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

Abstract

-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.

Meander reshaping – the formation process of a wintering riverbed depression of fish

“…During the wintering season, the basic habitat requirements of other species of fish also shift to the most important aspects: sufficient depth and low flow rates (Mochek et al, 2019;Wang et al, 2019). In the study of riverbed depressions, it was shown (Mochek et al, 2015) that aggregations of fish in these water areas are also found during open-water periods. The study (Borisenko et al, 2013) revealed that riverbed depressions mostly form on the vertices of meanders.…”

Excerpts from the biography of Michael Brody — an environmental scientist

“…Река Иртыш является важной составной частью Обь-Иртышского бассейна, обладая значительными водными биологическими ресурсами, в первую очередь рыбными. Для проведения фундаментальных и прикладных исследований в области ихтиологии в пределах бассейна Иртыша сотрудниками ТКНС УрО РАН и ИПЭЭ им А. Н. Северцова (борисенко и др., , Mochek et al, 2015 используется программно-технический комплекс «AsCor» (производитель ООО «Промгидроакустика», г. Петрозаводск). Данный комплекс предназначен для количественной оценки запасов гидробионтов гидроакустическим методом в реальном и отложенном времени в научных и прикладных рыбохозяйственных работах на внутренних водоемах и прибрежных районах морей.…”