Defining a fish bio-assessment tool to monitoring the biological condition of a cascade reservoirs system in tropical area

We studied five tropical reservoirs in the cascade system of the Tietê River (State of São Paulo, Brazil) in order to evaluate the suitability of a fish multimetric index (Reservoir Fish Assemblage Index—RFAI) as a bio-assessment tool. For this purpose, we investigated the spatial and temporal variability of the RFAI with the objectives of testing the effects of the unit measure (catch in numbers or individual weight) of abundance and trophic metrics categories on the index response and, to identify the best sampling season for a bio-assessment program. We located seventy-two sampling sites in three different reservoir zones (fluvial, transition, and lacustrine) and collected samples by habitats – lateral (L), tributary mouth (M) and central (C) – inside each reservoir zone. We recorded the fish assemblages and several physicochemical and habitat variables in the dry and rainy seasons. We tested candidate metrics for range, responsiveness, and redundancy and selected nine for inclusion in the final indices (RFAIN, the index calculated when we expressed fish in number of individuals, per species, and RFAIW, calculated when we expressed fishes in weight, per species). We used a discrete scoring criterion (1,3,5), and the final indices varied from 9 to 45. RFAIN and RFAIW were highly correlated and able to detect a clear spatial variability among the five reservoirs. RFAIN showed a higher temporal variability especially in the smaller reservoirs of the system (Bariri, Ibitinga and Nova Avanhandava), allowing us to consider the dry season as the preferable sampling period for the development of bio-assessment programs. RFAIW appeared to be more responsive than RFAIN, and was in agreement with the environmental gradients observed when analyzing the physicochemical and habitat variables collected in the present study. These findings indicate that the fish multimetric approach is a suitable tool for the assessment of the biological conditions of these reservoirs.