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We investigated whether isotopic niche partitioning could mediate the coexistence between native (Hoplias sp. B) and non‐native (Hoplias mbigua) trahira in a Neotropical floodplain. We hypothesised that during the dry season both species inhabit isolated sites (i.e. lakes) and thus rely on distinct food resources to sustain their coexistence. We found evidence of trophic segregation among native and non‐native freshwater fish in the Upper Parana River, Brazil. Even though both species rely on the same primary littoral carbon sources and present similar trophic diversity, trophic segregation was attained through larger niche amplitude and smaller isotopic niche width for the non‐native fish. Three decades after initial invasion, non‐native Hoplias have distinct foraging behaviour and do not compete for trophic resources with native trahira; this is likely due to morpho‐anatomical differences. Limited morphological similarity between these congeneric species might be the strongest factor contributing to their coexistence in the Parana River floodplain during the dry season.

This study evaluated the diet and feeding selectivity of the catfish Imparfinis mirini in streams with different degrees of urbanization and the effect of rainfall on the availability of prey. The diet was based especially on Chironomidae and Trichoptera. Significant spatial differences in diet were found between the streams; the diet of the fish was similar in the rural and peri-urban streams, and differed from that in the urban stream. Seasonality was an unimportant factor affecting the species' diet, which did not differ significantly between the rainy and dry periods in any of the streams. Fish from the urban stream fed more according to what was available in the environment, while fish from the peri-urban and rural streams showed higher degrees of selectivity. The results indicate that environmental conditions influenced the diet and prey selection of this species in response to the differences in diversity and abundance of the food organisms. They indicate that trophic studies of benthivorous fishes, such as I. mirini, may represent an alternative way to assess human effects on streams.