Trophic Structure of Fish Assemblages from Mamore´ River Floodplain Lakes

Ecology of Freshwater Fish 2004: 13: 245–257 Copyright Ó Blackwell Munksgaard 2004 Printed in Denmark Æ All rights reserved ECOLOGY OF FRESHWATER FISH Trophic structure of fish assemblages from Mamore´ River floodplain lakes (Bolivia) Pouilly M, Yunoki T, Rosales C, Torres L. Trophic structure of fish M. Pouilly1,2, T. Yunoki3, assemblages from Mamore´ River floodplain lakes (Bolivia). C. Rosales2, L. Torres3 Ecology of Freshwater Fish 2004: 13: 245–257. Ó Blackwell 1Institut de Recherche pour le De´veloppement Munksgaard, 2004 (IRD), France, 2Instituto de Ecologıá, Universidad Mayor de San Andres, La Paz, Bolivia, 3Univer- Abstract – The fish assemblage of the floodplain of the Mamore´ River sidad Tećnica del Beni, Trinidad, Bolivia (Bolivia) was estimated in eight lakes, corresponding to four habitat types, situated on an environmental gradient related to the river distance: lakes situated near the river, in the forested floodplain, at the floodplain edge and lakes isolated in the savanna. This paper documents the diet of 71 fish species (among the 140 recorded) and compares the taxonomic and trophic structure of fish assemblages between four lake types. The diet analysis was conducted to determine five trophic guilds: algivores/iliophages, herbivores, zooplanktivores, invertivores and piscivores. The taxonomic and trophic structures of the fish assemblages were not similar in the different lake types of the Mamore´ River. The trophic structure of assemblages showed a coarse pattern of dominance of algivores/iliophages Key words: Bolivia; river floodplain; tropical and invertivores, but different situations were observed in relative freshwater fishes; distribution pattern; diet abundance of the trophic groups in relation to the spatial position of the lakes (except for piscivores). Lakes close to the river appeared more Marc Pouilly, Institut de Recherche pour le De´veloppement (IRD), Universite´ Lyon 1, UMR favourable to the microphages (algivores/iliophages, zooplanktivores) CNRS 5023 Ecologie des Hydrosyste`mes although remote lakes appeared more favourable to the macrophages Fluviaux, 43, Bd du 11 Novembre 1918, Bat. (invertivores, herbivores). These results support the general idea that fish Forel, 69622 Villeurbanne Cedex, France; distribution follows a pattern linked to the ecology of the species, and e-mail: [email protected] related to environmental characteristics of the lakes. Accepted for publication April 13, 2004 Un resumen en espanõl se incluye detra´s del texto principal de este artıćulo. Until now little evidence exists about patterns of Introduction organization of neotropical fish assemblages in rela- The ecology of neotropical freshwater fishes has tion to environmental characteristics. Several studies, received increasing attention over the past 2 decades. based on taxonomic structure, lead to discordant However, the knowledge is still fragmented and results on the existence of a pattern and on the concentrated on a few major contributions coming parameters that control the fish distribution (Marlier from the central Amazon, Parana´ and more recently 1968; Bonetto et al. 1970; Cordiviola de Yuan 1980; from Orinoco. Little attention was paid to the Upper Rodrı´guez & Lewis 1997; Henderson & Crampton Amazon system. Because of the great ecological 1997). But it is likely that, even if taxonomic diversity and complexity of the structure and dynamics differences do not exist, ecological patterns of distri- of the Amazon and because of the current paucity of bution should exist. For example, Rodrı´guez & Lewis information, generalization of the species ecological (1997) demonstrated a pattern of piscivorous fish characteristics remains premature and needs to be distribution controlled by water transparency. reinforced. Researches that focus on new investigation This paper documents the diet of the most abundant areas should address basic ecology objectives, before fish species of the Mamore´ River floodplain lakes. The or at the same time as they look at more explicative or taxonomic and trophic structure of the fish assem- predictive goals. blages were determined in eight lakes corresponding 245 Pouilly et al. to four ecological situations ranging from lakes occurs at the end of the wet season (December to situated near the Mamore´ River to lakes isolated in April) and can last as long as 3 or 4 months with a the savanna. The objective was to test the hypothesis potential extension of ca. 150,000 km2 (Loubens et al. that taxonomic and trophic structures of fish assem- 1992). blages are influenced by the position of the lake in the The lakes studied in the present paper correspond to floodplain. four different ecological situations. Six corresponded to oxbow lakes and were situated in the forest gallery, at varying distances from the Mamore´ River. Materials and methods Mamore´ River Study area Two oxbow lakes situated near the Mamore´ River A 2-year fishing survey was undertaken in eight lakes (Tiuco and Verdun 1) were about 10 years old with a situated