Vertical and seasonal distribution of fish in Três Marias reservoir

Abstract

Understanding fish community dynamics close to dams can provide critical information about the necessity and feasibility of installing downstream fish passages. This understanding can also be used in reservoir operating plans to reduce the risk of fish being drawn into turbine intakes. To provide this background information for Três Marias dam we evaluated vertical and seasonal fish distribution immediately upstream from the Três Marias dam using hydroacoustics and gillnet sampling. We conducted hydroacoustic surveys and gillnetting on 18 separate trips. On each sampling trip, hydroacoustic surveys were done 4 times within a 24 h period at 6 h intervals. To validate the effectiveness of hydroacoustics, gillnets were set for 24 h and checked at 12 and 24 h. The gonadal maturity stage of migratory fish species was evaluated. The greatest concentration of fish was found in the sampling area at night during the rainy season, suggesting that this might be the most appropriate time to open spillway gates for downstream fish passage. This timing would also minimize the effect on reservoir storage. In addition, higher concentrations of fish and post spawning migratory species were captured during the rainy season. Our study results have the potential to enhance dam operation management plans to provide safer downstream passage for migrating fish.