 1979, 1980, 1981 during the period in which
the presentstudywasconducted(June25 to July 28). This period is in the early
stageof the major annual rainy seasonof this region.The streamsand riversof the
park flow over Cretaceousvolcanic and sedimentaryrocks and, to a lesserdegree,
Quaternaryalluvialdeposits(Janzen,1979).
The location of eachfield site is providedin Figure1. Fish speciesnumbers,physi.
cal parametersof the water, and stream length, width, and maximum depth are
provided in Table 1. The following are verbal descriptionsof the generalhabitat
conditionspresentat eachfield site.
Sites 1 and 3 are shallow,swampypools at the headwatersof smallstreamswhich
ultimately empty into the Rlo Sirena.The poolscontainedmuch emergentvegetation, including Hel/conia sp. Rotting Hel/conia leavesservedas refugiafor fishes
and substratefor SpyrogyrBsp. and other speciesof filamentousgreenalgae.The
bottoms of these pools were coveredwith thick laversof vegetativedetritus and
49
mud. At midday, direct sunlight was transmitted through the secondarygrowth
forest, producingscatteredpatchesof sunlit and shadedwater in eachpool. No observablewatercurrent waspresentat sites1 and3.
Sites2, 4, 5, 7, 8, 9, and 11 are pools in the QuebradasDantaand Camaronal.All
pools contained regionsof gravel, sand, and leaf litter substrate.Epiphytic bluegreenalgaeand diatomswereusuallypresenton rocks in the shallowmarginalareas
or deeper regionscontaining observablecurrent in thesepools. Eachpool is bordered at one side by an area of eroded (undercut) bank, usually with tree root
structuresor bedrock above.A small portion of most pools was exposedto direct
sunlight during midday, and thesewere often associatedwith limited algalgrowth
when suitablesubstrateswerepresent.
Site 6 is located at the extreme headwatersof the QuebradaCamaronal.This was
the last pool observedto contain fish (3 specimensof Agonostomusmont;cola) as
one traveled this fork upstream. Stream gradients are much steeperand many
moss-and lichen-covered boulders are present in this regionof the stream. Leaf
litter, bedrock, and gravel comprised the substrate,and very little observable
algaewas present at this site. The entire pool remainedshadedby primary forest
throughout the day.
Site 10 is a shallow riffle sectionof the O. Camaronalwith a coarsegravel,sand,
and leaf litter substrate.Mostof the riffle wasexposedto direct sunlightat midday,
and this appearedto support thin growths of epiphytic blue-greenalgaeand diatoms.
Site 12 is located at the mouth of the a. Camaronalwhereit emptiesinto the Rro
Sirenaestuary.Site 12 gradesfrom a deep, narrow streampool, borderedby bedrock, to a shallowsloughflowing over rocks,coarsegravel,andfinally sandand silt.
The entire site is exposedto direct insolation through most of the day and supported diatomson the rock surfaces.
Sites 13 and 14 are located in two separatesmall, shallow streamswhim flow
through areasof extensivesecondaryforest growth adjacentto the coast. These
streamsempty directly into the oceanover sandy beach.The substrateof site 13
consistedof sand and the pool marginswere composedprimarily of grassesand
sedges.Site 13 was exposedto insolation throughout most of the day, yet no
significant algal growth wasobservable.Site 14 wasentirely shadedand hasa sand,
soil, and leaf litter substrate.Clumps of emergent understory vegetation were
scatteredthroughout site 14. These streamsare probably ephemeral,with very
restrictedflow during the major dry season.
Sites 15 and 17 are large pools in the Rfo Claro which have bedrock and gravel
substrates. Thin growths of blue-green algae and diatoms were present in the shal-
so
low marginal areasand swift headwatersand tailwaters of each pool. Leaf litter
depositsweregenerallypresentamongthe rocks at the pool margins.Majorportions
of both pools wereexposedto direct sunlight at midday.
