Seasonal migration of fish into the floodplain of a large plain river: temperature, water level and light are among the driving forces of migration

Introduction This study examines the dynamics and characteristics of seasonal fish migration into the floodplain of a large lowland river. Methods For continuous recording of the number of fish migrating upstream and downstream, and their size composition, the hydroacoustic complex “NetCor” was used. The system was permanently installed in spring 2022 on a floating platform in the Varpak River (a small tributary of the Irtysh River), mounted in a side-scan setup from the left bank. Results The dominant role of water level and temperature for abundance dynamics of fish migrating into the floodplain is demonstrated. The earlier onset of high values for these factors positively correlate with the abundance of migratory fish. Their influence accounts for approximately 70%. On a daily basis, migration dynamics are determined by the daily variation in illumination. The abundance of upstream migrant fish correlated with diurnal variations in illumination. Migration intensity increases during periods preceding morning and, to a lesser extent, evening twilight. The lowest proportion of migratory fish was observed during the daytime, when illumination was highest. Discussion The influence of water temperature, water level and light on the dynamics of seasonal fish migration is revealed. At the same time, a number of specific features have been noted for seasonal migration to the floodplain: synchronous increases and decreases in the intensity of fish movements up and downstream are simultaneously recorded, while the general trend of migration to the floodplain is maintained; in the initial phase of the flood, the largest fish are the first to migrate into the floodplain, which is an adaptive behavioral response. The diurnal distribution pattern of migratory fish is a strategy which they use to counter visually oriented predatory fish species—zander, perch, and pike—to avoid optimal prey search conditions.