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A S P E C T S O F T H E D I E T S A N D F E E D I N G I C O L O G I E S O F F I S H F R O M N I N E F L O O D P L A I N

LAKES O F T H E L O W E R CAURA, V E N E Z U E L A N GUAYANA

Nirson G O N Z Á L E Z and Conrad VISPO

Abstract

We studied the diets of 39 common, méd ium to large fish species from nine lloodplain lakes of the lower Caura basin. Specifically, we present information on the diets of 16 fish species from the family Characidae; three species each from the families Anostomidae and Hemiodontidae; two species each from the families Clu-peidae, Curimatidae, Cynodontidae, Doradidae, Pimelodidae, and Prochilodon-tidae; and one species each from the families Cichlidae, Ctenoluciidae, Erythrinidae, Loricariidae and Sciaenide. A total of 1,339 stomachs were analyzed, 409 of these were empty. The most important diet item was material from other fish species (flesh, scales, fins), this was followed by plant material (seeds/fruits, flowers, leaves, other) and invertebrates. A few species fed only on detritus or zooplankton. Look-ing in more detail at the diets of the different trophic groups, we found that prochil-odontids, doradids, characids, curimatids and hemiodontids were the most impor­tant prey species for piscivores. For herbivores, vegetative material (as opposed to fruits, seeds or flowers) was the most important component. Among the insecti-vores, aquatic insects (largely, Ephemeroptera nymphs and dipterid larvae) were more important than terrestrial insects (mainly, Hymenoptera, Coleóptera and Isoptera). Certain other invertebrates were also consumed but rarely accounted for large portions of the diet. Geographic variation in diet within species was slight, however seasonal variation was noted. Fish and detritus consumption increased in low water, while plant material and insects became more important during high water. The diversity of food items consumed was greatest during high water.

Finally, we found that reproduction, fattening, and stomach fullness appeared to be associated with the hydrological cycle in many of the species that we studied. We conclude that the floodplain and the natural flood cycle were central to the ecology of many of these species.

Key Words: Caura River, diet, fish, feeding, floodplain lakes, Neotropical, river, trophic ecology, Venezuela.

N. ( il ) N / . A I I V A N I ) ( l.R. Vlsl'i )

Kesu men

Ecología alimentaria en las comunidades ícticas de las lagunas de inun­dación del bajo Río Caura, Edo. Bolívar. Se realizó un estudio sobre la alimen­tación de algunas especies de peces del área inundable del bajo Río Caura (cuenca del Orinoco, Venezuela), destacando la influencia que tienen las lagunas asociadas a estas áreas en la disponibilidad de alimento para los peces. Las dietas fueron anali­zadas en relación a las variaciones estacionales y áreas de capturas, mediante los métodos de frecuencia de aparic ión y porcentaje volumétr ico (Goulding 1980 y Goulding et al. 1988). Para esto, se analizaron 1.339 ejemplares de 39 especies cap­turadas entre los meses de junio 1998 y agosto 1999 en nueve lagunas de inun­dación. Se estudiaron las dietas de 16 especies de la familia Characidae; tres especies de cada una de las familias Anostomidae y Hemiodontidae; dos especies de cada una de las familias Clupeidae, Curimatidae, Cynodontidae, Doradidae, Pimelo-didae, and Prochilodontidae; y una especie de cada una de las familias Cichlidae, Ctenoluciidae, Erythrinidae, Loricariidae y Sciaenidae. De estas lagunas, tres se ubi­caron en el afluente Río Sipao, tres en el afluente Río Mato y las otras tres adya­centes al cauce principal del bajo Río Caura (ver Vispo et al, Cap. 8, este volumen para más información sobre el sitio; Fig. 8.1).

Las capturas se realizaron con redes de ahorque con cuatro tamaños de malla (2 a 5 cm de entrenudos). Los ítems alimenticios encontrados fueron: peces, material vegetal, insectos, arácnidos, otros invertebrados, detritus, zooplankton, restos de animales terrestres y material de animales no identificados. Los recursos peces, material vegetal e insectos fueron los ítems más consumidos por los peces (Tabla 9.1). Entre las principales especies con mayor preferencia por el consumo de peces (excepto Acestrorhynchusspp. que se consideran estrictamente piscívoras), se encuentran: Hy-drolicusspp., Hoplias malabaricus, Rhaphiodon vulpinus, Plagioscion squamosissimus, Pellona flavipinnis, Pellona castelneana, Pygocentrus caribay Serrasalmusspp. Con respecto a las especies que consumen principlamente material vegetal están Myleus rubripinnis, My-lossoma duriventris, Piaractus brachypomusy Pimelodus blochii. Las especies cuya dieta estu­vo basada principalmente en insectos son Triportheusangulatusy Triportheus elongatus. La mayor parte de la ictiofauna estudiada estuvo dominada por especies piscívoras, seguidos por las herbívoras y entomófagas, aunque se encontraron también espe­cies detritívoras (p.ej., Semaprochilodus spp. Psectrogasterciliata, Potamorhina altamazonica, Loricariichthys brunneus) y zooplanctófagas (p.ej., Hypophthalmusspp. y Anodus orinocen-sis), consideradas como especialistas en su dieta. Es posible que la dominancia de peces piscívoros esté relacionada en algunos casos por sesgos en las capturas.

De acuerdo a la estacionalidad, se encontró un mayor porcentaje de estómagos

I hap. 9: D I E T S < >i Fi < >< >i >PI \ I N I , A K I FISI I 331

\u ios durante aguas bajas, con predominio por el consumo de peces, y en menor grado de detritus. Durante aguas altas se observó un mayor consumo de material vegetal e insectos, y una mayor variedad de ítems en casi todas las especies estudia­das para ambas estaciones, principalmente en las especies P. caribay T. angulatus, a pesar del menor número de estómagos analizados (Figuras 9.4, 9.5, 9.6 y 9.7).

No hubo mucha variación con respecto a los í tems consumidos por los peces en las diferentes áreas de captura. A l parecer, las lagunas adyacentes al bajo Río ('aura presentan mayor variedad de recursos consumidos por los peces, aunque quizás sea por las altas capturas de peces realizadas en esta área (Tabla 9.2).

En la descripción detallada de los principales componentes consumidos, el recur­so peces estuvo representado por 16 familias. De éstas, los proqui lodónt idos re­sultaron ser más importantes en volumen; los dorádidos en números de ejemplares consumidos y los carácidos, curimátidos, prochilodontidos y hemiodontidos los más frecuentemente consumidos. Además, las presas de la familia Characidae fueron las más consumidas en aguas bajas y los dorádidos en aguas altas. Las presas de la familia Doradidae fueron importantes en numero de ejemplares en las dietas de P. squamosis­simus, P. cariba, P castelneanay Pflavipinnis. Asimismo, Hydrolicusspp. resultó con la más alta diversidad de familias de presas consumidas (ver Figura 9.1).

En relación al material vegetal, los recursos restos vegetales (hojas, tallos y raíces) y semillas obtuvieron los vo lúmenes promedios más altos con un 68% y 2 1 % respectivamente, siendo el primer recurso consumido principalmente por Laemoly-tóspp., Hemiodusgr. inmaculata, M. duriventrey Brycon spp. y el segundo por P. brachy­pomusy P. blochii. Los restos vegetales fueron más importantes en términos absolu­tos y relativos durante aguas bajas, mientras que el recurso semillas presenció un incremento relativo en aguas altas (Figuras 9.2 y 9.6).

Con respecto a los insectos, tanto acuáticos como terrestres, las especies del genero Triportheus presentaron la mayor diversidad en el consumo de insectos. Los insectos más frecuentemente consumidos fueron las ninfas de Ephemeroptera y larvas de Díptera, de origen acuático; y los Hymenoptera (hormigas), Coleóptera e Isoptera (termitas), de origen terrestre. La frecuencia más alta de ninfas de Ephemeroptera y larvas de Trichoptera consumidas correspondió a la especie Pla-tydoras costatus, mientras que larvas de Díptera correspondieron a Hassar iheringi. Del mismo modo, el consumo de ninfas de Ephemeroptera fue más frecuente en aguas bajas y de Coleóptera y Odonata, tanto acuáticos como terrestres, en aguas altas (Figuras 9.3 y 9.7).

Finalmente, es importante resaltar la importancia que tienen las áreas inundables asociadas al canal principal del bajo Río Caura y sus afluentes, en particular las lagunas y bosques ribereños, ya que proveen fuentes de energía y nutrientes vitales

N. GoN/Al Í/.ANDC.R. VlSI'O

p.u.i el sostenimiento ele las comunidades ícticas. La reproducción, el consumo de alimento y la condición lísica parecen ser todos relacionados con el periodo de inundación (Figura 9.8). Por lo tanto, la conservación de estas áreas es necesaria para la protección y el uso sostenido de los recursos acuáticos.

Palabras Claves: a l imentación, Río Caura, dieta, ecología trófica, Neotropical, peces, lagunas, planicie de inundación, Venezuela.

