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.

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This study focused on the distribution of fish with high reproductive activity along a basin, using a 430 km stretch of the Cuiabá River in Brazil as a model. The main objective of this study was to identify those fish that migrate long distances for reproduction, among all the basin species. Thus, a set of working criteria are proposed to classify species according to their reproductive behaviour (i.e., reproductive activity and distribution). Samplings were performed in the Cuiabá River basin, encompassing several environments (river, channels and lakes) during the reproductive periods (between October and February), from 2000 to 2004. Species occurrence (presence and absenceproxy of distribution) across the basin and index of reproductive activity values were used as criteria to identify the species that perform long-distance longitudinal migrations for reproduction. The study confirmed the classification of long-distance longitudinal migration species; nonetheless, some species were not classified as described in the literature. The proposed sequential criteria have proven to be effective in the classification of long-distance longitudinal migrations species and certainly contribute to filling some existing knowledge gaps of reproductive traits. This classification is of fundamental importance in planning new dam projects, in decision making and in the development of management and conservation actions for the ichthyofauna.