The energy budget of all fish populations was estimated in two small tributaries of the Paraná River (Paraná, Brazil). Total energy consumed by fish in the Caracu and the Agua do Rancho Rivers was 4.1 and 1.8 MJ m−2 y−1, and food items consumed were 2284 and 994.5 g wet weight m−2 y−1, respectively. The gross (K1) and net (K2) ecological efficiency coefficients were very low, but 43.2 and 59.6% of the total fish diet in these two streams, respectively, consisted of plant detritus. In both fish communities, omnivorous (opportunist) species dominated and specialists were rare. Although the Caracu River was more affected by human activity than was the Agua do Rancho, ecological efficiency coefficients calculated for the dominant fish populations were not significantly different.