Site 16 is a swift riffle areaof the R(o Claro. The substrateis comprisedof coarse
gravel and rocks. Most of this areais exposedto direct sunlight at midday, and a
thin layer of algae is presenton some rocks, particularly in shallowermarginal
regionsof the riffle.
Site 18 is located in a channel formed by the presence of a sand bar near the edge
of the Rfo Oaro estuary. The substrate here is comprised of sand and fine silt
deposits. The area is exposed to direct sunlight throughout the day, yet no significant epiphytic algal growth was detected. Forest litter was commonly deposited
along the banks of the channel following floods and high tides.
Site 19 is a large pool of the Rfo Pavo approximately 50 m upstream from the
crossing of the park's Casa Marenco trail. The substrate at this site was primarily
fine gravel and sand. Unlike the pools of the Rfo Claro (sites 15 and 17) several
fallen trees provided cover for the fishes. The entire pool was exposed to sunlight
at midday, and very limited growths of epiphytic algae were present in the shallower regions.
Site 20 is an isolated intertidal pool at low tide, which has a sandand bedrock
substrate.Numerousrocks were scatteredthroughout the pool, providingcoverfor
many small marine fishes. The pool is exposedto direct sunlight throughout the
day, and thin layersof epiphytic algaewere presenton many rocks. At high tide,
site 20 is inundated by over 3 m of seawater and is exposedto constantwave
action.
Observations
The number of speciesat eachfreshwaterfield site wasextremelyvariable,ranging
from one (sites 6, 13, 14) to 15 (site 19). In addition to providing total species
numbers,Table 1 summarizesthe number of primary division, secondarydivision,
peripheral,and marinefish speciesat eachsite. No continuouspattern emergedas
total speciesrichnesswas tracked from the headwatersof eachriver and streamto
its mouth. SPSS(StatisticalPackagefor the Social Sciences,Nie et al., 1975) was
usedto compute a multiple regressionanalysisof total numbersof fish speciesand
streamcurrent, maximumdepth, maximumwidth, andthe lengthof eachfield site.
The results,summarizedin Table 2, indicate that approximatelyhalf of the total
varianceis explainedby differencesin streamwidth. Additional variancewithin the
pattern of speciesrichnessis explainedby depth, current, and site length,in order;
and together these four variablesaccountedfor 73 percent of the variance.The
ratio of the numbersof peripheraland marinespeciesto the numberof freshwater
division speciesincreasedfrom 1.0 at the headwatersof the a. Camaronalto 12.0
)1
at its mouth. A simpleregressionof peripheraland marineto freshwaterspeciesfor
the sevensiteson the a. Camaronalresultedin a positivetrend (r2 =.494), however
this wasstatisticallynonsignificant(F = 4.873, df = 1, 5, p = .078). The distribution
of freshwater,peripheral,and marinedivision fishesin the park doesnot correlate
with salinity gradientssincedissolvedsolidswereuniformly low « 1 ppt) throughout the streamsand rivers (Table 1). The exceptionswere the two river estuaries
during high tides (Table1).
The distribution and habitat affinities of each fish species within the park are presented in Table 3. AstysnBX f.cistl/$ was by far the most abundant and widely
distributed primary freshwater fish species in the area. Brachyrhsphis rhsbdophora
was the most numerous secondary division species and was present at 14 of the 19
freshwater field sites. The distribution of Poeci/is sphenops was much more restricted to regions of filamentous green algal growth. Poeci/iopsis turrubarensis and
OxYZygOnectBSdovii were limited to the RIo Sirena estuary and adjacent stream
mouths where B. msbdophors was absent.
Among peripheral fishes, Awaous transandeanus, Sicydium pittieri, Gobiomorus
maculatus, Eleotris picta, and Agonostomus monticola were distributed far inland
in significant numbers. s. pittieri, in particular, may reproduce in freshwaters as
evidenced by the numerous juveniles throughout its distribution and the presence
of a brilliantly colored territorial male in the middle reaches of the Q. Camaronal.
