BRENESIA 21: 47-.66.1983

AN INTRODUCTION TO THE FRESHWATER FISH COMMUNITIESOF CORCOVADO NATIONAL PARK, COSTA RICA

Kirk O. WinemillerDepartment 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 ecology of each species of the freshwaterfish communities surrounding the Sirena station of Corcovado Parkwere investigated during the early rainy season of 1982. Thirty specieswere observed or collected in freshwaters of the park, and an additional10 species were restricted to intertidal regions. Peripheral division andinvading marine fishes were major integral components of the fresh-water ichthyofauna. These forms were particularly prevalent in therivers and lower stretches of small streams. Stream width was positivelycorrelated with species richness. Most of the primary and secondarydivision fishes appeared to be fairly restricted in their use of the rangeof available food resources. The habitat affinities and approximateabundances of each fish species was reported, based on visual censuses.The route of dispersal into the region by primary and secondary divi-sion fishes is discussed. Based on comparisons with earlier studies inCosta Rica and Panama, it is hypothesized that the marine elements inthe park's freshwater ecosystems decline during the dry seasons. Foodwebs for the Quebrada Camaronal and Rio Claro fish communitieswere constructed. The major sources of primary productivity for thecommunities are apparently allochthonous, indicating a dependencyof the present ichthyofauna on the tropical rainforest as a source ofenergy and nutrients.

The freshwater ecosystems of Corcovado National Park in Costa Rica are unusual inpermitting the study of freshwater tropical fish communities in a region of relative-ly low freshwater fish species diversity. Freshwater fish communities in the neotrop-ics are generally characterized by an abundance of primary freshwater fish species(i.e. members of families with little or no tolerance to saline conditions) of the

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suborders Characoidei and Siluroidei (Lowe-McConnell, 1975). By marked contrast,the freshwater fish fauna of Corcovado contains only three species of primary fresh-water fishes (two char.:oids and one siluroid), and this has been attributed togeographical factors inhibiting invasion by these forms onto the Osa Peninsula(Constantz et al., 1981). Secondary division freshwater species (i.e., members offamilies with limited degrees of tolerance to marine salinities) are underrepresentedat Corcovado as well, with only three reported (Constantz et aI., 1981) from themore than 25 secondary division species known from the Costa Rican mainland(Bussing, 1967). The remainder of the Corcovado freshwater fish fauna is com-prised of peripheral division and marine species which have invaded the streams,often many kilometers inland. I have chosen to define peripheral division species asmembers of families which are clearly marine in origin, yet they must reside in freshor brackish coastal waters for variable periods of time during their life histories.Peripheral fishes may require freshwater ecosystems for successful feeding duringjuvenile or adult life, or as an environment for reproduction and/or larval feeding.Marine fishes will be defined as species which may reside in fresh or br.:kish waters,often for extensive periods, yet they are fully capable of completing their entirelife cycles in the marine environment. Unlike peripheral division fishes, many popu-lations of marine fishes reside exclusively in the marine environment, either off-shore or in intertidal zones. The significance of this new distinction will becomeapparent in subsequent sections of this report.

Constantz et 8/. (1981) reported a total of 20 fishes from the freshwaters of Corco-vado Park. This represents relatively low species richness for neotropical freshwatersin a region of this size. The freshwater ecosystems of Corcovado Park provide goodopportunities for comparatively-based hypotheses concerning freshwater fishdispersal abilities, historical aspects shaping local species diversity, and tropicalfreshwater community dynamics. The present study was undertaken to: 1) providemore detailed information on the relative abundances, distributions, and habitatutilization of each freshwater fish species; 2) determine the food habits of each fishspecies; 3) obtain information on the trophic structure of the freshwater ecosys-tems; and 4) provide a preliminary record of intertidal and estuarine fish species.

