Factors of Variability in Downstream Migration of Juvenile Fish in Regulated Rivers

Unlike a natural river, where the conditions determining the characteristics of downstream migration change gradually from upper to lower reaches (river continuum), in a regulated river, environmental barriers are formed—a reservoir and a dam—which significantly change these conditions. In such transformed systems, downstream migration is not a random but a regular process, resulting in the selective removal of migrants (emigration) from the limnic to the lotic zone. Emigration is most pronounced among fish inhabiting pelagic habitats. Littoral habitats, rich in landmarks and shelters, significantly inhibit emigration. This selectivity is determined not only by differences in the migratory behavior of fish but is largely linked to the impact of the three-dimensional hydraulic structure in the water intake zone on adjacent ecological zones of the reservoir. The altered environmental situation has different effects on the migratory behavior and distribution of juvenile fish of different taxa. Thus, the relatively similar nature of the downstream migration of juvenile Leuciscidae and Percidae in natural rivers changes significantly and differently in regulated rivers. These changes are associated with both the transformation of the hydraulic and biotopic structure of the habitat and the emergence of new environmental barriers that act as selective mortality factors. Analysis and ranking of factors influencing variability in downstream migration and emigration of juvenile fish from reservoirs provides information necessary for developing approaches and technologies for the protection and reproduction of fish populations in regulated rivers. These approaches are based on ecological methods for protecting fish during migration, the most challenging period of their life cycle.