Larger, predatory peripheral division and marine fishes (Centropomus and Lutjanus
spp.) were present in significant numbers in the pools of rivers, even far inland
(site 19). The abundance of these forms fell off sharply as one traveled toward the
heedwaters of the smaller streams and pool volumes declined. Juveniles of Pomadasys bayanus were commonly encountered in pools of the streams. Several individuals were observed to exhibit intraspecific aggressionnear regions of cover such as
eroded stream bank. A single adult specimen (-1.5 m standard length) of the sawfish, Pristis sp., was observed at site 19 and one other location of the Rfo Pavo.
The most abundant fish speciesof the intertidal zonesappearedto Abudefduf
an8logus,Hypsoblenniusstriatus, and 88thygobiusramosus.The assessment
of the
abundancesof speciessuch as Gobiesoxsp., Gymnothor8x sp., and Mslacoctenus
zonifer was hamperedby the presenceof many deep, undercut bedrock crevices
utilized by theseforms.
Preliminary information on the food habits of the principal freshwater-dwelling
fishesof the park is presentedin Table 4. Epiphytic algaeand smallshrimpof the
family Palaemonidae
apparentlycomprisedthe major autochthonousbasisfor the
ichthyomasssustainedin the rivers and streams.Major allochthonoussourcesof
food input were forest vegetativedetritus, mature fruits and seeds,and terrestrial
insects,includingants.Aquatic snailswerevery abundantin the R(o Claro,but only
minor componentsof the R(o Pavoand streamecosystems.Other foods utilized
werecrayfish,aquaticinsectlarvae(Ephemeroptera),and microcrustaceans.
S2
A striking characteristicof the streamsat Corcovadois the paucity of autochthonous primary productivity, and the apparentextreme paucity of aquatic insects
and zooplankton. Preliminary schemesfor the trophic interactionsof the communities of the Q. Camaronal(Fig. 2) and Rro Claro (Fig. 3) were constructed
basedon the approximaterelative abundancesand food habits of eachspecies.In
both systemsthe major sourceof primary production takesthe form of allochthonous input from the rainforest.Snailsand shrimpcompriseda much greaterproportion of the "primary aquaticconsumer" trophic level in the Rfo Clarothan in the
Q. Camaronal.Both communitiesare representative
of a "food web" trophic structure, wherein many speciespopulationsutilize food resourcesfrom the samebroad
categories,or utilize food from two or more trophic levels.The specieswhich comprisedthe lower trophic levelsof the t~ systemswere quite similar, howeverthe
Rro Oaro supported severalmore predatory speciesin the higher trophic levels
(Figs.2 and 3).
Although the food habits data for Corcovadofishesshould be consideredpreliminary due to low samplesizes,severalspeciesexhibited fairly rigid selection for
specific items. Fruits and seedsconstituted the major portion of the diet of A
fasc;atus,and to a slightly lesserdegreeH. savage;.Severalspeciesappearedto be
strict algalgrazers,including P. sphenops(greenalgae,esp.Spyrogyrsspp.), P. turrubarens;s(diatoms), and s. p;tt;er; (diatomsand blue-greenalgae).The diets of
B. rhsbdophoraand A mont;co/a consistedof terrestrial insects,with B. rhsbdophora consumingmostly ants and tiny dipterans. The gut of the singleanalyzed
specimenof Sphoero;dessnnu/atuswas p.:ked with many aquatic snailsand shell
fragments.SeveralAMOUS trsnsandeanuswere observedsucking up portions of
fine gravel and sand substrateand allowing the inorganicparticlesto sift through
their opercular oPenings.One cKJultA. trsnsandeanusat site 16 was observedto
periodically dash up from the substrateto capture individual particlesof drifting
detritus. The gut contentsof two individualsrevealedonly fine particlesof detritus.