Introduction

The Caura River has been characterized as one of the most important tributar-ies of the Orinoco. Its drainage basin covers approximately 45,336 k m 2 (Peña and Huber 1996), and it is one of the last tropical watersheds of its size that remains largely pristine (Lewis 1986, Rosales and Huber 1996). Nonetheless, there have been few previous studies of the Caura's fish communities (Rodríguez and Lewis 1990, 1997; Balbás andTaphorn 1996, Machado-Allison et al. 1999). The present study describes the diets of fish from the Caura's floodplain lakes. Such lakes, and more broadly speaking the entire floodplain, play a fundamental role in the ecology of periodically-flooding river systems (e.g., Goulding 1980, Machado-Allison 1993, Goulding et al. 1996, Junk 1997). The importance may be especially great in nutri-ent-poor rivers, such as the Caura, where autochthonous production is less than in nutritionally-richer rivers. Large natural fluctuations in the flow of these rivers gives fish high-water access to an array of habitats where a diversity of foods are rela-tively abundant. These lakes also serve as centers of fish reproduction and as nurs-ery áreas for a variety of species, including several of the most important commer-cial species (Machado-Allison 1993, Goulding etal. 1988, Vispo and Daza, Chap. 6, this volume). These eggs, fry and juveniles are themselves a food source. At the same time, the tropical river floodplain is a highly threatened habitat both because of direct human use and because of the indirect effeets of hydrological engineer-ing {e.g., Goulding etal. 1996).

In this paper, we present a detailed description of the diets of some of the most common Caura fish species in this important floodplain habitat. Based on this information, we then try to answer the following questions:

1) What are the diets of some of the most common fish in the Caura's flood­plain lakes and how do these diets compare with what has been reported for these species elsewhere?

2) How do the diets vary with the hydrological cycle? 3) Is there evidence of geographic variation in diet within the fish species

studied?

< '.hap.l): 1 )ll is oí l i < >oi)]'l AIN 1.AK1 I'isii 333

4) Finally, how does the timing of the annual cycle of feeding, fattening and reproduction by these fish compare with the hydrological cycle?

Based on the answers to these questions, we consider the likely role of the floodplain in the nutritional ecology of these species. By better understanding the feeding ecology of these fish, including their dependence on the hydrological cycle and the flooded forest, we hope to more realistically evalúate the likely ecological (and henee sociological) consequences of future human alterations to the Caura basin.

STUDY ÁREA Samples were collected from nine floodplain lakes of the lower Caura found

between 7 o 09' and 7 o 36' N , and 65° 05' and 65° 1 V W, approximately 300 km west of Ciudad Bolívar, Edo. Bolívar. Three were along the Sipao tributary (Car-amacatico, Chiribital and Garzones), three along the Mato tributary (Patiquín, Cejal and Pozo Rico) and three along the banks of the Caura main stem (Naparaico, Brava and Aricagua). The lower Caura is formed by roughly the lower 280 km of the Caura's total 730 km. It is demarcated to the south by Salto Para, where the Caura falls 80 m in 2 km including free-falls of more than 50 m height. These falls apparently form a barrier to fish interchange. To the north, the lower Caura ends at the Caura's mouth on the Orinoco near the port of Las Majadas. During high water, the Caura's lower reaches are dammed by the flow of the Orinoco. This effect augments the Caura's floodplain área along its last 130 km (Vargas and Ran-gel 1996). We estímate the lower Caura's floodplain to be roughly 450 km 2 .

The Caura crests in July and August, and is shallowest during February and March. The waters are neutral to slightly acidic (pH 6-7) and carry few nutrients (García 1996). See Vispo and Knab-Vispo (Chap. 1, this volume) for more infor­mation on the general área, and Vispo etal. (Chap. 8, this volume) for a map of lake locations (Fig. 8.1).

Materials and Methods

Stomach contents were collected during scientific fishing conducted to study fish diversity in these lakes (see Vispo etal, Chap. 8, this volume, for greater detail on the methodology and Lasso et al., Chap. 7, this volume, for information on diversity). The fish were captured during four field trips between February of 1998 and August of 1999. The fish studied here were caught primarily wi th gilí nets of four different mesh sizes (1.7, 2.5, 3.5 and 5 cm between knots). During each field trip, the nets were in the water for 18 hours at each site. They were set at approxi­mately 17:00 hrs of the first day and removed at about 11:00 hrs of the second

N . GoN/.Ai.r./. A N D C.R. V I S P O

tl.iy- Ni i ihecks normally occurred at 22:00 hrs, 06:00 hrs and 11:00 hrs. After weig lúng and mcasuring, the digestive tracts of larger fish were removed and fixed in I 0% lormalin. It was not possible to collect the stomachs from all fish because ol limited manpower. A t the time of capture, fat abundance in the body cavity was evaluated on a scale of 1 (little or no fat) to 3 (fat surrounding much of the gut). Reproductive status was evaluated according to Nikolsky's (1963) categories of gonadal development. Because gilí nets were the primary capture method for the fish whose diets are described here, no information is presented on the smallest species found in these lakes (e.g., the smaller characids and cichlids; see Lasso et al. Chap. 7, this volume for a list that includes the smaller species). In the laboratory, each stomach was separated from the rest of the viscera, and the stomach fullness was estimated based on the perceived grade of stomach distention on a scale of 0 to 100, with 100 corresponding to 100% fullness and 0 to an empty stomach (Goulding 1980, Goulding etal. 1988).

The stomach contents were removed and placed in a petri dish. Total volume was then measured using water displacement, and the contents were inspected under a dissecting microscope and a binocular compound microscope. The dif-ferent items were identified to the most lowest taxa possible. The dietary results are presented in terms of frequency of appearance (Hyslop 1980, Prejs and Colomine 1981) and percent volume (Goulding 1980, Goulding etal. 1988, Marrero 1994).

We calculated all means based on the means from individual taxa as opposed to simply pooling all relevant data and averaging across individuáis regardless of taxa. In other words, means presented are not distorted by variation in the number of individuáis captured in each taxon.

Results and Discuss ion

GENERAL DIETS

A total of 1,339 stomachs from 39 species were included in our analyses. O f these, 409 were empty. In Table 9.1, we present the diet information according to frequency of appearance and percent volume. That table also presents taxa-specific data on the occurrence of empty stomachs. Both frequency and volume analysis resulted in similar conclusions in which fish, plant material and insects were the most important materials in the observed diets. We found a wide spectrum of food items and categorized them as follows (see Appendix 9.1 for a taxonomic listing of the food items identified):

Chap. 9: D I I . I S O I - F L O O D P L A I N L A K E F I S H 335

FISH: This was the most important food category in terms of total volume consumed, composing a mean of 35% of the diet of the individual species. Included in this category were bones, fins, scales and chunks of flesh from unidentifiable fish. This last item was the largest component of the "fish" cate­gory, although, whenever possible, fish prey were identified taxonomically.

PLANT MATERIAL: This was the second most important food category account-ing for a mean of 27% of diet volume in the individual species studied. Included here were the remains of leaves, stems, flowers, fruits and seeds, with the most important component being the remains of vegetative structures and seeds.

INSECTS: This food category accounted for an average of 13% of the total volume of consumed food. I t was very common in the stomachs of certain fish, principally fish of the genus Triportheus. This category was represented mainly by aquatic insects from an array of orders: Coleóptera, Diptera, Ephemeroptera, Hemiptera, Hymenoptera, Isoptera, Lepidoptera, Odonata, Orthoptera, Sypho-naptera and Trichoptera.

ARACHNUXS: Arachnids were represented mainly by terrestrial spiders of the order Araneae, although we also encountered an aquatic mite of the Hydraca-rina group in the stomach of a Triportheus angulatus.

OTHER INVERTEBRATES: This category included freshwater shrimp of the ge­nus Macrobrachium (Palaemonidae), small unidentified Brachiurid crabs, bivalves, gasteropods and the unidentified remains of other invertebrates.

UNIDENTIFIED ANIMAL MATERIAL: This category included all the animal mate­rial that we were unable to identify more precisely.

REMAINS OF TERRESTRIAL ANIMALS: Such remains were only found in the stom­achs of the piranha Pygocentrus cariba; they included bird remains (feet and feathers) and what appeared to be the remains of an unidentified reptile or amphibian.

DETRITUS: Herein we included all the finely-fragmented detritus of animal and/or plant origin.

ZOOPLANKTON: This resource was represented by the Cladocera (principally the family Daphnidae), Copepods (Cyclopoid group), Ostracods, and Branchio-pods. The first two groups were found primarily in the fish species Anodus orinocensis and Hypophthalmusspp., while the last two groups occurred mainly in Hemiodusgr. immaculatus. The majority of the species studied were piscivores followed by herbivores and

invertivores; only a few species were detritivores or zooplanktivores. Below, we describe the diets of the different trophic groups. We then discuss the diets of select species for which we had substantial data and for which comparisons were available in the literature.

