Effect of selective planktivory by fry of Rhamdia sapo (Pimelodidae: Pisces) on zooplankton community structure

1 The direct and indirect effects of predation by larval fish (Rhamdia sapo) on zooplankton in rearing tanks are analysed. Rhamdia sapo larvae showed an unusual species‐selectivity for Acanthocyclops robustus, the main invertebrate predator present. 2 Acanthocyclops robustus populations were markedly reduced, presumably as a direct consequence of species‐selective removal. Other zooplanktonic prey were not significantly affected by R. sapo predation. 3 Rotifers increased in tanks with fish, but this was not related to herbivorous crustacean variables (biomass, mean weight, abundance and species composition). On the contrary, rotifer biomass was negatively correlated with some A. robustus variables (biomass and mean weight of adults + copepodites and nauplii biomass). Thus, the rotifer increase appears to be an indirect effect of predation on A. robustus by R. sapo larvae.