Structure of fish assemblages in seven tropical reservoirs in southeastern Brazil during the rainy season; what matters: physico‐chemical or hydrological connectivity influences?

Closely connected reservoirs were hypothesized to have more similar fish compositions than do spatially isolated reservoirs, and that environmental variables play an important role in shaping assemblage structure on a local scale. Fish assemblages and environmental variables were sampled in seven reservoirs constructed to generate hydroelectricity, with most inflowing water coming directly or via transposition of the Paraiba do Sul River, in Southeastern Brazil. Using gill nets and environmental measurement variables, fish collections were carried out between January and February 2008 (wet season), when increases in temperature and rainfall increase hydrological connectivity, facilitating fish movements throughout the river‐reservoir system. Mean fish density, biomass and the number of species did not differ significantly among the seven reservoirs. On the other hand, the assemblage structure differed among almost all pairs of reservoirs. Of the reservoirs connected in series, only two (Santana and Vigário) showed a similar assemblage structure, suggesting that hydrological connectivity plays a minor role. Physico‐chemical variables changed among reservoirs and seemed to play an important role in determining the assemblage structure, with higher conductivity (average = 108 μS cm⁻¹) and TDS – total dissolved solid (0.07 g L⁻¹), and low pH (5.8) in the Pereira Passos Reservoir associated with a high number of the Siluriformes Loricariichthys castaneus, and with the opposite conditions in Tocos (conductivity = 27 μS cm⁻¹; TDS = 0.02 g L⁻¹; pH = 7) that were associated with a high number of the carnivorous Characiformes Oligosarcus hepsetus and Hoplias malabaricus. A direct relationship was found between abundance of the catfishes Pimelodus fur and Glanidium albescens and high turbidity (90.4 NTU) and low temperature (25.3°C) and transparency (0.1 m) in the Ilha dos Pombos Reservoir. Moreover, a close relationship was found between high abundance of the cichlid Cichla kelberi and the high temperature and transparency of Lajes (28.3°C; 2.2 m) and Santa Branca (27.1°C; 2.6 m) reservoirs. These findings support the hypothesis that physico‐chemical variables structure fish assemblages in reservoirs, but that hydrological connectivity seems to play a minor role in shaping assemblage structure. However, other unmeasured variables (e.g. physical habitat characteristics and biotic interactions) and other seasons need to be evaluated in further studies.