Material and Methods

Field sites were chosen along sections of the R(o Claro, Rio Pavo, Quebrada Cama-ronal, Q. Danta, two small streams which emptied directly into the ocean, theheadwaters of a small, unnamed stream, and one rocky intertidal pool. Additionalfish collections and notes were made in intertidal regions of bedrock substrate (A),an oxbow lake near the mouth of the R(o Sirena (B), the Rio Sirena estuary, andsections of streams between the selected field sites. Maps were constructed for eachfield site containing: Profiles of stream depth, poll or riffle length, stream width,substrate composition, and the location of potential fish refugia (i.e., undercutstream bank, roots, logs, boulders, etc.). The length of each field site was delineatedby the natUral boundaries of the pool or riffle under investigation. Consequently, a

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pool field site in a river was often much longer than a. pool in a smaller stream.Water current velocity was determined by tossing a35 mm film canister, half-filledwith water, into the region of greatest current and timing its passage rate over adesignated distance. At each field site, the following data were recorded: 1) pH; 2)salinity (ppt); 3) temperature; 4) maximum water current velocity; 5) the abun-dance of each fish species present; and 6) the relative ratio of juveniles to adults ofeach fish species present. The presence of epiphytic algae, zooplankton, crusta-ceans, insolation, and fruiting trees was also noted at each site. Each field site wasphotographed to supplement the mapped information in later analyses.

Fishes were collected by dipnetting, seining, fish traps, angling, and by hand. Habi-tat utilization and estimates of the relative abundances of fishes were made bydirect observation from above the water's surface and snorkelling below the water'ssurface during normal stream discharges. Special attempts were made to trap ordipnet rare or nocturnal fishes under sections of eroded stream bank. Selected fisheswere measured for standard length and their stomach morphology and contentswere examined. One to six specimens of each species collected were preserved in10 percent formalin and later deposited in the Texas Natural History Collection ofthe Texas Memorial Museum, University of Texas, Austin.

Descriptions of Field Sites

Corcovado park is located on the extreme western edge of the Osa peninsula ofCosta Rica's Pacific coast. The park is approximately 41,800 ha and lies at approxi-mately SO 30' N latitude and 8'jO 30' longitude. Corcovado Park contains extensiveareas of primary tropical rainforest plus patches of regenerating second growth,including dense stands of He/iconia, Ca/athes, Trema, Piper, Cecropia and speciesof other plant genera (Janzen, 1979). The park received 4701 mm of rainfall in1980 and averaged 364.4 mm over 1979, 1980, 1981 during the period in whichthe present study was conducted (June 25 to July 28). This period is in the earlystage of the major annual rainy season of this region. The streams and rivers of thepark flow over Cretaceous volcanic and sedimentary rocks and, to a lesser degree,Quaternary alluvial deposits (Janzen, 1979).

The location of each field site is provided in Figure 1. Fish species numbers, physi.cal parameters of the water, and stream length, width, and maximum depth areprovided in Table 1. The following are verbal descriptions of the general habitatconditions present at each field site.

Sites 1 and 3 are shallow, swampy pools at the headwaters of small streams whichultimately empty into the Rlo Sirena. The pools contained much emergent vegeta-tion, including Hel/conia sp. Rotting Hel/conia leaves served as refugia for fishesand substrate for SpyrogyrB sp. and other species of filamentous green algae. Thebottoms of these pools were covered with thick lavers of vegetative detritus and

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mud. At midday, direct sunlight was transmitted through the secondary growthforest, producing scattered patches of sunlit and shaded water in each pool. No ob-servable water current was present at sites 1 and 3.

Sites 2, 4, 5, 7, 8, 9, and 11 are pools in the Quebradas Danta and Camaronal. Allpools contained regions of gravel, sand, and leaf litter substrate. Epiphytic blue-green algae and diatoms were usually present on rocks in the shallow marginal areasor deeper regions containing observable current in these pools. Each pool is bor-dered at one side by an area of eroded (undercut) bank, usually with tree rootstructures or bedrock above. A small portion of most pools was exposed to directsunlight during midday, and these were often associated with limited algal growthwhen suitable substrates were present.

