In this paper we evaluate plasticity of fish concerning diet. We expect that sampling over a large temporal and spatial scale, including environmental changes such as impoundments, will allow us to cover most of the diet plasticity. We also evaluate the efficacy of ordination method in discriminating trophic groups based on fish species diet. Data were obtained from 17 sampling stations sampled monthly from March/96 to February/99 in the Corumbá river drainage, before and after the formation of the Corumba reservoir. Diet was determined analysing 9,177 stomach contents from 64 fish species. Trophic categories were discriminated by a non-hierarchic grouping analysis named k-means, applied to diet data. Most of the species presented great trophic plasticity, eating a great variety of food items. Resources availability, estimated from all fish stomach contents, was similar among environments, except in creeks, where it varied more with a large importance of terrestrial insects. K-means present satisfactory results, identifying nine trophic groups (detritivores, herbivore-piscivores, insectivore-herbivores, omnivores, invertivores, aquatic insectivores, piscivore-insectivores, piscivores and herbivores).Neste estudo avaliamos a plasticidade trófica em peixes. Nós esperamos que amostras obtidas com uma ampla escala temporal e espacial, incluindo mudanças ambientais como represamentos, nos permita cobrir a maior parte desta plasticidade. Foi avaliada também a eficiência do método de ordenação em discriminar os grupos tróficos baseado na dieta das espécies. As amostragens foram realizadas mensalmente de março/96 a fevereiro/99 em 17 estações de coleta na bacia do rio Corumbá antes e após a formação do reservatório de Corumbá. Foram analisados 9177 conteúdos estomacais, pertencentes a 64 espécies. As categorias tróficas foram discriminadas através de uma análise de agrupamento não hierárquica denominada K-means aplicada aos dados de dieta. A maioria das espécies apresentou elevada plasticidade trófica, consumindo uma grande variedade de itens alimentares. A disponibilidade dos recursos alimentares, estimada através de todos os conteúdos estomacais, foi similar entre os ambientes a exceção dos riachos, onde ocorreu maior heterogeneidade e os insetos terrestres tiveram grande importância. A análise K-means revelou resultados satisfatórios, identificando nove grupos tróficos (detritívoros, herbívoro-piscívoros, insetívoro-herbívoros, onívoros, invertívoros, insetívoros aquáticos, piscívoro-insetívoros, piscívoros e herbívoros).

The aim of this article is to evaluate whether alterations in flood pulses differentially affect diet composition, feeding niche breadth, and diet overlap of piscivorous fish. Species examined were Acestrorhynchus lacustris, Hoplias aff malabaricus, Plagioscion squamosissimus, Rhaphiodon vulpinus, and Salminus brasiliensis. These species were collected with gillnets (different mesh sizes) in the upper Paraná River floodplain, during four distinct flood events (four periods; A = 1992/ 1993; B = 2000; C = 2001; and D = 2002). The volumetric method was chosen to express diet results. Feeding niche breadth was calculated using Levins measure, and diet overlap was evaluated by the Pianka's Index. Flooding was more intense and lasted longer in the first period (1992/1993-A). Diet composition of the studied species was broad (47 total items consumed). For period A, Prochilodus lineatus was the main item taken by four out of five species. In the other periods, there were relevant alterations in diet, since P. lineatus was not recorded in any stomach of the five species; rather, it was replaced by the shrimp, Macrobrachium amazonicum. Diet overlap was low in all periods. The greatest overlap was obtained in period C for P. squamosissimus and R. vulpinus, due to high consumption of shrimps. There were no significant differences in niche breadth among species. However, the species presented distinct variation patterns in niche breadth. For example, H. aff. malabaricus showed a tendency toward increasing niche over the period, but the other species presented larger niches only during period A. Therefore, it can be concluded that the intensity and duration of the flood pulse influences: (i) the diet composition of piscivores; (ii) the breadth of their niches; and (iii) feeding overlap among species.

In order to investigate trophic interactions, the diets of peacock bass (Cichla kelberi) and dogfish (Galeocharax knerii) were studied in the Corumbá Reservoir between 1997 and 2000. This dietary study was performed to assess the niche breadth of each species and to determine the degree of niche overlap during different phases of reservoir colonization. During Period I, peacock bass were absent or recorded only in low numbers; during Periods II and III, peacock bass reached high abundances in the reservoir. Interactions between the species were weak during period I, but, during Periods II and III, they were found to interact intensively. The diet overlap was highest during Period II. The niche breadth fluctuated for both species in the different phases. Greater niche breadth was observed for dogfish during periods of low peacock abundance (i.e., Period I), and the lowest niche breadth value was observed during Period II. During the same period, the peacock bass exhibited a wide foraging niche. During Period III, the dogfish showed an increase of its niche breadth, while for the peacock bass a simultaneous decrease in the niche breadth, caused by increasing rates of cannibalism, was recorded. These results show that the presence of peacock bass induces changes in the diet of dogfish, probably due to a restricted number of prey items.

Major changes to local river habitats, after impoundment, are expected to restrict the reproductive activity of fishes with migratory strategies, although the changes are not expected to have relevant effects on fishes with other strategies. The reproductive activity of fishes with different reproductive strategies was evaluated along a longitudinal gradient before, and after, the formation of a reservoir on the Tocantins River (Tocantins State, Brazil). Samples were collected at six sites distributed along 80‐km stretch of the reservoir (R1 to R6 in the upstream–downstream direction) before and after the impoundment. The highest mean values of the reproductive activity index (RAI), considering all species in the river phase, were recorded for the intermediate segments (R4). The reproductive activity was classified as intense at the two sites located further upstream (R2 and R1). The highest mean RAI value after impoundment was observed for the site further upstream (R1). The reproductive activity was classified as intense, however, at every site, except for R6. Regardless of the reproductive strategy, mean RAI values were higher in this phase at site R1, particularly for long‐distance migratory species (LDM). The most lacustrine area (R6) was only relevant for the group of species with parental care. For non‐long‐distance migratory species, without parental care, the reproductive activity was intense at every site, except for R6. Overall, after the impoundment, the fish species tended to seek more lotic environments to spawn, including the riverine zone of the reservoir and side tributaries, regardless of the reproductive strategy. The results of this study provide evidence that impoundments affect the reproduction of the fish, independent of the reproductive strategy, although the intensity of the effect is different for each group.

Dams and associated reservoirs cause drastic changes in the composition and structure of ichthyofauna. Therefore, functional groups within a fish assemblage may respond distinctly to such human interventions in rivers. The objective of this study was to evaluate how fluctuations in fish abundance and biomass differ among trophic guilds. Fish were sampled before and after damming four reservoirs, in their inner areas (more heavily impacted areas) and upstream (less impacted areas). To evaluate possible changes, we used a before–after and control‐impact experimental design. Fish were classified into trophic guilds through diet analysis and based on information available from the literature. Six trophic guilds were identified. The effect of periods (before and after damming) and sites (more and less impacted) on abundance and biomass differed among the trophic guilds. Highly specialized trophic guilds, such as herbivores and detritivores, were negatively affected by damming. Invertivores were negatively affected in the impacted area and increased in abundance and biomass in the less impacted sites. The findings of this study show that modifications imposed by damming are reflected in the abundance and biomass of fish trophic guilds, but the degree of such impacts vary according to characteristics of the considered trophic guild.