Selective Removal of Fish from Reservoirs and Lakes: Interaction of Hydraulic and Ecological Factors

Abstract

Downstream migration (DSM) of fish through the different types of water intakes is not a random process, but rather a selective removal from the limnetic to the lotic parts of river–lake systems. Selectivity means that the assemblage of migrants does not exactly reflect the composition of the fish community in an upstream reservoir. Some fish are more prone to migrate compared to others, but this is not the only factor affecting DSM. We hypothesize that the interaction of a 3-D “hydraulic funnel” at the water intake with adjacent ecological zones results in the selective removal of fish. We tested our predictions by analyzing the data on DSM and spatial distribution in 13 reservoirs and lakes in Europe and Asia. Water intakes were surrounded by different habitats and located at different depths, from the surface layer to the depth of more than 50 m. Most vulnerable for entrainment by the outflow were the inhabitants of the pelagic zone. The share of fish emigrating from the littoral habitats was much lower compared to pelagic inhabitants. This suggests that littoral habitats saturated with landmarks and shelters hamper DSM more than the vertical physical gradients in the pelagic do. In conjunction with the factors operating on the scale of the whole reservoir, hydro-ecological barriers of different types associated with water intakes play an important role in the selective removal of fish from reservoirs and lakes.