Site 6 is located at the extreme headwaters of the Quebrada Camaronal. This wasthe last pool observed to contain fish (3 specimens of Agonostomus mont;cola) asone traveled this fork upstream. Stream gradients are much steeper and manymoss-and lichen-covered boulders are present in this region of the stream. Leaflitter, bedrock, and gravel comprised the substrate, and very little observablealgae was present at this site. The entire pool remained shaded by primary forestthroughout the day.

Site 10 is a shallow riffle section of the O. Camaronal with a coarse gravel, sand,and leaf litter substrate. Most of the riffle was exposed to direct sunlight at midday,and this appeared to support thin growths of epiphytic blue-green algae and dia-toms.

Site 12 is located at the mouth of the a. Camaronal where it empties into the RroSirena estuary. Site 12 grades from a deep, narrow stream pool, bordered by bed-rock, to a shallow slough flowing over rocks, coarse gravel, and finally sand and silt.The entire site is exposed to direct insolation through most of the day and sup-ported diatoms on the rock surfaces.

Sites 13 and 14 are located in two separate small, shallow streams whim flowthrough areas of extensive secondary forest growth adjacent to the coast. Thesestreams empty directly into the ocean over sandy beach. The substrate of site 13consisted of sand and the pool margins were composed primarily of grasses andsedges. Site 13 was exposed to insolation throughout most of the day, yet nosignificant algal growth was observable. Site 14 was entirely shaded and has a sand,soil, and leaf litter substrate. Clumps of emergent understory vegetation werescattered throughout site 14. These streams are probably ephemeral, with veryrestricted flow during the major dry season.

Sites 15 and 17 are large pools in the Rfo Claro which have bedrock and gravelsubstrates. Thin growths of blue-green algae and diatoms were present in the shal-

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low marginal areas and swift headwaters and tailwaters of each pool. Leaf litterdeposits were generally present among the rocks at the pool margins. Major portionsof both pools were exposed to direct sunlight at midday.

Site 16 is a swift riffle area of the R(o Claro. The substrate is comprised of coarsegravel and rocks. Most of this area is exposed to direct sunlight at midday, and athin layer of algae is present on some rocks, particularly in shallower marginalregions of the riffle.

Site 18 is located in a channel formed by the presence of a sand bar near the edgeof the Rfo Oaro estuary. The substrate here is comprised of sand and fine siltdeposits. The area is exposed to direct sunlight throughout the day, yet no signifi-cant epiphytic algal growth was detected. Forest litter was commonly depositedalong 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 thecrossing of the park's Casa Marenco trail. The substrate at this site was primarilyfine gravel and sand. Unlike the pools of the Rfo Claro (sites 15 and 17) severalfallen trees provided cover for the fishes. The entire pool was exposed to sunlightat midday, and very limited growths of epiphytic algae were present in the shal-lower regions.

Site 20 is an isolated intertidal pool at low tide, which has a sand and bedrocksubstrate. Numerous rocks were scattered throughout the pool, providing cover formany small marine fishes. The pool is exposed to direct sunlight throughout theday, and thin layers of epiphytic algae were present on many rocks. At high tide,site 20 is inundated by over 3 m of sea water and is exposed to constant waveaction.

Observations

The number of species at each freshwater field site was extremely variable, rangingfrom one (sites 6, 13, 14) to 15 (site 19). In addition to providing total speciesnumbers, Table 1 summarizes the number of primary division, secondary division,peripheral, and marine fish species at each site. No continuous pattern emerged astotal species richness was tracked from the headwaters of each river and stream toits mouth. SPSS (Statistical Package for the Social Sciences, Nie et al., 1975) wasused to compute a multiple regression analysis of total numbers of fish species andstream current, maximum depth, maximum width, and the length of each field site.The results, summarized in Table 2, indicate that approximately half of the totalvariance is explained by differences in stream width. Additional variance within thepattern of species richness is explained by depth, current, and site length, in order;and together these four variables accounted for 73 percent of the variance. Theratio of the numbers of peripheral and marine species to the number of freshwaterdivision species increased from 1.0 at the headwaters of the a. Camaronal to 12.0

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at its mouth. A simple regression of peripheral and marine to freshwater species forthe seven sites on the a. Camaronal resulted in a positive trend (r2 = .494), howeverthis was statistically nonsignificant (F = 4.873, df = 1, 5, p = .078). The distributionof freshwater, peripheral, and marine division fishes in the park does not correlatewith salinity gradients since dissolved solids were uniformly low « 1 ppt) through-out the streams and rivers (Table 1). The exceptions were the two river estuariesduring high tides (Table 1).

