Fish distribution in the riverbed depression under ice cover

The work was carrying out in the water area of the Gornoslinkinskaya riverbed depression of the Lower Irtysh in winter using a computerized hydroacoustic complex. Hydroacoustic surveying is performing during the daytime with “high” sun and in the evening in with dark. It has been establishing that fish of different ecological and size groups in the water area of the riverbed depression in the diurnal-light aspect demonstrate statistically reliable dynamics of density, the degree of development of water column horizons. The density of fish along the horizons of the water column (surface, pelagic, bottom) in the light and dark hours was 18, 105, 61 and 1378, 368, 411 ind / ha, respectively. During the daytime, fish develop bottom and pelagic horizons with smaller depths, closer to the shores, and in the dark - are most intensively distributing in the surface horizon, in the pelagic and bottom in the middle deeper part of the riverbed depression. The vertical distribution of taxonomic and size groups of fish within a single phase of the day and in the diurnal-light dynamics reflects the defensive behavior of peaceful and predatory fish species - avoiding the risk of predation and cannibalism. The cyprinids are distributing in the horizons with a smaller relative number of groups of percids and coregonids-esocids fish, percids are characterized by differentiation according to the dimensional feature. Establishing statistically significant difference in the fish’s choice of vertical horizons of the water column and their bathymetric characteristics (depths), depending on the light phase of the day. Vertical migration of fish in the absence of light landmarks shows a genetically fixed reaction of animals to the level of illumination realized through the circadian rhythm.