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The article describes features of the hydraulics of the Irtysh riverbed in the sections of significant fish concentrations -wintering holes - in the open water period. There have been explored the waters of the largest (in area and in depth) Gornoslinkinskaya and Kondinskaya riverbed depressions located in the Uvat and Khanty-Mansi districts of the Tyumen region and Khanty-Mansi Autonomous District, respectively. The bathymetric characteristics of wintering holes were studied using computerized hydroacoustic complex AsCor (Promgidroakustika, Ltd., Petrozavodsk). To create the bottom relief of riverbeds there were used geographic information software programs Surfer 9.0 and Map Viewer 6.0. The study of the species composition of the fish population was carried out conducting control catches with stationary and drift nets. It has been found that, as a result of the combination of features of the riverbed, on the investigated sectors there is formed a complex hydrodynamic and turbulent environment. The holes are located on meanders with coefficients of high curvature of the bend of a channel, there have been found the ranges with narrowing channels at the entrance into the turn. In the process of the channel narrowing there occurs deepening of dynamic river flow axis and eroding of the bottom, which may prevent silting of the wintering holes. Significant differences in the depths cause a compensating current. Due to the bend of the riverbed in the water areas there are formed whirlpool zones, and arising transverse water currents close the surface and bottom streams of the river flow. The heterogeneous hydrodynamic environment of the Kondinskaya depression is complicated by the fact that the Konda flows into the bend of the Irtysh; as a result, there takes place an exchange of river impulses. Thus, when the longitudinal flow velocity is imposed on the transverse flows, there appears a spiral movement of the water masses and vertical vortex structures - whirlpools resulting in optical (turbidity) and turbulent (hydrodynamic) heterogeneity in the water column of the riverbed wintering holes.

-Subarctic Siberian freshwater ecosystems are unique due to severe climatic conditions and strong seasonal changes of water supply, temperature and nutrient availability. In this work we evaluated the relative influence of the illumination level linked with the diurnal cycle, local environmental heterogeneity and water level changes during the receding flood on the distribution and migrations of fish in the Lower Irtysh basin. The data are based on hydroacoustic surveys conducted from boats in the main channel and long-term hydroacoustic recording from a stationary platform in a floodplain stream. The majority of the fish in the study area were represented by juvenile Cyprinidae. Illumination level is shown to have the strongest effect on fish density both in floodplain stream and near-surface areas of the main channel: fish tended to migrate at night and their density was always significantly higher at night. Effects of local ecological heterogeneity and the water level change were weaker: they affected the dynamic re-distribution of the fish. Daily reduction of the water level significantly influenced the intensity of fish migrations in a small floodplain stream, although with a 4 days lag. We suggest that the high environmental variability in lower Irtysh dampens local environmental effects on fish assemblages. Two-way (downstream and upstream) migrations of juvenile fish in a floodplain stream are documented, pointing the potentially important role of adaptive behavioural tactics.

The paper presents the technique of hydroacoustic equipment operation for ichthyologic studies in the period of reservoir icing and minimum ambient temperature. The stages of carrying out a mobile sonar survey are described: creating a route with drilling holes in the ice and conducting the survey. For trouble-free operation of the software-technical hydroacoustic complex it has been proposed to use warm sledge-trailer for the snowmobile. A metal frame and a geotextile awning are installed on the top of the sledge-trailer to protect it against precipitation and low temperatures. Inside the sledge there is installed a gas-electric generator, a hydroacoustic complex; in addition, in the front part of the sledge there is a workplace for the operator making the survey. To discharge the exhaust gases of the generator there is an opening made in the sidewall of the awning; to prevent melting of generator sides there are set up asbestos sheets. The awning is located at the rear of the sledge and takes 2/3 of its length. In the middle of the sledge-trailer there is an amortization table, on which a hydroacoustic complex and a field tablet are installed. The table is attached to the sledge body with rubber ties, due to which shaking of the equipment is minimized when the snowmobile and sleigh are moving over the uneven snow-ice cover of the investigated water area. Due to the air-cooling of the generator under the awning, a positive temperature is maintained, which ensures the uninterrupted operation of computerized scientific research equipment and allows the operator to work comfortably. The described method was successfully tested in ichthyologic studies in the field at min ambient temperatures in the winter-spring period on the watercourses of the Tyumen region within the Tobolsk and Uvat regions in the period 2016-2018.

The studies were performed by the hydroacoustic method in the section of the lower reaches of the Irtysh River (Western Siberia, Tyumen Region). The density dynamics of a group of fish with a symmetrical swimming bladder (Coregonidae and Esocidae) in the water areas of the riverbeds depression was studied. Among coregonids, inconnu dominates. The share of this group of fish is in the range of 10-11% of the total density of the fish population, and the density is an average of 0.3 thousand individuals ha1. Fish density has an inverse statistically significant correlation with the water level factor and the inverse correlation with the water temperature. Lower Irtysh is a wintering and feeding section for inconnu individuals; here, inconnu are formed aggregations in river areas called riverbeds depression.