Migrations of Fish Juveniles in Dammed Rivers: the Role of Ecological Barriers

Analysis of previously published characteristics of the downstream migration of juvenile cyprinids (Cyprinidae) and percids (Percidae) from the Volga and Ivankovskoe, Volgogradskoe, and Tsimlayanskoe reservoirs is presented. In case of river damming, migrations of river fish are modified first of all due to transformation of the hydrophysical and morphological structure of the water course creating a different biotopic pattern influencing life activities of fish, including migratory behavior. In contrast to the natural river, where the conditions controlling the characteristics of the downstream migration change gradually from the upper reaches to the lower reaches, ecological barriers are formed in the dammed river—the water reservoir and the dam—which essentially change these conditions. A leading role in formation of these barriers and in regulation of the downstream migration is performed by the morphological complexity of a water body and by the intensity of water exchange. These factors act both on the scale of the whole water reservoir and on the scale of local biotopes where the downstream migration takes place and behavioral mechanisms act efficiently. The synergistic effect of these factors (a high ramification index of the reservoir at a low water exchange) may decrease the intensity of emigration of fish juveniles from a reservoir by several orders of magnitude.