in the central Mamore´ River floodplain (near morphology similar to the river channel, and with a the city of Trinidad, latitude 14°30¢S–14°52¢S, longi- water depth varying from 17 m during high waters to tude 64°51¢W–65°01¢W; Fig. 1). The Mamore´ River 4 m during low waters. They were connected to the is one of the main tributaries of the Madeira, that Mamore´ River by the way of a short channel (<100 m). drains white waters (Sioli 1964) from the south Bolivian Andes. In the plain of Beni, the Mamore Forest River flows on a landscape dominated by savanna with Two oxbow lakes situated in the middle of the forested some patches of forest confined to the elevated part of floodplain (Siquero and Verdun 2) were more than the plain, and some forest galleries along the rivers. 20 years old with a water depth varying from 8 m Local climatic conditions are marked by the alterna- during high waters to 1 m during low waters. They tion of a wet (October to March) and a dry season were connected to the Mamore´ River by the way of a (April to September). A big annual flood generally small temporal tributary that drains the savanna and Fig. 1. Study area in the Mamore´ River floodplain, Bolivia. 246 Trophic structure of Bolivian fish assemblages the floodplain. The channel distance is over 1 km long from the Museo Nacional de Historia Natural of La and flowed through one or two other lakes before Paz, and at the Museum National d’Histoire Naturelle reaching the Mamore´ River. of Paris (Lauzanne & Loubens 1985; Lauzanne et al. 1991). Edge Two oxbow lakes situated at the forested floodplain Fish diet analysis and trophic classification edge (Potrero and Florida) were estimated to be more than 50 years old. The water depth varied between Estimation of diet was based on the analysis of 4 m during high waters and 0.5 m during low waters. stomach contents of fishes covering a range of sizes Florida was connected only by inundation although that we assumed to correspond to the adult stage. After Potrero was connected by the way of a short channel identification of the fish, stomachs were extracted by that converged with a small temporal tributary. They dissection. Empty stomachs or stomachs with almost were situated more of 4 km from the Mamore´ River. fully digested contents were excluded from further analysis. The contents of the remaining stomachs were Savanna examined under a microscope and items were separ- The last two lakes (Coitarama and Suarez) are ated into seven categories: soft substrate; algae or situated in the savanna adjacent to the annual periphyton; aquatic or terrestrial vegetation, fruits or floodplain. They were estimated to be more than seeds; zooplankton (cladocers, rotifers or copepods); 100 years old. The water depth varied from 1 to aquatic invertebrates; terrestrial invertebrates; fish. 2 m throughout the years. During regular hydrologic Invertebrate categories (terrestrial and aquatic) corres- cycles they were isolated all the year round, but ponded mainly to insect prey. The soft substrate connections are probable during strong hydrologic category did not correspond to a biological feeding cycles. resource. It is likely that fish that ingest soft substrate Lakes close to the river may be subjected to are in fact consuming periphyton and algae, which are whitewater invasion (Loubens et al. 1992; Iban˜ez aggregated in the substrate and/or other associated 2000), and are largely influenced by the variations of microorganisms. However, the soft substrate category water level. On the contrary, endogenous waters, local was retained as an indicator for a particular benthic rainwater with characteristics intermediate between feeding. white and blackwaters (Sioli 1964), supply lakes In order to obtain a general qualitative diet for the remotely situated from the river (Loubens et al. 1992; species, results were expressed by the occurrence Iban˜ez 2000). Savannah lakes are less influenced by method (Hyslop 1980). The relative importance of an water level variations (Pouilly et al. 1999). item in the diet was estimated by the number of stomachs that contained that item divided by the total number of nonempty stomachs analysed in the partic- Fish sampling ular species. This method is adapted to describe Fish were sampled using 13 gillnets with a wide range general food habits at an interspecific level and to of mesh sizes (25 m long · 2 m high, mesh size: 10, define broad trophic groups. 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90 and 110 mm). In order to show relative level of diet specialization Sampling was conducted during eight periods from of the species, diet breadth was estimated using 2 1 March 1998 to March 2000. For each sampling, Levin’s standardized index Bi j Pij À 1 1 ¼ ½ð Þ À gillnets were left in place for 2 h in the evening n 1 À , where Pij is the proportion of the food (17.00–19.00 hours) and for 2 h in the morning categoryð À Þ j in the diet of species i, and nPis the number (05.00–07.00 hours) for gillnets of small mesh size of categories (Hurlbert 1978).

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