N . GONZÁLEZ ANDC.R. VISPO

l'ablc 9. 1: l )icts or those species having five or more stomachs with at least some contents, presented according to % frequency of appearance (% FA) and % volume (%V).

labia 9. / ; / Vicia de ¿as especies con cinco o mas estómagos con alimento de acuerdo al análisis defrecuencia de aparición (%F.A) y porcentaje volumétrico (%V).

nonempty FISH PLANT INSECTS ARACHNIDS SPECIES stomachs MAT.

at. % %FA %V %FA %V %FA %V %FA %V

Acestrorhynchus spp. 20 25.3 100.0 100.0 A. microlepis 1 1 100.0 100.0 100.0 _ _ _ _ _ _ Anodus orinocensis 11 61.1 Argonectes longiceps 10 76.9 20.0 14.7 30.0 15.3 50.0 40.9 Boulengerella maculata 8 32.0 62.5 62.5 12.5 12.5 Brycon bicolor 1 1 84.6 18.2 14.7 90.9 72.4 27.3 3.0 18.2 9.9 Brycon sp. 8 88.9 37.5 34.0 75.0 53.5 12.5 12.5 Cichla temensis 14 51.8 85.7 85.7 7.1 7.1 Hassar iheringi 9 90.0 22.2 12.3 77.8 58.8 Hemiodusgr. immaculatus 15 83.3 53.3 48.0 46.7 42.0 Hoplias malabaricus 23 51.1 78.3 72.1 30.4 16.3 8.7 7.2 Hydrolicus spp. 1 14 50.2 93.9 ' 92.8 2.6 1.1 7.0 4.3 Hypophthalmus spp. 7 70.0 Laemolyta spp. 22 84.6 27.3 27.3 72.7 72.7 Leporinas fasciatus 11 78.6 9.1 5.6 54.5 35.2 54.5 34.3 Leporinus friderici 12 92.3 16.7 11.7 58.3 39.6 50.0 42.4 Loricariichthys brunneus 24 46.2 Myleus rubripinnis 6 100.0 83.3 83.3 16.7 16.7 Mylossoma duriventris 7 100.0 85.7 83.9 28.6 16.1 Pellona castelneana 8 88.9 87.5 77.9 12.5 12.5 62.5 9.6 Pellona flavipinnis 27 65.8 70.4 58.8 44.4 24.9 Piaractus brachypomus 16 100.0 31.3 5.1 87.5 80.1 25.0 13.3 Pimelodusgr. blochii 5 83.3 20.0 11.1 80.0 45.4 60.0 23.4 Plagioscion squamosissimus 26 68.4 84.6 76.4 23.1 9.6 30.8 14.0 Platydoras costatus 6 100.0 33.3 10.4 33.3 18.1 83.3 71.5 Potamorhina altamazonica 66 93.0 Pristobrycon spp. 6 100.0 66.7 45.5 66.7 35.6 66.7 18.8 Pristobrycon striolatus 6 60.0 66.7 53.3 50.0 28.1 16.7 2.0 Psectrogaster ciliata 16 100.0 Pygocentrus cariba 131 89.7 84.0 69.8 61.1 21.0 13.7 3.0 Rhapiodon vulpinus 30 58.8 90.0 89.7 6.7 3.6 Semaprochilodus kneri 22 64.7 S. laticeps 65 79.3 Serrasalmus altuvei 7 100.0 71.4 71.4 14.3 2.0 14.3 2.0 Serrasalmus elongatus 20 90.9 95.0 86.4 30.0 10.0 Serrasalmus rhombeus 44 89.8 90.9 83.8 11.4 2.5 31.8 13.5 Triportheus angulatus 22 91.7 4.6 0.1 36.4 25.6 77.3 46.5 4.6 4.6 Triportheus elongatus 34 94.4 2.8 0.9 50.0 41.4 69.4 55.6 11.1 2.1

(aiap. 9: On: i s oi ; F L O O D P L A I N L A K E F I S H 337

Table/Tabla9.1 (cont.)

üTHFk MülDENT REMAINS DETRITUS zoo-SPECIES INVERT. ANIM.MAT. TERR.ANIMS. PLANKTON

%FA %V %FA %V %FA %V %FA % v %FA %V

Triportheus sp. 30 62.5 40.0 33.7 76.7 65.3 3.3 1.0 Acestrorhynchus spp. _ _ _ _ — - -A. microlepis _ _ _ _ _ Anodus orinocensis _ _ 100.0 1 00.0 Argonectes longiceps _ 30.0 23.6 _ _ 10.0 5.5 _ -

Boulengerella maculata 12.5 12.5 12.5 12.5 _ _ _ _ _ Brycon bicolor _ _ _ _ _ - - -Brycon sp. _ _ _ _ _ - - -Cichla temensis 7.1 7.1 _ _ _ _ _ Hassar iheringi 33.3 28.9 _ _ _ _ _ _

Hemiodusgv. immaculatus 6.7 5.3 _ _ 20.0 4.7 Hoplias malabaricus 4.3 4.4 _ _ _ _ _ Hydrolicusspp. 2.6 0.2 1.8 1.5 _ _ _ _ _ —

Hypophthalmus spp. _ _ 28.6 28.6 _ _ 71.4 71.4 Laemolyta spp. _ _ _ _ _ — - - - -Leporin usfasciatus 9.1 9.1 18.2 15.9 _ _ _ _ _ _ Leporinus fiderici 8.3 6.2 _ _ _ _ _ Loricariichthys brunneus _ _ 100.0 94.3 33.3 5.7 Myleus rubripinnis _ _ _ _ _ _ _ Mylossoma duriventris _ _ _ _ _ Pellona castelneana _ _ _ _ _ Pellona flavipinnis 11.1 8.9 7.4 7.4 _ _ _ - -

Piaractus brachypomus 12.5 1.5 _ _ _ _ _ _ Pimelodusgr. blochii _ _ 20.0 20.0 _ _ — _ - -Plagioscion squamosissimus _ _ _ _ - - - - -Platydoras costatus _ _ _ _ _ — - - -Potamorhina altamazonica 4.5 4.6 _ 95.5 95.3 1.5 0.2 Pristobrycon spp. _ _ _ _ _ _ — -Pristobrycon striolatus _ 16.7 16.7 _ _ - -Psectrogaster ciliata _ _ _ _ _ _ 1 00.0 1 ()().() - -

Pygocentrus cariba 0.8 0.3 3.1 2.2 5.3 3.5 _ - -

Rhapiodon vulpinus _ 6.7 6.7 _ _ _ _ _ Semaprochilodus kneri _ _ _ _ _ 100.0 100.0 -S. laticeps _ _ _ _ _ 100.0 100.0 -Serrasalmus altuvei 28.6 24.5 _ _ _ Serrasalmus elongatus 5.0 3.6 _ _ _ _ _ Serrasalmus rhombeus 2.3 0.1 _ _ _

Triportheus angulatus _ _ 18.2 9.8 _ _ — 18.2 13.4 Triportheus elongatus _ _ _ _ _ — — — - -Triportheussp. - - - - - - - - -

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Fig. 9.1: Presas de peces por familia consumidospara nueve especies que se alimentan con mayor frecuencia depeces y que tenían más de cinco estómagos con alimento. La importancia de las familias se representa por el porcentaje de los individuos consumidospor un depredador dado.

CALUCH • CALLICHTHYIDAE, CICHL * CICHLIDAE, CLUPEI = CLUPEIDAE, CURIM = CUR1MATIDAE, CYNOD = CYNODONTIDAE, CHARAC = CHARACIDAE, DORAD = DORADIDAE, ENGRA • ENGRAUUDAE, ERYTH • ERYTHRINIDAE, HEMIOD = HEMIODONTIDAE, HYPOPH HVPOPHTHALMIDAE, LORICAR = LORICARIIDAE, PIMELOD = PIMELODIDAE, PROCHIL = PROCHI LODO NTIDAE

• lx)w Waicr = 391 aomachs • HighWaltr =88 stomachs

N. CiON/.ÁI IX AND C ].R. VlSI'O

i( juveniles). Novoa et al (1989) found that consumption of Cichlidae (Cichlasp. .iiul (¡cophagus sp.) was followed by that of Erythrinidae (Hoplias sp.) and Charac-ul.ic (Ilemigrammus sp. and Serrasalmus sp). Novoa (1993) and Gil etal. (1993) also reported cannibalism. Zaret (1977, 1980), Taphorn and Barbarino (1993), and Machado-Allison (1990) indicated that despite being voracious predators as adults, consumption of conspecific juveniles was avoided by the adult's recognition of species-specific color patterns (oceli) on the juveniles, although Winemiller (1990a) questioned the assumptions of this hypothesis. Goulding etal. (1988) found Cichl­idae (Cichlasoma, Crenicichla), Characidae (Acestrorhynchussp., Brycon sp., amongoth-ers) and Anostomidae (Leporinus sp.) to be the most important species in the diets of Amazonian Cichla. Rodríguez-Olarte and Taphorn (1997) reported on the pis-civory of Cichla species in the Aguaro River of the central Venezuelan Llanos. Wine­miller et al. (1997) reported that Hemigrammus analis was the most common species identified in C. temensis stomachs from the Cinaruco River; Crenicichla sp., Nannosto-mus and anostomids were also found. These authors reported observations of this species attacking Semaprochilodus kneri and Acestrorhynchus heterolepis. Jepsen et al. (1997) reported that during high water C. temensis in the Cinaruco consumed more catfish and juvenile Cichlasp. than duringlow water.

Hoplias malabaricus: Hoplias malabaricus preyed primarily on Curimatidae (Psec­trogaster sp. and Curimatella immaculata) followed by Cichlidae (Chaetobranchus sp.), Characidae (Moenkhausia collettt), Hemiodontidae (Hemiodussp.) and Hy-pophthalmidae (Hypophthalmus sp.). A juvenile H malabaricus was also found in one stomach. Goulding etal. (1988) reported consumption of the families Anos­tomidae (lleúdanos trimaculatus), Characidae (Moenkhausia lepidura and other un­identified species), Rivulidae (Rivulus sp.) and Hypopomidae (Hypopomus sp.). They also reported predation of congenerics. Oliveros and Rossi (1991) re­ported very similar diets, w i th Characidae (especially Astyanax sp.), Cichlidae (Crenicichlasp.), Prochilodontidae (Prochilodussp.) and unidentified Curimatids being the most important.

Hydrolicus spp.: Hydrolicus showed the highest diversity of prey species consumed, although this was probably due in part to the high number of Hydrolicus stomachs examined. The most important prey species included Prochilodontidae (Semaprochilodus spp.), Curimatidae (Psectrogaster ciliata and other unidentified species), Hemiodon­tidae (Hemiodus sp.) and unidentified doradids. Goulding (1980) mentioned the con­sumption of Curimatidae in H. pectoralis. Novoa etal. (1989) found Cichlidae (Cichla sp. and Geophagussp.) and erythrinids to predomínate in the diet of H. scomberoides. Lasso etal. (1989) reported Cichlidae (Cichlaspp.) and Sciaenidae (P. squamosissimus) to be the most common fish consumed.

