The importance of a conservation unit in a subtropical basin for fish spawning and growth

Abstract

Studies for location of breeding sites and early development areas of freshwater fish in subtropical basins are still incipient. Most of the existing knowledge is based on sampling in environments impacted by dams of hydroelectric power plants and studies in areas with characteristics similar to the original are lacking. In this way, this study evaluated the spatial distribution of the ichthyoplankton in a Conservation Unit (Turvo State Park, Rio Grande do Sul State, Brazil) on the subtropical basin of the Uruguay River, in a stretch with no hydroelectric power plants and at best preservation condition. The density distribution of fish eggs and larvae was evaluated quantitatively through plankton nets samples, while the analysis of the species composition of larvae was assessed qualitatively through plankton nets and light traps sampling. There were significant differences in two stretches situated inside the park separated by a waterfall, both in the distribution of eggs and larvae and in species composition of the larval assemblage. Our results indicate that periods without the total flood of Yucuma Falls influences the composition of the ichthyoplankton and the existence of two distinct reproductive environments over an approximate length of 50 km. This result supports the hypothesis that in subtropical basins the alternation of flood and dry periods can influence the structure of fish community and evidences the importance of maintenance of hydrological conditions to fish reproduction in this river section, because showed the importance of the habitat diversity in the river channel to fish recruitment.