Daily dynamics of vertical distribution and amount of sturgeons in riverbed depression in summer-autumn period

Abstract

The article presents the results of studying the fish distribution in the water column of the riverbed depression using remote echometric sounding with a software-hardware acoustic system with a vertical view. The study was conducted in the lower reaches of the Irtysh River. In the appointed period (July-October) under a decrease in the level regime and water temperature the increasing density of fish in the water area of the riverbed depression is explained by fish gathering in the period preceding wintering. The share of sturgeons varied from 5.46 to 10.28% of the total fish density, the indicators of which were in the range 1.70-5.05 thousand sp/ha in the daytime, 2.78-6.77 thousand sp/ha in the night. There has been stated the daily vertical migration of sturgeons, which is more uniform throughout the water column in the night, including the near-surface and near-bottom water horizons. In the dark near-surface water horizons are most intensively explored by large fish (25-30, 30-35, > 35 cm). The trigger of the daily vertical migration of sturgeons is supposed to be the light brightness, despite the poor eyesight of the fish under study, i.e. changing vertical position of sturgeons occurs due to endogenous circadian rhythms, as in many hydrobionts. The recorded features of sturgeon distribution because of preference of the direction and intensity of various factors can contribute to the optimization of bioenergy losses in hard turbulent conditions of riverbed depressions.