( liap. 9: l )n I S O I l ; i < >< >l >i'i A I N 1 , A K I I-'ISII 34

Pellona castelneana and P. flavipinnis: These species were only captured during high water. Doradids were the most common prey for both species, and Curimatidae and Characidae were also found in the stomachs of both species. Goulding (1980) reported that P. castelneana from the Rio Machado ate mainly Boulengerella spp., t r i -chomycterids and small characids; he commented that he had seen this species hunting small fish near the water surface.

Plagioscion squamosissimus: P. squamosissimuswas predominantly piscivorous, but included some insect and plant remains in its diet. The fish diet of P. squamosissimus was composed primarily of unidentified doradids followed by Hemiodontidae (Hemiodussp.), Prochilodontidae (Semaprochilodussp.) and unidentified curimatids. In the Guri Reservoir, Lasso et al. (1989) and Novoa et al. (1989) found the highest predation to be on Characidae (principally Hemigrammus spp.), followed by Erythrinidae (Hopliassp.), Cichlidae (Cichlasp. and Geophagussp.) and uniden­tified Gymnotiformes. Goulding (1980) identified prey from the Characidae (Tri­portheus &ná Serrasalmus), Hemiodontidae (Anodussp.), Curimatidae (unindenti-fied), along wi th small pimelodids and loricariids. Lasso-Alcalá etal. (1998) found juvenile P. squamosissimus to be a common item along with Characidae (Roeboides dayi, Ctenobrycon spilurus), Erythrinidae (Hoplias malabaricus) and Cichlidae (Aequidens sp). In white-water (turbid) rivers, P. squamosissimusappears to include more fresh-water shrimp in its diet, while in nutrient-poor rivers, where food production is probably lower, this species appears to be more strictly piscivorous (Goulding et al. 1988). Araujo-Lima etal. (1995) described this species as one of the principal predators in the Paraná river.

Pristobrycon spp.: In our specimens of Pristobrycon spp., fish made up the major-ity of the diet although over 40% of diet volume was composed of plant matter (leaves, flowers, and seeds) and insects. Other authors have reported similar feeding habits for this fish (Machado-Allison and García 1986, Machado-Allison 1993 and Lasso 1996).

Rhaphiodon vulpinus: R. vulpinus ate. mainly Engraulidae (Anchoviellasp.), followed by Hemiodontidae (Hemiodussp. and unidentified) and Prochilodontidae (S kneri). Goulding (1980) reported curimatids, anostomids (Rhytiodussp.), hemiodonts (An­odus sp.) and small pimelodids.

Piranhas (Pygocentrus cariba ana Serrasalmus spp): Doradidae, Characidae, Prochil­odontidae, Engraulidae and Clupeidae were, in descending order of importance, the families identified in the diet of P. cariba. However, much of this flesh probably carne from fish trapped in our nets and so did not represent an "independent" sample of the diet; indeed, monofilament nylon from our nets was occasionally found in the stomachs.

I I ' N . ( ¡( )N/.AI I /. AND C !.R. VlSI'O

I 11rii- species in the genus Serrasalmus were included in this study: S. altuvei, S. rhombeus and S. elongatus. These were fundamental ly piscivo-IOUS fish, wi th a diet composed primari ly o f scales and pieces o f fin (with ihe exception o f S. altuvei who ate main ly small f ish). The next most common foods were plant matter and insects, w i th insects being the most impor tan t secondary food for S. rhombeus, and p lant matter the most important for S. elongatus. Lasso (1996) reported S. altuvei to be a carniv-orous species w i t h a high frequency o f fin consumption and S. elongatus to have more than a t h i r d o f its diet composed o f scales. According to Goulding (1980), the second most important item in the diet o f S. rhombeus, after fish f ins, was fru i ts and seeds, wh i l e S. elongatus was found to consume both fish scales and fins, w i th the first being the most important i t em a l though d i f fe rent i a l d i g e s t i ó n may have affected these results. Machado-All ison and G a r c í a (1986) found the stomachs o f S. rhombeus to contain principal ly fish fins and seeds. Addi t iona l works such as those o f Sazima (1984), Sazima and Zamprogno (1985), Machado-Allison (1993, 1994), Machado-Al l i son and G a r c í a (1986), N ico and Taphorn (1988), and Araujo-Lima et al. (1995) mentioned the importance o f fin or scale consumption for almost all species o f this genus. Goulding (1980) identi­fied fish f rom the genera Brycon, Triportheus, Mylossoma, Serrasalmus, Rhaphiodon, Leporinus, and Cichla along w i t h various curimatids in the diet o f S. rhombeus; he reported the genera Triportheus and Colossoma in the diet o f S. elongatus. Nico and Taphorn (1988), working w i th Pygocen­trus notatus ( = P. cariba) and S. rhombeus reported that most prey items were from the family Characidae. I n P. notatus, they found Aphyocharax erythrurus,C. spilurus, Gymnocorymbus thayeri, H. marginatus, Odon-tostilbe pulcher, Brachychalcinus orbicularis and R. dayi, fo l lowed by curimatids (Curimata metae and Curimatella sp.), lebiasinids (Pyrrhuli-na cf. lugubris), and cichlids (Mykrogeophagus ramirezi) among others. For S. rhombeus, they identif ied characids, mainly Astyanax and Charax, along w i th fish fins possibly from an Erithrynidae (Hoplias).

In sum, certain morphological and taxonomic characteristics seem to repeat among the prey species consumed by the piscivores we studied. The most com-monly consumed families were those o f rather generalized body form (Hemi­odontidae, Curimatidae, Prochilodontidae, and Characidae), and there was notable overlap between the predators in terms of genera and species consumed. None-theless, distinctions were also apparent. For example, doradids were important components in the diets of Pellona spp., P. squamosissimus, and P. cariba, whereas they

( Ihap . l ) : l >n i s o i ' h o o D l ' l . A l N LAKE FlSH 343

were absent from the diets o f Acestrorynchus spp., C. temensis, H. mala­baricus and R. vulpinus. Likewise, the high intake o f cichlids by C. temen-sis and of engraulids by R. vulpinus was marked. A review o f the literature does not , in most cases, support these particular species-specific tastes, and our generally small sample sizes may have resulted i n appreciable sampling effect.

The high number of piscivorous fish in our captures, both in terms of species and biomass (unpubl. data) may have been due in part to the relatively high suscep-tibility of these fish to capture. In addition, we did not investígate the diets of the smallest species (i.e., <15 cm) among whom there were probably fewer piscivores. l'urthermore, floodplain lakes are not closed trophic systems, and so a convention-al trophic pyramid should not be expected when their aquatic environments are considered in isolation as we do here.

T H E HERBIVORES Those species in which plant material was the most important single food cate­

gory and for which we had at least five non-empty stomachs were Brycon spp., Hemiodusgx. inmaculata, Laemolyta spp., Leporinus fasciatus, Myleus rubripinnis, Mylossoma duriventre, Piaractus brachypomus ana Pimelodusgv. blochii (Fig. 9.2). Plant material ac-counted for more than half of the diet in all these species except for Laemolyta sp. and H. gr. inmaculata. " Vegetative remains" accounted for the highest average mean volume in the diets of the individual species (68% of the observed plant material), followed by seeds (21%), flowers (8%) and fruits (2%). "Vegetative remains" in­cluded leaves, stems and roots from unidentified species. Flowers carne principally from the families Caesalpiniaceae (Campsiandrasp.) and Rubiaceae. Identified fruits were mainly from the Ebenaceae (Diospyros sp.) and Polygonaceae (Ruprechtia sp.); while seeds carne mainly from the families Euphorbiaceae (Alchornea discolor), Cae­salpiniaceae (Macrolobium sp.) and Rubiaceae. For Laemolyta spp., Hemiodus gr. inmac­ulata, Mylossoma duriventre ana Brycon sp. no id. , vegetative material was the most important plant component, while for Piaractus brachypomus and Pimelodusgx. blochii

seeds were most important. Brycon spp.: Brycon bicolor vías the only species in which flowers dominated the

plant portion of the diet. More than 70% of the diet volume was composed of plant remains (primarily flowers and leaves), followed by the remains of small fish and insects. Brycon sp. no id . showed lower consumption of plant matter as a whole, and greater importance of vegetative material. Similar diets have been re­ported for Brycon by Angermeier and Karr (1984), Machado-Allison (1993) and Lasso (1996). A somewhat different diet composition was reported by Goulding (1980) and Goulding et al. (1988), who stated that these species consumed prima-

I 1 1 N . CloN/.Ái.i;/. A N D C . R . V I S I

nlv h mis and secas (around 90% of the total food volume), although terrestrial invertebrates captured near the water surface and small fish were also present. While on a visit to one of the study lakes during falling water (but at the time of á l a t e , brief surge in water level), one of us (CV) observed individuáis of this genus jumping from the water to take bites from the new leaves of a Passiflora vine that was dan-gling cióse to the water surface; on the same trip, similar damage to overhanging young leaves was seen along other parts of the river.

Myleus rubripinnis and Mylossoma duriventre:T\\e diet of Myleus rubripinnis and Mylossoma duriventre was composed mainly of the remains of seeds, leaves and flowers, and the occasional insect remains. A similar diet was found by Goulding (1980). Machado-Allison (1993) and Lasso (1996), reported that these species mainly eat seeds.

