ИСТИНА

The Don River is an important river for its fishing resources. The current climatic changes result in reduction of spring floods. The areas of floodplain spawning regions where phytophilous fish species spawn on flooded vegetation are decreasing. Fish reproduction is affected, and commercial fishing decreases. To characterize the occurring changes, we evaluated the conditions for spawning of phytophilous fish species by duration of the spawning period (DSP). DSP is the time of flooding of a plain with water more than 20 cm high and its temperature higher than that at which fish begin to spawn. DSP should last about a month for spawning of many phytophilous fish species. As a model object, the bream was chosen, beginning to spawn at water temperature 8 оС. The river section was chosen below of Tsimlyanskoye reservoir (151 km from the river mouth). DSP showed that in the period of 1936-2018 only 16 years were beneficial for spawning, when DSP was longer than 30 days. During the first 15 years of observations before the appearance of Tsimlyanskoye reservoir in 1951), 9 years were beneficial, and in the following 66 years, only 7 years were good. In 51 cases (out of 83), water did not even enter the flood plain. That was due to the reservoir-caused spring runoff reduction and prolonged reduced runoff, which appeared beginning with the early 1970s. Ivanchenko demonstrated close connection between the water level in the river and the number of bream fingerlings. In our evaluations, if water rises 1 m above the channel edge, the number of fingerlings doubles, and if it rises 2 meters, their number increases fivefold (compared to the unflooded plain). This is related to increased spawning areas. Thus, fish spawn under hard conditions, worsened by low water and runoff regulation.