The distribution and habitat affinities of each fish species within the park are pre-sented in Table 3. AstysnBX f.cistl/$ was by far the most abundant and widelydistributed primary freshwater fish species in the area. Brachyrhsphis rhsbdophorawas the most numerous secondary division species and was present at 14 of the 19freshwater field sites. The distribution of Poeci/is sphenops was much more re-stricted to regions of filamentous green algal growth. Poeci/iopsis turrubarensis andOxYZygOnectBS dovii were limited to the RIo Sirena estuary and adjacent streammouths where B. msbdophors was absent.

Among peripheral fishes, Awaous transandeanus, Sicydium pittieri, Gobiomorusmaculatus, Eleotris picta, and Agonostomus monticola were distributed far inlandin significant numbers. s. pittieri, in particular, may reproduce in freshwaters asevidenced by the numerous juveniles throughout its distribution and the presenceof a brilliantly colored territorial male in the middle reaches of the Q. Camaronal.Larger, predatory peripheral division and marine fishes (Centropomus and Lutjanusspp.) 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 theheedwaters of the smaller streams and pool volumes declined. Juveniles of Pomada-sys bayanus were commonly encountered in pools of the streams. Several individu-als were observed to exhibit intraspecific aggression near regions of cover such aseroded stream bank. A single adult specimen (-1.5 m standard length) of the saw-fish, Pristis sp., was observed at site 19 and one other location of the Rfo Pavo.

The most abundant fish species of the intertidal zones appeared to Abudefdufan8logus, Hypsoblennius striatus, and 88thygobius ramosus. The assessment of theabundances of species such as Gobiesox sp., Gymnothor8x sp., and Mslacoctenuszonifer was hampered by the presence of many deep, undercut bedrock crevicesutilized by these forms.

Preliminary information on the food habits of the principal freshwater-dwellingfishes of the park is presented in Table 4. Epiphytic algae and small shrimp of thefamily Palaemonidae apparently comprised the major autochthonous basis for theichthyomass sustained in the rivers and streams. Major allochthonous sources offood input were forest vegetative detritus, mature fruits and seeds, and terrestrialinsects, including ants. Aquatic snails were very abundant in the R(o Claro, but onlyminor components of the R(o Pavo and stream ecosystems. Other foods utilizedwere crayfish, aquatic insect larvae (Ephemeroptera), and microcrustaceans.

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A striking characteristic of the streams at Corcovado is the paucity of autochtho-nous primary productivity, and the apparent extreme paucity of aquatic insectsand zooplankton. Preliminary schemes for the trophic interactions of the com-munities of the Q. Camaronal (Fig. 2) and Rro Claro (Fig. 3) were constructedbased on the approximate relative abundances and food habits of each species. Inboth systems the major source of primary production takes the form of allochtho-nous input from the rainforest. Snails and shrimp comprised a much greater propor-tion of the "primary aquatic consumer" trophic level in the Rfo Claro than in theQ. Camaronal. Both communities are representative of a "food web" trophic struc-ture, wherein many species populations utilize food resources from the same broadcategories, or utilize food from two or more trophic levels. The species which com-prised the lower trophic levels of the t~ systems were quite similar, however theRro Oaro supported several more predatory species in the higher trophic levels(Figs. 2 and 3).