Piaractus brachypomus: The juvenile individuáis of Piaractus brachypomus that we cap­tured in the lakes ate primarily tree seeds in addition to small quantities of fruits and leaves. Aquatic insects and small fish were also occasionally consumed. Machado-Alli­son (1993), Lasso (1996), Goulding (1980), and Junk etal. (1997) considered this fish to be herbivorous, consuming primarily seeds and fruits. Juveniles are apparently more insectivorous than adults (Lasso 1996, Knab-Vispo etal, Chap. 10, this volume).

Brycon Brycon sp. ¡xtemohlosp. Lepo rimú Myhxsoma Píame tus Mvleus Hemiodus gr. limehiius gr bicolor fasciatus durñentre bmchvpitmun tvbnpinnñ Inmaculata bhchii

Fig. 9.2: Diet composition according to % volume for those species consuming mainly a plant diet and with five or more non-empty stomachs. The first six species were captured during low water only, the last two during high water and M. rubripinnis was captured relatively commonly during both seasons.

Fig. 9.2: Composición de la dieta de los peces que tienen mayorpreferencia en el consumo del recurso vegetal con cinco o más estómagos llenos mediente el método porcentaje volumétrico. Las primeras cinco especies fueron capturadas durante aguas bajas, mientras que lasdos ultimas fueron capturadas durante aguas altas. Myleus rubripinnis fue la única especie encontrada con un número representativo de estómagos en ambas estaciones.

< li.ip. (): DiiTsoi Hi.ooniu.AiN L A K F . F I S H 345

Pimelodus blochii: Pimelodus blochii had a diet consisting primarily of seeds and iquatic insects. Castillo (1981), Castillo etal. (1988), Fernández and Kossowski (1997),

t ioulding (1980) and Lasso (1996) also reported this species (or very similar species) to be omnivorous, with a marked tendeney to eat animal matter, mainly aquatic in­sects.

REMAINING SPECIES: We found l i t t le published information on Laemolyta ipp.j Leporinus fasciatus, and Hemiodus immaculatus. I n the six stomachs oi Leporinus fasciatus that he inspected, Gould ing (1980) d id f ind plant material to be important, but observed seeds and fruits rather than vegeta­tive matter. From the Río Negro, Goulding et al. (1988) report H. immacu­latus to be a consumer o f autochthonous plants, especially fi lamentous algae. We could not distinguish the source o f the vegetative material we lound in the stomach o f this species.

Among the herbivores, it may be appropriate to distinguish between scavengers and vegetative specialist with P. brachypomus and P. blochii representing the former and Laemolyta spp., L . fasciatus, and H. immaculatus the latter, while the remaining species fall in between. A better idea of the likely source of the vegetative material (i.e., does it come from live vegetation growing in or extending into the water, or is it largely from fallen, dead matter?) is necessary before the ecological relevance of this differentiation can be adequately evaluated.

THE INVERTIVORES The fish species showing preference for invertebrates and having at least five

stomachs with contents were Argonectes longiceps, Hassar iheringi, Leporinus friderici, Platydoras costatus, Triportheus angulatus and T. elongatus (Fig. 3). In this section we consider primarily the diets of the insectivores, because no species specialized on other types of invertebrates. We did find mollusks in the diets of H. iheringi and P. brachypomus, freshwater shrimp in the diets of Boulangerella maculata and Pellona fla­vipinnis, and arachnids in the diets of Brycon spp. and Triportheus elongatus..

Insects were represented by the following 11 orders, listed alphabetically: COLEÓPTERA (BEETLES): Both aquatic and terrestrial Coleóptera were found,

although aquatic individuáis dominated, primarily from the families Dytiscidae, Hydrophilidae and Elmidae. The most important terrestrial families were Gy-rinidae, Scarabidae, Chrysomelidae and Curculionidae.

DÍPTERA (FLIES): Díptera were the second most important insect order, and were represented mainly by larvae from the Chironomidae (Subfamilies Tanipo-dinae and Chironominae) and Ceratopogonidae; larvae from the Culicidae were also found occasionally.

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Fig. 9.3: % Frequency of appearance (% F.A.) for insects consumed by the six species who most consumed insects and who had five or more non-empty stomachs.

Fig. 9.3: % defrecuencia de aparición (% FA) de insectos consumidos por seis especies con cinco o más estómagos llenos que se alimentan con mayor frecuencia de este ítem.

I ÍH N . ( ¡ O N / . Á I . I . X A N D C . R . V I S P C

inse< is which fell into the water. Almeida (1984) reported these species to be om-nivorous with a tendency towards herbivory. Goulding (1980), working in the Ma­dura River (Amazonas), described both T. elongatus ana 77 angulatus as, omnivores who consumed substantial amounts of fruit. In contrast, we found the stomachs of '/.' elongatus to contain no fruits and those of 77 angulatus to contain only 10% fruit by volume. Goulding commented that these species regularly consumed flowers and terrestrial invertebrates, food items which we also encountered. He suggested that 77 angulatus especially large pectoral muscles were an adaptation for its more insectivorous diet and that its many fine gilí rakers helped filter zooplankton.

Among the insects most commonly eaten by both these species were Ephemeroptera, Díptera, Hemiptera and Coleóptera , with Ephemeroptera con­sumption being highest for T. angulatus and Diptera, Hemiptera and Coleóptera being most common in the diets o f 77 elongatus. Terrestrial insects were represented by Hymenoptera, Coleóptera and Lepidoptera with Hymenoptera and Lepidoptera consumption being highest in 77 elongatus. Almeida (1984) described similar results, documenting that Coleóptera, Ephemeroptera and Hymenoptera were the most important terrestrial taxa, while the nymphs of the Ephemeroptera and adult Hemi­ptera were the most important aquatic taxa. Equally, Goulding (1980) found that the only insects consumed by these fish were terrestrial species. The most common were Coleóptera , Lepidoptera, Orthoptera and Hymenoptera during high water, and Ephemeroptera during low water. Nevertheless, in subsequent work along the Brazilian Rio Negro, he reported that both aquatic and terrestrial insects were con­sumed (Goulding etal. 1988).

THE REMAINING INSECTIVOROUS SPECIES: Hassar iheringi and Leporinus friderici con-sumed primarily Diptera larvae and, secondarily, Ephemeroptera nymphs. These similar diets were found even though H. iheringi was only captured during high water and L . friderici was captured primarily during low water. Similar diets have been reported by Lasso (1996), and Goulding etal. (1988) concurred in reporting the importance of aquatic invertebrates in the diets of Hassar spp. However, he did not find any insect consumption in L . friderici from the Rio Machado (Goulding 1980). Diptera and Ephemeroptera were also important in Platydoras costatus, while Trichoptera were uniquely important in this species. Surprisingly, Goulding et al. (1988) report only seed and scale consumption for this genus. Dipterids were most common in the Argonectes longiceps we studied, although Goulding etal. (1988) re­ported Ephemeroptera and not Diptera from the diet of this genus.

Generalizing, we can single out two feeding strategies amongst these insectivorous fish species. In the first, observed by us only in Triportheus spp., both aquatic and terrestrial insects occurred in the diet, indicating that, at times, the fish fed from the

1 hap, 1 >n rs< »i l ' i i K >iipi \ I N I,\ki Fisii 349

M. i surface. Body form is such as to allow rapid snatching of floating prey. A similar Feeding strategy probably occurred in several of the microcharacid genera (e.g., Asty-anax, Bryconops, Hemigrammus, Moenkohausia), but, although we captured these spe-. ics, we did not study their diets. The second generalized form of invertebrate feeding, • .1 >sei ved in Argonectes longiceps, Hassar iheringi, Leporinus friderici and Platydoras cos-:, mis, involved the almost exclusive consumption of aquatic invertebrates. However, \i i. ilion in body form and habits even among species sharing diets of the same insect i irders (e.g, Hassar ana Leporinus), suggests differentiation in feeding ecology that was not apparent with our basic analyses and relatively small sample sizes.

THE DETRITIVORES

The detritivorous fish were Loricariichthys brunneus, Potamorhina altamazonica, Psec-11 ogaster ciliata, Semaprochilodus kneri and Semaprochilodus laticeps. The alimentary system of diese species appears to be adapted to their detritus diet (Vispo pers. obs.), and it is thus not surprising that other authors have coincided with these findings (e.g., < loulding etal. 1988, Lowe-McConnell 1975, Lasso 1996).

Obviously, "detritus" is a gross category which lumps together such materials as grazed "aufwuchs", sieved sediments and filtered organic matter from the water column. According to stable isotope work by Martha Yossa (unpublished report), the majority of the organic matter in the detritus of the sediments of these lakes was derived from terrestrial plant material, but we have no additional information on this food item. A more complete biochemical and/or microscopic analysis would be necessary before further ecological conclusions could be made (e.g. see Yossa and Araujo-Lima 1998 for detailed work on detrivory in Amazonia).

THE ZOOPLANKTIVORES O f all the species we studied, only Anodus orinocensis and Hypophthalmus spp.

were found to be zooplankton specialists. These two species consumed princi­pally planktonic crustaceans (i.e., Cladocera and Copepoda), wi th the cladocerans being more common than the copepods. For Hypophthalmus species, this compo-sition was supported by Carvalho (1980) in a study of H. edentatus. This author suggested that the relatively slow movement of the cladocerans, in comparison to the copepods, made them especially easy prey. Relative abundance may have also affected the diet of zooplanktivores in the Caura where Saunders and Lewis (1988) reported a low copepod abundance. Lowe-McConnell (1975), Castillo et al. (1988), Araujo-Lima etal. (1995) and Lasso (1996) all reported zooplanktivo-ry in Hypophthalmus, and several stressed the importance of cladocerans. Appre-ciable levéis of zooplankton were also found in the diets of Hemiodus gr. inmacu-latus, L . brunneus, and Triportheus angulatus.