Discussion
With the addition of Oxyzygonectesdov;;, the presentstudy increases
the number
of known freshwaterdivision fishesat Corcovado(Constantzetal., 1981) to seven.
To the previous list of 14 fisheswith marine affinities presentin the park (Constantz .:...jl., 1981), this study reports 13 additional peripheraldivision and marine
species,plus 10 additional marine speciesfrom intertidal zones.The three pipefish
specimens,which were returned to the Texas Natural History Collection, were
determinedto be Pseudopha//us
starks;. P.wudopha//us
elcaptansnsewas reported
as moderatelyabundantin the a. Camaronalby Constantzet al. (1981), wheretwo
of my specimenswerecollected.Gob;esoxpotan;usand RhamdiaMgnsr; were not
observedduring the study although both werepreviouslyreportedasrare in the Q.
Camaronal.Despitethe fact that they were reported in slightly different manners,
there appearto be major disparitiesin the approximationsof the abundanceand
distribution of manyof the fishesbetween1979 (Constantzet al., 1991)and 1982.
~~
This may be due to differencesin season,sincethe 1979 observationswere made
during the major dry seasonand the 1982 observationsduring the early portion of
the major wet season.Differencesin collecting techniquesand censusingmethods
may havealsocontributed to differencesin abundanceestimates.
The ichthyofauna of the park's freshwater ecosystems is a mixture of freshwater,
peripheral, and marine division forms. Certainly the composition of these communities is temporally dynamic in nature, and they will ultimately require detailed longitudinal studies before the factors influencing their structure are fully understood.
If, as indicated by Table 2, stream volume has a major influence on speciesrichness,
then major differences in the number of peripheral and marine division fishes might
be expected between wet and dry seasoncensuses.Several investigations in a variety
of temperate and tropical lotic systems have also supported the view that stream
volume, width, and/or discharge may explain a major portion of the variability observed within fish speciesdiversity patterns (Gorman and Karr, 1978; Smith, 1981;
Livingstone et al., 1982). I hypothesize that many more individuals of the marine
division species join the ichthyofaunas of small coastal tropical streams, such as
those at Corcovado, during wet seasonswhen stream volumes, discharge rates, arxl
food resources are higher. Allochthonous food input to the Tortuguero estUary of
eastern Costa Rica was found to be much higher during the wet seasons(Nordlie
and Kelso, 1975). In a small Panamanian stream, fish foods were more abundant
during the wet season (Zaret and Rand, 1971). During the dry seasonsat Corcovado, many marine fishes may be excluded from freshwaters due to competition with
freshwater and peripheral division fishes. In reviewing the distributional patterns of
African freshwater fishes, Roberts (1976) noted that, when present, freshwater
division fishes tend to exclude more euryhaline forms from inland habitats. Tiny
palaemonid shrimp were extremely numerous in the streams and rivers of Corcovado during the early rainy season, and these should provide a major potential
source of food for temporary marine invaders as well as permanent freshwater residents. The view, that primary and secondary division freshwater fishes dominate the
ecosystems at Corcovado when stream volumes are lower, is also supported by the
observation that freshwater division fishes comprised a greater proportion of the
speciesrichness as one traveled away from the mouths of streams.
