The impact of a biomanipulation experiment on the fish assemblage trophic structure in a Neotropical semiarid reservoir.

ABSTRACT. Aim: This study aimed to analyze the influence of the exotic species Oreochromis niloticus Linnaeus, 1758 before and after its removal in the fish assemblage, characterizing its trophic guilds over the months in a reservoir of the Brazilian semiarid. Methods: Data were sampled in two periods, before removal of exotic species (May 2012 to January 2013) and after removal (April 2013 to November 2013). Six species were analyzed ( Oreochromis niloticus ,  Prochilodus brevis Steindachner,1875, Hypostomus cf. paparie Fowler,1941 , Hoplias gr. malabaricus Bloch,1794 , Astyanax bimaculatus Linnaeus,1758 and Leporinus piau Fowler,1941) whose values of the Feeding Index (IAi) were used. The Morisita-Horn similarity coefficient was used to assess the feed similarity between species and establish four trophic categories (insectivorous, omnivorous, detritivorous/iliophagous and piscivorous). To assess the possible variations due to the presence of the species O. niloticus , we used the Feeding Index of species in Bray-Curtis similarity matrix, using the ordination analysis (NMDS) that detects if there was difference in items consumed by the species. Results: The result of Multivariate Analysis Canonical Correlation (CCA) between abiotic factors (rainfall, pH, turbidity, dissolved oxygen) and biotic factors showed that the diet is associated with abiotic factors with little correlation with the presence of detritivorous/iliophagous species. The species continued inserted in the same categories, in both periods analyzed, and therefore the number of guilds did not change. Conclusion: It is possible that the species O. niloticus had no time to settle in the environment during the study period, it is therefore necessary a long-term study to verify the relationship between abiotic factors and the establishment of tilapia in protected areas.