Factors affecting the distribution of fish during receding flood in lower Irtysh: effects of water level and diurnal cycle

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. Juvenile Cyprinidae represented more than 80% of all fish in the lower Irtysh. Illumination level had the strongest effect on fish density, fish density was significantly higher at night. Effects of local ecological heterogeneity and the water level change were relatively weak. Downstream and upstream migrations of fish in a floodplain stream were documented.