Corcov~o Park lies within the isthmian faunal province of Miller (1966) and Bussing (1976). This region of CentralAmericais by far the richestin primary division
freshwaterfish species(Myers,1966; Miller, 1966;Bussing,1967),with most forms
derivedfrom South Americanfaunal elements.Of the three primary division sp«:ies
presentat Corcov~o, Astyanax fa.w:;stusand Rhamdiawagner;are reportedto have
limited toleral~es to salineconditions (Miller, 1966; Bussing,1976). Both of these
speciesexhibit ubiquitous rangesin lower CentralAmerica,andA. fa.w:;stusin particular extends from Argentina to southern Texas on the Atlantic slope (Miller,
1966). It is therefore quite likely that these specieshaveutilized a coastalroute
for dispersalinto this distal regionof the Osapeninsula.If streamheadwatercaptures h~ played a significant role in the dispersalof primary freshwaterfishesinto
the park, more specieswould be expected.The degreeof salinitytoleranceby
~
Hyphessobrycon savagei is unknown. Like the primary freshwater fishes, the secondary division fishes of Corcovado are represented by several species which are
ubiquitous in Central America. The presence of these seven primary and secondary
division freshwater fishes suggests that these forms have dispersed into the Osa
fairly recently (i.e., compared to major dispersals of these families along the mainland Central American slopes) and by way of coastal routes, possibly during periods
of unusually high rainfall. The recent arrival of freshwater division fishes into the
Corcovado region is further supported by Bussing's (1967) observation that a large
number of indigenous speciescomprise the Costa Rican freshwater fish fauna. The
peripheral and marine division fishes of Corcovado are primarily species which are
widely distributed along the Pacific coast of Central America. Severalspecies, such
as Agonostomus monticola, Centropomus undecimaJis, and Mugil curema are
known from both coasts of Central America. Apparently, the presenceof Abudefduf analogus in my collections represents the first report of this species for the
Pacific coast of Central America.
The preliminary data on the diets of Corcovadofishesindicatedthat severalfresh.
water and peripheraldivision specieshavefairly restrictedfeedinghabits (Table4).
One marine invader, Sphoeroidesannulatus, may speCializeon snails, using its
"beak-like" dentition to crush shells. Most of the marine speciesin freshwaters
at Corcovadoare large predatorswhich apparently exploit the numerousshrimp
and small fishes.Much more information is neededon the diets of thesefishesbefore it can be determinedif food resourcesarea limiting factor for marinefish bio.
massduring the wet season.The descriptiveinformation on streamhabitat use by
the fishesof Corcovado(Table3) suggests
that habitat selectioncould be onemeans
by which fishesecologicallysegregate
during periodsof depressedresources.This is
an emerginggeneralizationfrom recent studiesof tropical stream(Zaret and Rand,
1971; Roberts, 1972; Bishop, 1973; Gormanand Karr, 1978) aswell astemperate
freshwaterfish ecology(Winn, 1958;Allen, 1969;Mendelson,1975;Symons,1976;
Werneret al., 1977). The distributional patternsof fisheswithin the park (Table3)
probably relatesmost directly to habitat affinities. Consequently,the total species
richnessat each site should relate, not only to streamvolume (Table 2), but also
to environmentalheterogeneityin termsof habitat diversity at eachsite. For example, Poeciliasphenopswas only abundant in environmentswhere there was low
water current velocitiesand significantgrowth of fil~entous greenalgae.
The freshwater ecosystemsof CorcovadoPark appearto be consistentwith the
physical characteristicsof many tropical rainforest headwaterstreams (Myers,
1947; Roberts, 1972; Bishop, 1973; lowe-McConnell, 1975). Their waters are
apparentlyquite nutrient poor, contain little primary production, areoften entirely
shaded,exhibit low thermal variability, and receiveseasonalpatterns of rainfall.
One major difference is the absenceof high freshwaterfish speciesdiversity at
Corcovadodue to the peninsulareffect. The Rio Claro and Rio Sirenaalso lack
extensivestretchesof lowland floodplains and extensiveestuariesprior to their
dischargeinto the Pacific Ocean. It is this unique combination of physical and
biotic similarities and dissimilaritiesto other tropical regionswhich givesthe fresh~~
water ecosystemsof CorcovadoPark a tremendouspotential for comparativetests
of the factors governingthe evolution of tropical streamfish communities.For example, I would predict that the large numersof palaemonidshrimp, supported
largely by terrestrially-derivedvegetativedetritus, could support many morefreshwater fishes in higher trophic levelsduring the rainy season,if they were introduced. The reproductivebiology of the peripheraldivision fish speciesof the park
remain virtually unknown. Thesefishes are very important componentsof many
freshwatercommunities,and additional life history information could contribute
much to clarifying the oPerationaldefinitions of fish divisions basedon marine
dispersiveabilities and ecology.
