Fish Distribution in the Middle Course of a Large Lowland River under the Effect of Hydrophysical Factors

The spatial distribution of fish in two 9-km-long sections of the Khoper River, both in the Khoper Nature Reserve and outside it, has been studied using a horizontal-beam echo sounder. Fish densities in the river-bed stretches with a different degree of meandering and depths and the effect of islands and accumulations of woody debris on fish distribution have been investigated. During the period of high water level (end of spring), all factors under consideration affect the spatial distribution of fish. In autumn, as the water level drops, the difference in densities of fish assemblages decreases in stretches with different degrees of meandering, as well as in those with and without islands, but it increases in parts of the river with different depths.