Migrations of Young Fish in Regulated Rivers. Effects of Ecological Filters (Review)
Compared to non-regulated rivers new ecological filters are formed in reservoirs, the structures (gradient zones between biotopes) which affect the behavior and distribution of juvenile fish and change parameters of the downstream migration. A distinct feature of filters is the selective effect on different species and age groups of migrants when they pass boundaries between biotopes. The differential transparency of boundaries considerably affects the composition of migrants and dispersal of juvenile fish over biotopes. These suggestions are confirmed by the data of long-term studies on downstream migration of juvenile Cyprinidae and Percidae in several reservoirs in Eurasia: Sheksna (Sheksna River), Ivankovo (Volga River), Kapchagay (Ili River), and Saryyazinsk (Murgab River) Reservoirs. The formation of ecological filters is shown in the zone of wedging out backwaters (lotic-limnetic transformation), in the zone between the littoral (resident) and pelagic (migratory) biotopes in the reservoir and in the near-dam part. Emigration of juveniles from the reservoir depends on the location of the water intake (deep-water or surface) and overlapping of water intake zones with spatial distribution of juvenile fish. Different selectivity of water intake sites of the hydroelectric power station located not far from each other and a navigation lock forms different (in species and age composition) complexes of emigrants. Mechanisms and consequences of functioning the ecological filters and ecological barriers are considered.