Acknowledgments
I would like to thank Frank Pezoldfor his extensiveefforts in verifying anddetermining the identification of speciescollected at CorcovadoPark. Clark Hubbsand
LawrenceGilbert provided important logistical assistancewhich otherwisewould
havemadethe study impossible.ThomasGrohmanand LawrenceGilbert aidedin
the collection of severalfish species.Frank Pezold, LawrenceGilbert, and John
Rawlinsprovidedimportant criticismsand suggestions
during the preparationof the
manuscript.I would especiallylike to thank the efforts of the CostaRica National
Park Servicefor their provision of a collecting permit and hospitality during the
study. The data was compiled during participation in the U. T. Zoology 384L
coursein tropical ecology.
Resumen
5e investigaronlas comunidadesde pecesde aguadulce en el ParqueNacional
Corcovadodel 25 junio al 28 de julio. 5e observaron0 recolectarontreinta especies
de pecesde aguadulce, asi como diez especiesque seencuentransolamenteen zonascon influenciade lasmareas.
Los pecesde lis divisionesde aQuadulce mas abundantesy extensamentedistribuidos son Asryanax fasciatusy Brachyrhaphisrhabdophora.De 105pecesde la
divisi6n periferica, Sicydiumpittieri, Awaous trangndeanus,Gobiomorusmaculatus y Eleotris picta fueron 105mas extensamentedistribuidos en lis aQuasdulces
de la regi6n. Variasespeciesde la divisi6n marinatambien se encontraroncon fre.
cuencia en aQuadulce, entre otras, Mugil curema, Pomadasysbayanus,Lutjanus
colorado, L no~mfa.:iatus, y Spheroidesannulatus.Estasespeciesseencontraron
en abundanciasobretodoen 105rfos y las regionesbails de 105arroyos. EI unico
ejemplarde pel sierra (Pristissp.) observado,seencontr6en el rfo Pavocasia kilo.
metro y mediode la desembocadura.
Examinamoslos contenidosde los est6m~os de varios ejemplaresde las especies
dulceaculcolasmas comunes.Los alimentos terrestresles eran muy importantes;
'6
entre otros, pepitas,frutas, insectos(especialmente
hormigas)y material organico.
Ahora bien, la mayor parte de la comida aut6ctonaen 105regrmenesde 105peces
fue algas,camaronesy pecespequenos.La rflayoria de 105pecesde lasdivisionesde
aQuadulce parecentener habitosde alimentaci6nparticulares.
.
La particularidaddel habitat en el arroyo de cads especiesugierela posibilidadde
que estefactor puedaserun metodoadicionalde segregaci6n
ecol6gica.
Tomando en cuenta la informaci6n preliminar de que dispon(amos sabre los habitos
de alimentaci6n, se construyeron redes de alimentaci6n para las comunidades de
Quebrada Camaronal y r(o Claro. Se encontr6 que el ancho del arroyo se correlaciona positivamente con la riqueza de especiesde peces. Con base en los estudios preliminares hechos en Costa Rica y Panama,se ofrece la hip6tesis de que los elementos
marinas en las aQuasdulces del parque bajan en nCAmero
durante las estaciones secas.
Tambien que par 10 menos cinco de los pecesde las divisiones de aguedulce emigraran hacia el interior de la regi6n par rutas costeras.
Literature Cited
Allen, K. R. 1969. Distinctive aspectsof the ecology of stream fishes: A review. J. Fish. Res.
Bd. Canada26: 1429-1438.
Bishop, J. E. 1973. Limnology of a sm.1 Malavan river, Sungai Gombek, Monogr~h. Bioi. 22.
