Seasonal Dynamics of the Spatial Fish Distribution in the Estuary Area of the Tributary of Plain Reservoir

The seasonal dynamics of the spatial distribution of the fish population along the longitudinal section of the estuary area of a small tributary of the plain reservoir has been studied based on the example of the Il'd' River. The spatial distribution of fish was assessed by the hydroacoustic method and the surveys were performed from a motorboat in May, July, and November 2012 during the day time. The data we obtained show an increase in the distribution density of fish from the freshwater estuary mouth to the frontal zone of the estuary area. The fish density in shallow floodplain biotopes was 1.5-8.2 times higher than in deep-water channel habitats in all zones of the tributary estuary during all seasons. The manifestation of the marginal effect, which is recorded by the species composition and parameters of quantitative development of aquatic invertebrates in the frontal zone of the estuary, obviously favors the fish concentration, as evidenced by its maximal distribution density.