Major Ecological Threats for Young Fish in Human-Made Reservoirs: Modified Downstream Migration Causes Reduced Fitness and Increased Mortality

Abstract

Downstream migration (DSM) of young fish is an efficient dispersal method in rivers. Regulation of rivers transforms them into lotic–limnetic cascades fragmented by dams. Novel conditions of transformed riverine habitats influence the routes and behavior of migrants, creating hazardous ecological situations, which lead to enhanced mortality and reduced fitness of fish. To mitigate these effects and develop efficient management programs, the gaps in our knowledge on the ecology of fishes in transformed rivers should be filled. Most studies on the ecological effects of river regulation on DSM are focused on relatively small areas adjacent to dams. We believe that upstream sections of reservoirs also significantly impact the survival of juvenile fish and the intensity of their transportation to dams and water intakes; thus, these sections deserve special consideration. This review addresses ecological effects of river regulations, focusing on the heterogeneity of habitats as one of the key factors controlling DSM throughout the novel system of the human-made reservoir. Our analysis shows that habitat heterogeneity influences foraging, defense, and migratory behavior at different spatial scales. In reservoirs, the topography of the whole reservoir (macroscale), stretched hydraulic gradients between migratory and residence habitats (mesoscale), and local shelters and landmarks (microscale) may hamper DSM. Homogeneous open waters are less suitable for feeding, communication, defense, and other behaviors of migrants than heterogeneous inshore habitats. We distinguish three main ecologically distinct zones within reservoirs and describe their specific modifications of DSM. Fish from different taxonomical and ecological groups, e.g., Cyprinidae and Percidae, were found to respond differently to newly emerged structures and modified habitat heterogeneity. When creating fish-friendly migration routes and developing conservation measures for fish populations in regulated rivers, not only transformed habitats but also the characteristics of different ecological groups of fish should be taken into account.