Although the food habits data for Corcovado fishes should be considered prelimi-nary due to low sample sizes, several species exhibited fairly rigid selection forspecific items. Fruits and seeds constituted the major portion of the diet of Afasc;atus, and to a slightly lesser degree H. savage;. Several species appeared to bestrict algal grazers, including P. sphenops (green algae, esp. Spyrogyrs spp.), P. tu-rrubarens;s (diatoms), and s. p;tt;er; (diatoms and blue-green algae). The diets ofB. rhsbdophora and A mont;co/a consisted of terrestrial insects, with B. rhsbdo-phora consuming mostly ants and tiny dipterans. The gut of the single analyzedspecimen of Sphoero;des snnu/atus was p.:ked with many aquatic snails and shellfragments. Several AMOUS trsnsandeanus were observed sucking up portions offine gravel and sand substrate and allowing the inorganic particles to sift throughtheir opercular oPenings. One cKJult A. trsnsandeanus at site 16 was observed toperiodically dash up from the substrate to capture individual particles of driftingdetritus. The gut contents of two individuals revealed only fine particles of detritus.

Discussion

With the addition of Oxyzygonectes dov;;, the present study increases the numberof known freshwater division fishes at Corcovado (Constantzetal., 1981) to seven.To the previous list of 14 fishes with marine affinities present in the park (Cons-tantz .:... jl., 1981), this study reports 13 additional peripheral division and marinespecies, plus 10 additional marine species from intertidal zones. The three pipefishspecimens, which were returned to the Texas Natural History Collection, weredetermined to be Pseudopha//us starks;. P.wudopha//us elcaptansnse was reportedas moderately abundant in the a. Camaronal by Constantz et al. (1981), where twoof my specimens were collected. Gob;esox potan;us and Rhamdia Mgnsr; were notobserved during the study although both were previously reported as rare in the Q.Camaronal. Despite the fact that they were reported in slightly different manners,there appear to be major disparities in the approximations of the abundance anddistribution of many of the fishes between 1979 (Constantz et al., 1991) and 1982.

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This may be due to differences in season, since the 1979 observations were madeduring the major dry season and the 1982 observations during the early portion ofthe major wet season. Differences in collecting techniques and censusing methodsmay have also contributed to differences in abundance estimates.

The ichthyofauna of the park's freshwater ecosystems is a mixture of freshwater,peripheral, and marine division forms. Certainly the composition of these commu-nities is temporally dynamic in nature, and they will ultimately require detailed lon-gitudinal studies before the factors influencing their structure are fully understood.If, as indicated by Table 2, stream volume has a major influence on species richness,then major differences in the number of peripheral and marine division fishes mightbe expected between wet and dry season censuses. Several investigations in a varietyof temperate and tropical lotic systems have also supported the view that streamvolume, width, and/or discharge may explain a major portion of the variability ob-served within fish species diversity patterns (Gorman and Karr, 1978; Smith, 1981;Livingstone et al., 1982). I hypothesize that many more individuals of the marinedivision species join the ichthyofaunas of small coastal tropical streams, such asthose at Corcovado, during wet seasons when stream volumes, discharge rates, arxlfood resources are higher. Allochthonous food input to the Tortuguero estUary ofeastern Costa Rica was found to be much higher during the wet seasons (Nordlieand Kelso, 1975). In a small Panamanian stream, fish foods were more abundantduring the wet season (Zaret and Rand, 1971). During the dry seasons at Corcova-do, many marine fishes may be excluded from freshwaters due to competition withfreshwater and peripheral division fishes. In reviewing the distributional patterns ofAfrican freshwater fishes, Roberts (1976) noted that, when present, freshwaterdivision fishes tend to exclude more euryhaline forms from inland habitats. Tinypalaemonid shrimp were extremely numerous in the streams and rivers of Corco-vado during the early rainy season, and these should provide a major potentialsource of food for temporary marine invaders as well as permanent freshwater resi-dents. The view, that primary and secondary division freshwater fishes dominate theecosystems at Corcovado when stream volumes are lower, is also supported by theobservation that freshwater division fishes comprised a greater proportion of thespecies richness as one traveled away from the mouths of streams.