I S O N . Ci( >N/.AI I / A N D C R . VlSI'O

SEASONAL VARIATION IN DIET

OI .ill the stomachs analyzed from the high-water period, a mean of 77% had at least some contents while during the dry season, this valué was 7 1 % . Despite this relative parity, mean stomach volume during high water averaged more than twice that found during low water. Figure 9.4 illustrates diet in relation to season. A l ­though the valúes presented represent the means of individual species diets and thus are not affected by the numbers o f i n d i v i d u á i s captured in each spe­cies, not all species were captured in both seasons. Thus this figure repre-sents our gross description o f seasonal changes in the overall diet o f the fish communi ty captured dur ing each season and not a direct description of diet change within species (see below for such data). Fish and plant material dominated in both high and low water, followed by insects and detritus. The fre­quency and relative volume of plant material, insects, other invertebrates and uni­dentified animal material increased during the high water period, the last two cate-gories also showed increases in absolute volume. Only fish and detritus increased in relative (and absolute) importance during the dry season. Zooplankton consump­tion appeared to be basically constant between the two seasons. Terrestrial animal matter was not included in the analysis because so little was consumed.

•

HIGH WATER LOW WATER HIGH WATER LOW WATER FREQU1 Ni ^ o l \PPEARANCE % V O L U M E

L E G E N D

• FISH • PLANT M A T T E R • INSECTS

O O T H E R INVERTS. • Z O O P L A N K T O N • D E T R I T U S

• UNID. ANIM. REMAINS

Fig. 9.4: Variation in diet during the hydrological seasons. Fig. 9.4: Variación de los componentes alimenticios durante las dos estaciones hidrológicas según los

métodos de frecuencia de aparición y % volumétrico.

Hydrolicus sp.

No of itoojftctu witb foo« Plagioscion squamosissimus *•««,-12

U w W « . r - U

1-ígh Water Low Water High Water Low Water Hgh Water Low Water Hgh Water Low Water

High Water Low Water High Water Low Water

Pygocentrus cariba ««bw.

High Water Low Water High Water Low Water

Triportheus angulatus

High Water Low Water Hgh Water Low Water

% FREO APPEAR % V OLLME

High Water Low Water Hgh Water Low Water

% FRBQ- APPEAR % V C U * E

LEGEND:

|H Fish ü hseets H Mal anmri unldent

• Plant Material • Other inverts. §¡ Terrest ankn mat

Detritus

Zooplankton

Fig. 9.5: % Frequency of appearance and % volume during the high and low water periods for six species with five or more stomachs with contents from each period.

Fig. 9.5: % de frecuencia de aparición y % volumétrico durante los períodos de aguas altas y aguas bajas para las seis especies con más de cinco estómagos con alimento.

352 N . G O N Z Á L E Z A N D C R . V I S P O

I igure 9.5 summarizes our seasonal comparisons among those species having live or more stomachs with contents from both hydrological periods. In Hydrolicus spp., Plagioscion squamosissimus, Pygocentrus cariba and Rhaphiodon vulpinus, "fish" was the most important item in both seasons, this was followed by insects, plant material, and unidentified animal material. In Hydrolicus spp. and R. vulpinus, "insects" and plant material were relatively more important in the high water period, accounting for nearly 20 and 30% o f the diet respectively. In P squamosissimus, in terms o f frequency o f appearance, the consumpt ion o f plant resources increased dur ing high water, and that o f insects stayed nearly constant, although according to relative and absolute volumes, plant material and insects were only important dur ing low water. P. cariba was the species that showed the most diverse diet ( in terms o f the food categories we identified) dur ing high water.

Many of the primarily piscivorous species, such as Pristobrycon, Pygocentrus, Ser­rasalmus, Pellona, Hoplias, Plagioscion ana Cichla, complimented their diets with leaves, seeds, fruits and insects during high water. This may have been due in part to the reduced densities of prey fish. A similar feeding pattern was reported by Machado-Allison (1993). During the same season, there was an increased consumption of doradids, curmatids and engraulids. Fish from the families Callichthyidae and Cyn-odontidae were only consumed during high water but even then in relatively low amounts. During low water, the Characidae were the most important prey family

HIGH WATER: 54 stomadis LOW WATER: 214 stomachs

Plant Material Rowets Fruits Seeds

Fig. 9.6: Mean % volume of plant material consumed by fish during the different hydrological seasons.

Fig. 9.6: % promedio en volumen del material vegetal consumido por los peces durante las dos estaciones hidrológicas

hap.l): 1 ) ! !• r s o i ' F I . O O D I M . A I N I . A K I . F I S H 353

• Torounal Instas

Fig. 9.7: % Frequency of appearance for insects consumed during the hydrological season. Fig. 9.7: % de frecuencia de aparición de insectos consumidos por los peces durante las estaciones hidrológicas.

followed by Hemiodontidae and Prochilodontidae (Fig. 9.1). These apparent sea­sonal differences in specific diets were no doubt partially due to the fact that iden-tifiable material from certain piscivorous species (i.e., Acestrorhynchus spp., C. temensis, P. castelneana and P. flavipinnis) vías only available from one season.

Among the herbivores taken as a whole, a relative increase in fruits and seeds consumed was observed during high water, while during low water, there was both a relative and absolute increase in the volume of vegetative remains and flow­ers consumed (Fig. 9.6). Again, as with Fig. 9.4, certain of the fish species studied during low water were not present during high water, and so these differences were due not only to changes in plant consumption but also to interspecific diet differ­ences. Unfortunately, our sample sizes did not permit us to conduct species-specific seasonal comparisons for the herbivores and thereby present more precise informa­tion. Goulding (1980) and Goulding etal. (1988) indicated that certain primarily herbivorous species (e.g, Brycon, Myleus, Mylossoma, Piaractus), who feed mainly on fruits and seeds during high water, eat considerable quantities of leaves during low water. We found similar results in terms of volume of food consumed during the dry

154 N . ( .((N/.AI I /. A N D C ].R. VlSI'C)

season h\e species; in addition, we also noted an increase in the percentage of empi \.

For the insectivores as a whole, Figure 9.7 shows the seasonal variation in the i onsumption of aquatic and terrestrial Arthropoda. Aquatic insects dominated during both seasons, but terrestrial insects were notably less important during low water. During low water, the highest consumption was of Ephemeroptera, followed by arachnids, Hymenoptera and Isoptera. During high water there was an elevated consumption o f Co l eóp te r a and Odonata, both aquatic and terrestrial, fol­lowed by Lepidoptera and Hemiptera . The consumption o f Diptera was more or less constant dur ing the two seasons. The consumption o f T r i -choptera and Hydracarina was only observed dur ing low water. Triporth­eus angulatus was the only pr imar i l y insectivorous fish for w h o m suffi-cient stomachs were available from both season so as to permit stronger, species-specific comparisons. This species had an increased consumption of plant material during high water and of insects (primarily larvae of aquatic Ephemeroptera) dur ing low water (Fig. 9.5). Similar results are reported from Lago de Castanho in the Amazon by Almeida (1984) who proposed that the high consumption of Ephemeroptera was due largely to the fact that the sub-merged tree trunks, where many of these insects lived during high water, began to dry out. The fleeing Ephemeroptera then became easy prey. A similar possibility was also proposed by Ligia Blanco for the Caura itself (pers. comm.).

Many of the fish species studied showed seasonal variation in diet. Lowe-Mc­Connell (1975), Goulding (1980), Goulding etal. (1988), Machado-Allison (1990, 1993), Lasso (1996) and Junk etal. (1997) amongst others, all singled out the high water period as a time of an enormous increase in food quantity and quality, and, as a result, increased feeding by many species. Our data similarly show that peak stomach volume generally occurred during this period {Le., in our June and August samples, Fig. 9.8). For almost all the fish species we compared, the high water diet1

was also the more diverse despite the fact that fewer stomachs were analyzed during this period. In a detailed study of P. brachypomus diets in the lower Caura, Knab-Vispo et al. (Chap. 10, this volume) found botanical diversity of the diet to be highest during the flood season.

In trying to interpret the role of diet differences in the natural history of these fish species, it is useful to look at the seasonal changes in other aspeets of the fish. Figure 9.8 also illustrates the seasonal distribution of fat deposition and reproduc-tion. Although samples were sometimes few and for certain species in certain months we had no data, a general pattern is readily evident: most but not all of the fish we studied entered into reproduction during the end of the dry season (March) or the

i ¡hap. l>: 1 )IE rsoF FLOODPLAIN LAKE FISH 355

ndicates the month in which the highest fat índices were remoled for the given species

indicares the month in whidl the highest % of reproducing fish were recorded.

lindieates month with peak volume f diet contents

X |indicates months from which no data were available for the given species

Fig. 9.8: Peak months for fat, reproduction and stomach fullness for those species for which we had at least some data. The lower Caura usually peaked around August and was lowest around March.

Fig. 9.8: Calendario indicando los meses durante los cuales observamos los picos de deposición de grasa, reproducción y llenura de estómago en las especies de la des tuvimos algo de información. En el bajo Caura, las aguas más altas normalmente se•encoinraron alrededor de agosto, mientras que las aguas más bajas se encontraron alrededor de marzo.

