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SynopsisThe feeding ecology of three characids ( A . fasciatus, A. bimaculatus and A. schubarti) was studied monthly during 1988 in Lake Inferngo, a major floodplain lake of the Mogi-Guaqu River in the State of SBo Paulo. River flooding directly influenced the diet of the omnivorous A. bimaculatus and A. fasciatus which responded to maximum inundation (March) by consuming predominantly allochthonous insects. In contrast, A. schubarti was less influenced by the river flood cycle on qualitative changes in diet and relied basically on aquatic macrophytes and periphytic algae. The importance of zooplankton in the diet of the three species was low, and may be attributed to its low density in the water column (< 1 ind 1.'). Ontogenetic diet changes were evident for the three species. For A. fasciatus and A. birnaculatus the importance of zooplankton was high at the early stages, decreasing with size. A. schubarti at younger stages consumed diversified items which gradually decreased quantitatively up to the size class of 65 mm; from this size onwards, the diet became restricted to the consumption of periphytic algae and macrophytes.

Resource partitioning and the seasonal patterns of food intake of four characids (Astyanax fasciatus, A . bimaculatus, A . schubarti and Cheirodon stenodon) were studied during 1988 in a floodplain lake of MogiGuacu River, Sao Paulo State . For interspecific comparisons, data of the previously studied Moenkhausia intermedia have also been used. A . fasciatus and A . bimaculatus can be considered omnivorous species, while A . schubarti and C. stenodon are predominantly herbivorous . The river flood cycle (following the wet season October-March and dry season April-September) seems to influence both seasonal food intake and food overlap between species . February-March and September-October are probably the months of highest food availability, and the months preceding these periods have the lowest resource amount . Our study shows that both omnivorous and herbivorous species modified the intensity of their interactions according to this cycle .Omnivorous species maintained fairly segregated diets when resources were presumably limited, and showed a rapid change from distinct food niches in the dry season to widely overlapping ones when resources were presumably more abundant . Conversely, the herbivorous species showed higher overlap during the dry season . Acute cyclic changes in the environment, as observed in this study, suggest the necessity of small interval sampling in food partitioning studies in tropical freshwater floodplains, in order to understand how species interact and cope with changes in food availability .

This study investigated the fish fauna of a stream in southeastern Brazil that runs through sugar cane-growing and urban areas that are subject to sewage discharge. It aimed to determine the importance of spatial and environmental factors on fish distributions and to assess the use of fish as indicators of the environmental conditions. A longitudinal pattern of chemical water quality was observed, with more disturbed conditions at the urban sites. Several opportunistic or tolerant and omnivorous species such as Astyanax altiparanae, Poecilia reticulata, and Astyanax fasciatus were numerically abundant at these urban sites, which resulted in a consistent increase in species density and biomass along the stream. Redundancy analysis extracted three axes, identifying a nutrient-productivity gradient associated with different species composition patterns, as also detected by the indicator species analysis. No species could be considered a potential indicator of the agricultural area in the dry season, whereas four different species were judged potential indicators of three urban sites. In order to explain species distribution, variation in species abundance was partitioned into spatial and environmental components. The latter was more important, explaining 25.4% of the variation. We concluded that the fish assemblage of the Guamium stream is undergoing the initial stages of disturbance, with pronounced effects at the urbanized area, but no detectable changes at the sugar cane sites. Therefore, fish assemblages might be used as ecological indicators to detect urban impacts in similar rivers. However, the potential use of fish to diagnose stressors in areas of sugar cane plantations should be complementary to more comprehensive studies aimed at understanding the influence of this crop on freshwater resources.

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This research characterized the feeding ecology of the fish community of the upper-middle course of Paraitinga River, located within the Upper Tietê River Basin, a peculiar Atlantic Forest area, regarded as a hotspot for fish conservation. Considering the several anthropogenic modifications, knowledge of the trophic structure might contribute to a better understanding of the factors that maintain the present fish community. Fish were collected with electrofishing equipment at 16 sites with different riparian vegetation, including native forest, secondary forest, pasture, and Eucalyptus, during the dry and rainy season of 2004/2005. Results obtained for 15 species indicated a predominance of insectivores and herbivore-detritivores along the course and an increase of total biomass, specifically of the herbivores-detritivores at the pasture sites, which seemed to be mediated by specific habitat features, which included open canopy, high water speed, and deeper areas. Strategies of resource use indicated that 47% of total combination pairs showed high overlap, but competition seemed to be minimized through low co-occurrence, spatial segregation, and abundance of food resources. Niche width was broad for all species, with no significant differences occurring among sites, seasons, and upper and lower course. With regard to the ongoing modifications in riparian zone conditions in this area, the implications of these findings for regional biodiversity conservation are discussed.

Diet of 364 specimens of the dourado (Salminus maxillosus) was studied monthly at Cachoeira das Emas from November/1996 to May/1998, considered the breeding ground of this and other migrant fishes of Mogi-Guaçu River. Only adults were found at this site, and diet consisted exclusively of fish, with predominance of Leporinus sp. Seasonal variation in feeding activity was highest during some months of the rainy and hot season (September-December), when fish composition and number of prey species consumed also changed. These modifications were primarily attributed to an increase in food availability at the site of study during the rainy season, once most of the migrant species, including small Characidae concentrate there at this time of year. Apparently diet shifts, prey availability and rainfall are intimately related, representing a combination of factors that may account for the observed patterns of resource exploitation.Key words: Salminus maxillosus, feeding, Mogi-Guaçu River, prey availability. A dieta de 364 exemplares do dourado (Salminus maxillosus) foi estudada mensalmente na região denominada Cachoeira das Emas de novembro/1996 a maio/1998, local considerado o lar de reprodução desta e de outras espécies migratórias do Rio Mogi-Guaçu. Foram capturados apenas adultos, sendo a dieta composta exclusivamente por peixes com predominância de Leporinus sp. Foi observada variação sazonal da atividade alimentar, que foi maior na estação quente e chuvosa (setembro a dezembro), época na qual a composição e o número de presas consumidas foram distintas. Essas mudanças foram basicamente atribuídas a um aumento da disponibilidade de presas no local de estudo durante o período chuvoso, visto que a maioria das espécies migratórias, incluindo pequenos Characidae, se concentra aí nessa época. Aparentemente, as mudanças na dieta de S. maxillosus, a disponibilidade de presas e a pluviosidade estão intimamente correlacionadas, representando uma combinação de fatores que explicam os padrões de exploração de recursos aqui observados. RESUMOPalavras-chave: Salminus maxillosus, alimentação, Rio Mogi-Guaçu, disponibilidade de presas.

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