Dispersal of fish eggs and larvae in a cascade of small hydropower plants with fish ladders

This study was conducted to investigate fish eggs and larvae dispersal in a river stretch influenced by small hydropower plants in Southeast Brazil. The main hypothesis is that the downstream dispersal of free-flowing eggs and larvae is likely to occur given the small size of the studied reservoirs and that passage through the dam may occur, with the fish ladder contributing to it. Eggs and larvae were collected fortnightly, between November 2016 and February 2017, from locations upstream of two dams, including lotic, transition and lentic zones. Additional samples were collected inside the fish ladders at each dam. Downstream dispersal of eggs and larvae was influenced by a combination of rainfall/flow variation and stage of the reproductive cycle, with the peak of rainfall during the summer coinciding with higher abundances. Under these conditions, eggs and larvae drifted downstream through the reservoirs and reached the dams. Once in the dam, they could pass through the fish ladder. This passage was correlated with larval density in the reservoir immediately upstream, although at very low densities compared with the other sampling sites located upstream of the fish ladders.