156 N. ( ÍONZAI I / AND ( !.R. Visi't >

stan ol the wct season (June), whereas peak fat deposition lagged behind, oceurring during peak water level (August) or during falling water (November). A very similar pattern was noted in the more detailed study of P. brachypomus in the same drainage (Daza and Vispo, Chap. 11, this volume; Knab-Vispo et ai, Chap. 10, this vol­ume). Although other factors may be involved (e.g., predation levéis, dispersal mech-anisms, diet for juveniles), i t is probable that in most cases reproduction coincides with the beginning of the period of nutritional abundance. This contention is sup-ported by the pattern of stomach volume in the species studied, with high water representing the period of greatest food abundance. Nonetheless, certain species, e.g, those which exploit the rocky rapids or the pools that appear during low water, may encounter a reverse pattern of nutritional abundance. Low water is likely the period when vegetation or "aufivuchs" on the rocks is most abundant, because low water levéis and increased water clarity permit greater photosynthesis by substrate-bound primary producers. Our informal observation suggest that species such as Mylesinus schomburgki and certain rock-loving loricariids may show an inverse pat­tern of fattening, reproduction and dietary abundance.

Winemiller (1989), studying the fish of the Llanos and of the Andean foothills of Venezuela, proposed the existence of at least three life history strategies among South American fish in seasonal environments: seasonal (life histories synchronized closely with the hydrological cycle), equilibrium (a more steady-state pattern of re­production and body condition) and opportunistic (life history adapted to taking advantage of unpredictable resource abundance). It appears that the majority of the species that we studied were seasonal breeders, although a more extensive and de­tailed study would be necessary to quantify participation for the entire fish commu-nity. Winemiller (1990b) went on to describe seasonal changes in diet. He noted the wet season importance of aquatic plants at his white-water Llanos site. We believe that our contrasting conclusions are due to the fact that we studied a forested, low-nutrient water system. In the Llanos, Winemiller (1990b) reported that a rich flora of aquatic plants grew up on the sunny floodplains during highwater; at our sites, poor soils, shaded banks and large, abundant fruiting trees (see Knab-Vispo et al. Chap. 10, this volume) may have meant that the majority of nutrients encountered by fish during the rainy season were formed outside of the water. Indeed, Winemil l­er (1990b) found an increased importance of allochthonous production in the sandy and rocky forest stream that he studied in the Andean foothills. The natural trophic ecology of black- and clear-water environments is probably especially dependent on natural riparian vegetation.

SPATIAL VARIATION IN DIET The capture sites were divided into the Mato, Sipao and Caura lake groups

Chap.9: D I I I S Q I F Í Q Q I M ' I . A I N F A K I . F ' I M

357

I .ihlc 9.2: Comparison oí diets among Caura (C), Mato (M) and Sipao (S) lake groups using Irequency ol appearance and % volume (in parentheses). — : the given species was not captured in the given lake group.

labia 9.2: Resultados de los análisis de las dietas de los peces capturados más frecuentemente en los diferentes sitios de muestreo: Caura (C), Mato (M)y Sipao (S), utilizando los métodos de frecuencia de aparición (%)y porcentaje volumétrico (mireparéntesis). —: especies no capturadas en un determinado grupo de lagunas.

nr. of nr. of SPECIES n

ni.

on-e mpty empty FISH F LANT MAT. INSECTS

s rnmachs stomachs

c : N 1 S c M 5 C M S c M S C VI s

11 cstrorhynchus spp. 5 i 3 2 20 29 1 0 100 100 100 0 0 0 0 0 0 (100)(100)(100)

I Whla temensis i r 7 3 1 0 100 _ 71 0 - 14 0 _ 0 (100) (71) ( 14)

11 oplias malabaricus 7 13 0 15 7 33 71 92 33 14 39 33 14 0 (33) (62) (87) ( 33) (14) (13) [33) 11UJ

1 lya'rolicus spp. 6 i i 1 42 33 5 ' 5 98 100 86 2 0 5 5 u tz (97)(100)(86) (0.04) (3) 1.8) (9)

/ aemolyta spp. ^ i 4 7 3 0 l 46 0 14 55 100 86 0 0 0 (46) (14) ( 55)(100)(86)

I .oricariichthys brunneus 8 8 8 5 17 6 0 0 0 0 0 0 0 0 0

1'cllona flavipinnis 0 3 14 10 1 3 60 100 71 0 0 0 10 33 71

(60) (86) (53) (U.3)U-U^o;

Piaractus brachypomus 6 10 0 0 33 _ 30 67 - 100 50 _ 1 U

í C\ \ (4) (5) [56) (94) (35) (0.1)

1 'otamorhirut 5 2 29 0 4 1 0 0 0 0 0 0 0 0 0

altamazonica Pygocentrus cariba S2 28 41 9 2 4 86 86 81 69 36 66 19 14 5

(70) (75) (66) (25) (12) (21) (3) (6) (0)

Rhaphiodon vulpinus 20 2 8 6 0 15 85 100 100 0 0 0 10 0 0 (85)(100)(100) (5.4)

Semaprochilodus kneri 1 1 4 7 6 4 2 0 0 0 0 0 0 0 0 0

Semaprochilodus laticeps 39 5 21 10 2 5 0 0 0 0 0 0 0 0 0

Serrasalmus elongatus 9 1 10 1 1 0 89 100 100 22 0 40 • 0 0 0

(81)(100)(90) ( ID (10)

Serrasalmus rhombeus 25 18 1 3 2 0 84 100 100 20 0 0 48 11 0 (74) (96)000) (4.5 ) (21) (4)

1 riportheus spp. 20 1 9 5 1 12 0 0 0 40 0 44 80 100 67 (31 > (43)

/ riportheus angulatus 10 6 6 l 0 1 10 0 0 50 17 33 90 (0.3) '(32) (12) (29) (5b) w

158 N . (i()N/.AI.I-:/ANI)C .R. VlSI'C)

l.ihlc / Hiblti 9.2 (cont.)

cppriPQ T ? 7 H E R UNIDENTIF REMAINS DETRITUS ZOO SPECIES INVTT<T^ A N I M ^ A T ^ "JTi^^ANIMS. PLANKTON

C M S_ c M s i . 1

c ü r v K

: jvi

• A l \

5 c A C

PLAT \[KTC >N

Acestrorhynchus spp. 0 0 0 0 0 0 0 0 0 0

¡s,

0

i s

0

C

0

M

0

S

0

Cichla temensis 0 _ 0 0 14 0 0 0 0 0 A

IT,, i. 1' _ / / „ (14) — u

Hoplias malabaricus 0 0 0 0 14 0 0 0 0 0 0 0 0 n A

(14, u u

nyaro licus spp. 5 0 0 2 0 2 0 0 0 0 0 0 0 0 A

(0.4) (1) (2) u

Laemolyta spp. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Loricariichthys brunneus 0 0 0 0 0 0 0 0 0 100 10( ) 100 50 50 n 0 // (91) (92 ) (100) (91 (8) / ellonaflavipinnis 20 33 0 20 0 0 0 0 0 0 0 0 0

\ ° /

0 A

(20)(13) (20) u

¿ luracius orachypomus 33 - 0 0 0 0 0 0 0 0 A

(4) — U

Potamorhina 0 0 0 60 0 0 0 0 0 40 inn 1 nn n •x altamazonica (60) (40) Í99.7)íl nm u

(0 3) j 0

Pygocentrus cariba 0 4 0 3 0 5 0 7 7 0 0 0 0 0 n ( 2) (1) (5) (6) (7) u

Rhaphiodon vulpinus 0 0 0 10 0 0 0 0 0 0 0 0 0 n A

(10) \ u

Semaprochilodus kneri 0 0 0 0 0 0 0 0 0 100 100 inn 0 n n

(100) (100) finm yt u

Semaprochilodus laticeps 0 D 0 0 0 0 0 0 0 100 ion inn 0 n A

( ioo)(ioo)nno) w u

Serrasalmus elongatus 0 1 ) 0 11 0 0 0 0 0 0 0 0 0 n A

(8) u U

Serrasalmus rhombeus 4 ( ) 0 0 0 0 0 0 0 0 0 0 n A

(0.2) u U 0

1 riportheus spp. Q ^ 0 0 0 0 0 0 0 0 0 0 0 0 0

Triportheus angulatus 0 0 0 30 0 7 0 0 0 0 0 0 0 A

(12) 0 7) ( 0 /

49) U

(see Study Site). Comparisons were made uñthin species, and only those species havmg five o r more sromachs from two or more o f these three sites were in cluded in the analysis.

( ¡ hap . 9: Dii.Ts O F F L O O D P L A I N L A K F , F I S H 359

As seen in Table 9.2, the Caura lakes had the highest levéis of invertebrate con­sumption while the lakes of the remaining two áreas showed lower and roughly cquivalent valúes. Plant material was, on average, two times more common in the diets o f Caura and Sipao fish than in the diets o f Mato fish. O n the other liand, fish from Mato lakes showed a more frequent consumption of zoop­lankton.

While diet within species may have varied little geographically, initial analyses indicated that the relative trophic composition of the fish communities associated with each lake group may have differed somewhat with piscivores being relatively rare in Mato, and detritivores relatively common therein (Vispo and González, unpubl. data).

I t would be logical to suppose that these geographic patterns are associated with patterns in food abundance but any conclusions about the ecological interac-tions implied either by the within species diet differences or the differences in the geographic occurrence of the trophic groups, wi l l need to await more information on the relative densities of the various food items across our study sites.