Th. Hague: W. Junk. 435 pp.
Bussing. W. A. 1967. New species Md new records of Costa Rican freshwater fishes with a tentetive list of ~ecies. Rev. Bioi. Trap. 14: 206-249.
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ConstMtz, G. D., W. A. Bussing.and W. G. Saul. 1981. Freshwater fishes of Corcovado Nation. Park, Costa Rica. Proc. Acad. Nat. Sci. Phil.. 133: 16-19.
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S7
Miller, R. R. 1956. Geogr~hic.
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petitive exclusion principle. Ecology 52: 336-342.
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Table 2. Resultsof a stepwisemultiple regressionanalysisof fish speciesnumbers
and measures
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Contribution
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Partial F
Overall
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62
Table 4. Resultsof the stomachcontents analysesfor the principal fish speciesinhabitingthe freshwatersof CorcovadoNational Park.
St.nd.rd
Stomach cont8nts
Species
A$tyanax faa:istu$
HyphB$$Obrycon savagei
Poscilis sphenops - ,ill;
Brschyrhsphi$
rhabdophora
Posciliopsi$ turrubsrensi$
Oxyzygonectes dovii
."',-
Sicydium pitt;.,i
Awaau$ transandsanU$
Gobiomoru$ msculetu$
E/eotris picts
".,
Alimenury c_1
No.
length
(mml
Fruits. seeds.insects
Sm., seeds.ants
Green alg.
(esp.Spyrogyra spp.)
Short, sc-like
Short. sac-like
Long. coiled
13
7
3
38-95
35-43
58-90
Ants. small insects
Epiphytic .gee
Short, sac-like
Long. coiled
Ants. sm.1 insects.epiphytic .g.. detritus
Epiphytic .g.
Detritus
Sm.1 fish. shrimp.
Ephemeroptera nymphs
Larval fish. shrimp.
crayfish. snails. seeds,
fruits. detritus. insects
Short. sac-like
7'
3
2
18-46
20-28
48-80
Long. coiled
Long. coiled
Short, sac-like
2
2
5
56-71
74-86
46-168
Short. sac-like
7
38-200
2
3
4
7
l'
3
2
2
2
1
1
81-123
25-88
133-188
50.85
130
132-166
132-150
66-140
92.110
12
15-63
Agonostomus monticols
Mugil CUrBmS
Pomstlssys bBysnus
Insects
Detritus, algae
Short, sac-like
Long, coiled
Shrimp, crayfish
SphoBl'oidsssnnulstus
LutiBnus colorado
LutiBnus novemfssciBtus
LutiBnus srgsntiventris
Pseudophsllusstarks;
Eucinostomus sp.
Bsthygobius sndrsi
Snails
Shrimp. fish(7)
Fish, shrimp(?)
Shrimp. fish(?)
Microcrustaceans(?)
Microcrustaceans(7)
Shrimp
Short, sac-like
Short, sac-like
S8:-like, long. coiled
Short, sac-like
Short, sac-like
Short, s-=-like
Short, sac-like
?
Short. sac-like
Csntropomus
undecims/is Fish. shrimp(~)
63
"",
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,
SIRENA
ParQue Nacional Corcovado
~
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Fig. 1. Map of the area surrounding the Sirena station of Corcovado Park, Costa
Rica and the locations of field sites.
64
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&
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&
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algae
I
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01
3:
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~
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c
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Autochthonous
Fig. 2. Block diagram representing the trophic relationships within a typical community of the Quebrada Camaronal. The sizes of the blocks represent the approxi.
mate relative abundances of each component in terms of biomass, based on visual
censuses.
.
65
l. novemfasciatus
I
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u I
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V
.
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I
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&
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I
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(detrjws)
Allochthonous
I
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I.
I
Autochth.onous
Fig. 3. Block diagram representing the trophic relationships within a typical community of the RIo Claro.
66
.
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