Corcov~o Park lies within the isthmian faunal province of Miller (1966) and Bus-sing (1976). This region of Central America is by far the richest in primary divisionfreshwater fish species (Myers, 1966; Miller, 1966; Bussing, 1967), with most formsderived from South American faunal elements. Of the three primary division sp«:iespresent at Corcov~o, Astyanax fa.w:;stus and Rhamdia wagner; are reported to havelimited toleral~es to saline conditions (Miller, 1966; Bussing, 1976). Both of thesespecies exhibit ubiquitous ranges in lower Central America, and A. fa.w:;stus in par-ticular extends from Argentina to southern Texas on the Atlantic slope (Miller,1966). It is therefore quite likely that these species have utilized a coastal routefor dispersal into this distal region of the Osa peninsula. If stream headwater cap-tures h~ played a significant role in the dispersal of primary freshwater fishes intothe park, more species would be expected. The degree of salinity tolerance by

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Hyphessobrycon savagei is unknown. Like the primary freshwater fishes, the secon-dary division fishes of Corcovado are represented by several species which areubiquitous in Central America. The presence of these seven primary and secondarydivision freshwater fishes suggests that these forms have dispersed into the Osafairly recently (i.e., compared to major dispersals of these families along the main-land Central American slopes) and by way of coastal routes, possibly during periodsof unusually high rainfall. The recent arrival of freshwater division fishes into theCorcovado region is further supported by Bussing's (1967) observation that a largenumber of indigenous species comprise the Costa Rican freshwater fish fauna. Theperipheral and marine division fishes of Corcovado are primarily species which arewidely distributed along the Pacific coast of Central America. Several species, suchas Agonostomus monticola, Centropomus undecimaJis, and Mugil curema areknown from both coasts of Central America. Apparently, the presence of Abudef-duf analogus in my collections represents the first report of this species for thePacific coast of Central America.

The preliminary data on the diets of Corcovado fishes indicated that several fresh.water and peripheral division species have fairly restricted feeding habits (Table 4).One marine invader, Sphoeroides annulatus, may speCialize on snails, using its"beak-like" dentition to crush shells. Most of the marine species in freshwatersat Corcovado are large predators which apparently exploit the numerous shrimpand small fishes. Much more information is needed on the diets of these fishes be-fore it can be determined if food resources are a limiting factor for marine fish bio.mass during the wet season. The descriptive information on stream habitat use bythe fishes of Corcovado (Table 3) suggests that habitat selection could be one meansby which fishes ecologically segregate during periods of depressed resources. This isan emerging generalization from recent studies of tropical stream (Zaret and Rand,1971; Roberts, 1972; Bishop, 1973; Gorman and Karr, 1978) as well as temperatefreshwater fish ecology (Winn, 1958; Allen, 1969; Mendelson, 1975; Symons, 1976;Werner et al., 1977). The distributional patterns of fishes within the park (Table 3)probably relates most directly to habitat affinities. Consequently, the total speciesrichness at each site should relate, not only to stream volume (Table 2), but alsoto environmental heterogeneity in terms of habitat diversity at each site. For exam-ple, Poecilia sphenops was only abundant in environments where there was lowwater current velocities and significant growth of fil~entous green algae.

The freshwater ecosystems of Corcovado Park appear to be consistent with thephysical characteristics of many tropical rainforest headwater streams (Myers,1947; Roberts, 1972; Bishop, 1973; lowe-McConnell, 1975). Their waters areapparently quite nutrient poor, contain little primary production, are often entirelyshaded, exhibit low thermal variability, and receive seasonal patterns of rainfall.One major difference is the absence of high freshwater fish species diversity atCorcovado due to the peninsular effect. The Rio Claro and Rio Sirena also lackextensive stretches of lowland floodplains and extensive estuaries prior to theirdischarge into the Pacific Ocean. It is this unique combination of physical andbiotic similarities and dissimilarities to other tropical regions which gives the fresh-

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water ecosystems of Corcovado Park a tremendous potential for comparative testsof the factors governing the evolution of tropical stream fish communities. For ex-ample, I would predict that the large numers of palaemonid shrimp, supportedlargely by terrestrially-derived vegetative detritus, could support many more fresh-water fishes in higher trophic levels during the rainy season, if they were intro-duced. The reproductive biology of the peripheral division fish species of the parkremain virtually unknown. These fishes are very important components of manyfreshwater communities, and additional life history information could contributemuch to clarifying the oPerational definitions of fish divisions based on marinedispersive abilities and ecology.