Conclus ions

The importance of the floodplain and its forests was evident in our data: plant material, almost exclusively terrestrial, was the second most important food class for the species we studied; terrestrial insects, while less important than aquatic ones, nonetheless figured prominently in some species' diets; feeding, fattening and re­production seemed to be keyed to the flood cycle wi th the high water period gener-ally being the most nutritionally beneficial; and, finally, initial isotopic evidence from the Caura suggested that the remains of terrestrial vegetation is a key compo­nent of at least certain detritus (Yossa unpubl. report). While piscivory was the most important trophic habit in the fish we captured, many of their prey species probably derived some of their nutrients from allochthonous matter {e.g., the importance of terrestrial insects for the microcharacids). In sum, it is apparent that an important energy input to the lower Caura's aquatic ecosystem comes via the floodplain and in the form of allochthonous organic material which is used by many species of fish. This abundance of nutrients in the floodplain not only increases the availability of new food resources, but also the possibility for certain fish groups to develop dietary specialization. Large scale alterations to the drainage (e.g, deforestation, contami-nation, or water regulation) that changed the structure and composition of the r i -parian forest or the natural hydrological fluxes would bring serious impacts to many fish species (see also, e.g., Goulding 1980, Goulding et al. 1996, Waldhoff et al.

UA) N . C i O N X Á I . K Z A N I ) G R . Vl.M'O

I 996, funk 1997). This is perhaps especially true when, as is the case for the Caura, (he rivers waters are nutrient poor. Similarly, Knab-Vispo et al. (Chap 2, this vol­ume) documented the importance of the Caura's riparian vegetation for many wild-life species; and Knab-Vispo etal. (Chap. 10, this volume) detailed its importance for the Caura's most important commercial fish species P brachypomus. Sustainable management in the Caura's faunal resources should give high priority to the preser-vation of riparian forests.

Acknowledgements

The authors wish to thank Ligia Blanco-Belmonte and Claudia Knab-Vispo for their help in identifying insect and plant material, respectively. Kaires Prieto, Alexis Rodríguez and José Mej ías were our will ing field assistants. This work formed part of the project "Estudio integral de la ecología acuática del bajo Río Caura'' which was co-financed by Fundación para el Desarrollo de la Ciencia y la Tecnología de Guayana ( F U N D A C I T E - Guayana), Fundación La Salle de Ciencias Naturales, and the Wildlife Conservation Society (WCS). Kirk Winemiller and students provided use-ful comments on the manuscript; remaining errors are our own.

L I T E R A T U R E C I T E D

Almeida, R. G. 1984. Biología alimentar de tres especies de Triportheus (Pisces: Characoidei, Characidae) do La go do Castanho, Amazonas. Acta Amazónica 14 (1/2): 48-76.

Angermeier, P. L. andj. R. Karr 1984. Fish communities along environmental gradients in a system of tropical streams. In: Evolutionary Ecology ofNeotropicalFreshwater Fishes (T. Zaret, ed.), pp. 39-57. W. Junk Publishers, The Hague, The Netherlands.

Araujo-Lima, C , A. Agostinho and N . Fabré 1995. Trophic aspects of fish communities in Brazilian river and reservoirs. In: Limnology in Brazil. (J.C Tundisi, C. Bicudo, andT. Matsumura, eds.), pp. 105-136. ABL/SBL, Rio de Janeiro.

Balbás, L. and D. Taphorn 1996. La fauna: Peces. In: Ecología de la cuenca del Río Caura, Venezuela. I. Caracteriza­ción general(J. Rosales and O. Huber, eds.). Scientia GuaianaeG: 76-79.

Cala, P 1995. Trophic levéis of the most abundant fishes of the Betania reservoir, upper Río Magdalena, Colombia. Acta Biol. Venez. 16 (1): 47-53.

Carvalho, M . F. 1980. Alimentacao do Mapará {Hypophthalmus edentatus Spix, 1829) do Lago do Castanho, Amazonas (Siluriformes, Hypophthalmidae). Acta Amazónica 10(3): 545-555.

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I Mi I S (II Fu '< il >l'l AlN I AKl FlSl I 3 6 5

M ' l ' l N I ) I X 9.1: Systematic list of items encountered in the stomachs of the fish studied. I i'ciidice 9. /: Lista sistemática de items encontrados en los estómagos de los peces estudiados.

k I N G D O M A N I M A L I A Phylum MOLLUSCA

( l.ssCASTROPODA Order Mesogastropoda Tam. Pilidae

( l.ivs mVALVIA ()rder Veneroida

Fam. Sphaeriidae

lum ARTHROPODA ¡ubphylum CRUSTACEA

( lass Branchiopoda División Eubranchiopoda División Oligobranchiopoda Oder Cladocera

Fam. Daphnidae Daphnia sp.

( lass Maxillopoda Subclass Ostracoda

Order Podocopa Subclass Copepoda

Order Cyclopoida Fam. Cyclopidae

(llass Malacostraca Order Decapoda

Fam.Palaemonidae Macrobrachium sp.

Suborder Brachyura

Subphylum CHELICERATA Class Arachnida

Order Araneae Order Acariformes

Suborder Hydracarina

Subphylum UNIRAMA Class Insecta

Order Ephemeroptera Fam. Polymitarcidae

Order Odonata Fam. Coenagrionidae Fam. Gomphidae Fam. Aeshnida Fam. Libellulidae

K I N G D O M A N I M A L I A Order Orthoptera Order Isoptera Order Hemiptera

Fam. Corixidae Fam. Notonectidae Fam. Pleidae Fam. Gerridae

Order Trichoptcra Fam. Hydropsychidae

Order Lepidoptera Order Coleóptera

Fam. Dytiscidae Fam. Hydrophilidae Fam. Elmidae Fam. Gyrinidae Fam. Scarabidae Fam. Chrysomelidae Fam. Curculionidae

Order Hymenoptera Fam. Formicidae

Order Diptera Fam. Chironomidae

Subfamily Tanipodinae Subfamily Chironominae

Fam. Ceratopogonidae Fam. Culicidae

Order Siphonaptera

Phylum CHORDATA Class Osteichthyes

Order Clupeiformes Fam. Clupeidae

Pellona sp. Fam. Engraulidae

Anchoviella sp. Order Characiformes

Fam. Anostomidae Laemolyta sp.

Fam. Characidae Acestrorynchus spp.

, Iguanodectes spilurus

N . G O N Z Á L E Z A N D C R . VI.SI-O

Appendix / Apéndice (cont.)

K I N D O M A N I M A L I A

Moenkhausia collettii Moenkhausia sp. Serrasalmus sp.

Fam. Curimatidae Curimatella immaculata Potamorhina altamazonica Psectrogaster ciliata Psectrogaster sp.

Fam. Cynodontidae Hydrolicus sp.

Fam. Erythrinidae Hoplias malabaricus

Fam. Hemiodontidae Hemiodus sp. Anodus orinocensis

Fam. Prochilodontidae Semaprochilodus spp.

Order Siluriformes Fam. Callichthyidae Fam. Doradidae

Platydoras costatus Fam. Pimelodidae

Hypophthalmus sp. Sorubim lima

Fam. Loricariidae Hypoptopoma sp.

Order Perciformes Fam. Cichlidae

Cichla spp. Chaetobranchusflavescens Crenicichla sp.

Fam. Sciaenidae Plagioscion sp.

K I N G D O M P L A N T A E

División SPERMATOPHYTA Class Angiospermae Subclass Dicotyledoneae

Fam. Caesalpiniaceae Campsiandra sp. Macrolobium sp.

Fam. Rubiaceae Fam. Ebenaceae

Diospyros sp. Fam. Polygonaceae

Ruprechtiasp. Fam. Euphorbiaceae

Alchornea discolor Fam. Lentibulariaceae

Utricularia sp.

ScitwriA (¡UMAN.-r 12: 3 6 7 - 3 9 1 . 2003. c k 11 a b (?" f acs t a f r. w i se. ed u

I H E D I E T O F MOROCOTO {Piaractus brachypomus) IN T H E L O W E R R I O CAURA I N R E L A T I O N T O ITS

E C O L O G I C A L R O L E A N D ITS C O N S E R V A T I O N

Claudia KNAB-VISPO, Félix DAZA, Conrad R. VISPO and Nirson G O N Z Á L E Z

Abstract

In this paper we present detailed observations on the diet of morocoto (Piaractus brachypomus) in the lower Caura River of south-central Venezuela and document the mterdependence between this economically important fish species and a diverse and lunctional floodplain forest. Our observations are based on the analysis of stomach and/or intestinal contents of 188 adultand 16 juvenile morocoto caught at42 different sites along 250 km of the lower Caura's main channel, 115 km of its tributaries and in (ive floodplain lakes throughout the low, rising, higí^ and falling water seasons of the years 1996-2000. While morocoto consume a wide rapge of items, and there is consid­erable variation throughout the year, among sites and among individuáis, allochtho­nous plant material always composed the largest proportion of morocoto stomach contents. Fruit and seeds became most important with rising water when large áreas of flooded forest and the fruits produced therein became accessible to morocoto. While juvenile morocoto consumed a larger proportion of aquatic insects throughout the sea­sons, they also consumed proportionally more fruit than the adults during the dry season. We were able to distinguish 120 morpho-species of fruit and seeds in the stomachs and/or intestines of adult morocoto. O f these, 77 species were found to be physically undamaged in at least some samples. This indicates a high potential for adult morocoto to serve as seed dispersers. In juveniles, we did not find any seeds that had not been masticated and therefore, juvenile morocoto seem to be poor seed dis­persers. Seed dispersal by adult morocoto includes the lateral transport of seeds from the shallow and stagnant waters inside the flooded forest to the deeper and faster-flowing waters of the main channel, where the seeds get defecated and then further dispersed by water current. In sum, our dietary information has demonstrated the importance of the floodplain forest to morocoto nutritional ecology in the Caura, and the probable importance, at least in terms of lateral seed dispersal, of the morocoto to the floodplain forest. The fate of the species is thus both dependent on and signifi-cant for the conservation of that forest.