Fish distribution in riverbed depression and conjugated reservoirs (effects of lighting and turbulence)

Abstract

The paper describes the studies carried out by the hydroacoustic method in the open water period (spring) in the water area of the riverbed depression of the Irtysh River and its conju-gated water bodies (Western Siberia, the Russian Federation) in 2017. Collectively investigated reservoirs form a laminar-turbulent system, as they vary in intensity and direction of currents which form a certain turbulence and turbidity. It has been stated that in the dark time the fish population density increases in the waters of the reservoir with the maximal turbulence and turbidity rates - 166 NTU (the Irtysh river), and decreases in other water bodies (in the Konda river - 20.75 NTU and in the lentic section of the riverbed depression - 25 NTU). In the waters of conjugate reservoirs an increasing share of cyprinids is accompanying by a decrease in the proportion of predatory fish, while with an increase in predatory fish, on the contrary, the proportion of cyprinids decreases. A significant difference (P < 0.001) in terms of fish density was noted in the dark time. It was found that of the two factors (time of the day and turbulence) the second factor has a significant reliable effect (F = 18.518; P < 0.001) on the variable of fish population average density, both factors reliably interacting (F = 3.982; P = 0.037). There has been shown a transition of non-predatory fish to the site that is visually and hydrodynamically the most difficult for finding prey, which seems to be an element of reducing the risk of predation. Thus, there was found a significant influence of an abiotic factor of turbulence and, consequently, arising turbidity when interacting with the illumination factor on forming the increased fish concentration in the waters of the riverbed depression during the open water period.