Acknowledgments

I would like to thank Frank Pezold for his extensive efforts in verifying and deter-mining the identification of species collected at Corcovado Park. Clark Hubbs andLawrence Gilbert provided important logistical assistance which otherwise wouldhave made the study impossible. Thomas Grohman and Lawrence Gilbert aided inthe collection of several fish species. Frank Pezold, Lawrence Gilbert, and JohnRawlins provided important criticisms and suggestions during the preparation of themanuscript. I would especially like to thank the efforts of the Costa Rica NationalPark Service for their provision of a collecting permit and hospitality during thestudy. The data was compiled during participation in the U. T. Zoology 384Lcourse in tropical ecology.

Resumen

5e investigaron las comunidades de peces de agua dulce en el Parque NacionalCorcovado del 25 junio al 28 de julio. 5e observaron 0 recolectaron treinta especiesde peces de agua dulce, asi como diez especies que se encuentran solamente en zo-nas con influencia de las mareas.

Los peces de lis divisiones de aQua dulce mas abundantes y extensamente distri-buidos son Asryanax fasciatus y Brachyrhaphis rhabdophora. De 105 peces de ladivisi6n periferica, Sicydium pittieri, Awaous trangndeanus, Gobiomorus macula-tus y Eleotris picta fueron 105 mas extensamente distribuidos en lis aQuas dulcesde la regi6n. Varias especies de la divisi6n marina tambien se encontraron con fre.cuencia en aQua dulce, entre otras, Mugil curema, Pomadasys bayanus, Lutjanuscolorado, L no~mfa.:iatus, y Spheroides annulatus. Estas especies se encontraronen abundancia sobretodo en 105 rfos y las regiones bails de 105 arroyos. EI unicoejemplar de pel sierra (Pristis sp.) observado, se encontr6 en el rfo Pavo casi a kilo.metro y medio de la desembocadura.

Examinamos los contenidos de los est6m~os de varios ejemplares de las especiesdulceaculcolas mas comunes. Los alimentos terrestres les eran muy importantes;

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entre otros, pepitas, frutas, insectos (especialmente hormigas) y material organico.Ahora bien, la mayor parte de la comida aut6ctona en 105 regrmenes de 105 pecesfue algas, camarones y peces pequenos. La rflayoria de 105 peces de las divisiones deaQua dulce parecen tener habitos de alimentaci6n particulares.

.La particularidad del habitat en el arroyo de cads especie sugiere la posibilidad deque este factor pueda ser un metodo adicional de segregaci6n ecol6gica.

Tomando en cuenta la informaci6n preliminar de que dispon(amos sabre los habitosde alimentaci6n, se construyeron redes de alimentaci6n para las comunidades deQuebrada Camaronal y r(o Claro. Se encontr6 que el ancho del arroyo se correlacio-na positivamente con la riqueza de especies de peces. Con base en los estudios preli-minares hechos en Costa Rica y Panama, se ofrece la hip6tesis de que los elementosmarinas en las aQuas dulces del parque bajan en nCAmero durante las estaciones secas.Tambien que par 10 menos cinco de los peces de las divisiones de ague dulce emigra-ran hacia el interior de la regi6n par rutas costeras.

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Roberts, T. R. 1976. Geogr~hic distribution of Afrlc., freshwater fishes. Zool. J. Linn. Soc.57: 249-319.

Smith, G. R. 1981. Effects of habitat size on species richness and ~ult body sizes of desertfishes. Pages 125-171 In R. J. Nai~and D. L SoItz, eds. Fishes in North Americandeserts. New York: John Wiley & Sons,

Symons, P. E. K. 1976. Behavior and growth of juvenile Atl.,tlc salmon (SaI/1X)saI.-).,d threecompetitors at two stream velocities. J. Fish. Res. Bd. Canada 33: 1766-1773.

Werner, E. E., D. J. Hall, D. R. Laughlin, D. J. Wagner, L. A. Wilsrn.,n, and F. C. Funk. 1977.Habitat partitioning in a freshwater fish community. J. Fish. Res. Bd. Cenada 34: 360-370.

Winn, H. E. 1968. Comparative reproductive behavior and ecology of fourtMn species of darters(Pisces- Percidael. Ecol. Monogr. 28: 155-191.

Zaret, T. M. and A. S. Rand 1971. Competition in tropical stream fishes: Support for the corn.petitive exclusion principle. Ecology 52: 336-342.

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Contributionto R2

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Table 4. Results of the stomach contents analyses for the principal fish species in-habiting the freshwaters of Corcovado National Park.

St.nd.rdlength(mml

Alimenury c_1 No.Species Stomach cont8nts

1373

38-9535-4358-90

18-4620-2848-80

56-7174-8646-168

38-200

7'32

225

7

A$tyanax faa:istu$HyphB$$Obrycon savageiPoscilis sphenops - ,ill;

Brschyrhsphi$rhabdophora

Posciliopsi$ turrubsrensi$Oxyzygonectes dovii

Sicydium pitt;.,i . "',-Awaau$ transandsanU$Gobiomoru$ msculetu$

E/eotris picts

Fruits. seeds. insects Short, sc-likeSm., seeds. ants Short. sac-likeGreen alg. Long. coiled(esp. Spyrogyra spp.)

Ants. small insects Short, sac-likeEpiphytic .gee Long. coiledAnts. sm.1 insects. epi- Short. sac-likephytic .g.. detritusEpiphytic .g. Long. coiledDetritus Long. coiledSm.1 fish. shrimp. Short, sac-likeEphemeroptera nymphsLarval fish. shrimp. Short. sac-likecrayfish. snails. seeds,fruits. detritus. insectsInsects Short, sac-likeDetritus, algae Long, coiledShrimp, crayfish Short, sac-likeFish. shrimp(~) Short, sac-likeSnails S8:-like, long. coiledShrimp. fish(7) Short, sac-likeFish, shrimp(?) Short, sac-likeShrimp. fish(?) Short, s-=-likeMicrocrustaceans(?) Short, sac-likeMicrocrustaceans(7) ?Shrimp Short. sac-like

2347l'322211

81-12325-88

133-18850.85130

132-166132-15066-14092.110

1215-63

Agonostomus monticolsMugil CUrBmSPomstlssys bBysnus

"., Csntropomus undecims/isSphoBl'oidss snnulstusLutiBnus coloradoLutiBnus novemfssciBtusLutiBnus srgsntiventrisPseudophsllus starks;Eucinostomus sp.Bsthygobius sndrsi

63

"", -.,

SIRENAParQue Nacional Corcovado

~ ' .~M.'...

'. '-'." .:

Fig. 1. Map of the area surrounding the Sirena station of Corcovado Park, CostaRica and the locations of field sites.

64

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I Terrestrial Fruits I Diatoms ~& I & 01

Vegetation Seeds I Blue-green iOalgae c

(detritus) I 3:I ..I (!)

Allochthonous I AutochthonousI

Fig. 2. Block diagram representing the trophic relationships within a typical com-munity of the Quebrada Camaronal. The sizes of the blocks represent the approxi.mate relative abundances of each component in terms of biomass, based on visualcensuses.

65.

l. novemfasciatus

I!!. ( 0)u I -Q) U., I Q).E .,Ii r .E.- I 0)::. ~., I 0Q) IU

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Fruits I DiatomsTerrestrial & I &V . Seeds Iegetatlon Blue- green algae

I(detrjws) I

II

Allochthonous I. Autochth.onousI

Fig. 3. Block diagram representing the trophic relationships within a typical com-munity of the RIo Claro.

66 ~.

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