The regulation of river flow by dams of hydroelectric power plants (HPPs) has a significant impact on the populations of migratory fish species, preventing their movement between the upstream and downstream sections. Violation of the level and thermal regime of the river in the downstream of the HPP can also lead to a modification of the migration process, both within the main channel and between the main channel and its adjoining system. It is assumed that after the construction of the dam of the Krasnoyarsk hydroelectric power station, a significant part of the population of the Baikal grayling Thymallus baicalensis in the downstream section switched to a sedentary lifestyle. However, quantitative assessments of this phenomenon have not yet been carried out. The middle course of the Yenisei River in the downstream of the HPP is a specific watercourse with a smoothed seasonal amplitude of water temperatures and a high biomass of benthic invertebrates, up to 10-30 g/m2. Such conditions provide an increase in the growth rate of grayling in the main channel of the Yenisei, and lead to the formation of the structure of its scales, which differs from fish from the tributaries of the Yenisei. An additional feature of the Yenisei in the downstream of the HPP is that the content of technogenic radionuclides, in particular, radiocesium, 137Cs, increased compared to the background levels, below the point of radioactive discharges of the Mining-and-Chemical Combine ROSATOM (MCC) located near the town of Zheleznogorsk (Fig. 1). Thus, high growth rates, a unique structure of scales, and an increased concentration of radiocesium can be considered as markers of grayling that constantly inhabit the main channel of the Yenisei in the downstream of the Krasnoyarsk HPP. Within the framework of this work, we for the first time attempted to assess the ratio of settled and migrated individuals of the Baikal grayling from the tributaries in the thermally altered section of the Yenisei on the basis of the structure of the scales. As a verification of the proposed marker, we used the linear dimensions of fish and the content of radioactive cesium in their tissues.
A sample of 161 individuals of the Baikal grayling was used in this work. Fish were collected in the section of the main channel of the Yenisei near the mouth of the Kan River, near the village. Khloptunovo (56º28′05″ N, 93º38′53″ E) in July–November 2019. Fork Length and total weight of fish were measured. Grayling scales taken in the lateral line region were photographed and then used to count the number of sclerites in the completed annual rings. The principle of differentiation of grayling individuals by their origin is based on previously obtained results on an increased number of sclerites on the scales of fish (primarily in the second annual ring) inhabiting a thermally altered section of the middle reaches of the Yenisei. Within the framework of this study, we assumed that individuals of grayling with more than 15 sclerites in the second annual ring of scales constantly inhabit the main channel of the thermally altered section of the Yenisei. Further, these individuals are designated by the term "sedentary". Individuals with a smaller number of sclerites, designated as “migrants,” spent at least the second year of their life in tributaries. To measure the content of technogenic radionuclides, fish of the two indicated types were isolated from the total sample. In the pooled samples of sedentary grayling for each month, the number of individuals varied from 12 to 31 (total 125), in the samples of migratory ones - from 3 to 8 individuals (total 18). Samples were prepared from muscles taken from the axial skeleton of fish, dried and ashed in a muffle furnace at 450°C. The content of γ-emitting radionuclides in the ash samples was measured using a gamma spectrometer with a hyper-pure germanium detector GX2320 (Canberra). The spectra were analyzed using the Genie-2000 software (Canberra).
The number of sclerites in the second annual ring of the studied grayling varied from 9 to 24. The share of individuals with from 9 to 14 sclerites, and designated in our work as migrants, was 18.6% of the entire sample (Fig. 2). The highest percentage of migrating individuals was recorded in July (24%), in August-September it was about 20-22%, in October - 15%, and in November such grayling was not found. To assess potential differences in the growth rates of sedentary and migratory fish, the samples were divided into groups by age and month of capture. The linear dimensions of the sedentary grayling aged 2–3 + were 3-5 cm higher (p <0.05) than the linear dimensions of migrating fish in July–August (Fig. 3). There were no differences in FL in 4+ year old fish in July, as well as in 3-year-old fish in September–October. Of the technogenic gamma-emitting radionuclides, only radiocesium was registered in the muscles of grayling (137Cs, half-life - 30.1 years). The content of 137Cs in grayling muscles varied in the range of 0.9–7.2 Bq/kg. The maximum content of 137Cs was noted in sedentary fish in August, which exceeded the content of this radionuclide in the sample of migrants by six times (Fig. 4). In the rest of the months, the samples of sedentary and migrating individuals contained a similar specific activity of 137Cs. In July, the content of radiocesium in the sample of migrating individuals was not measured. The content of radiocesium in the muscles of grayling caught in the background site of the Yenisei did not exceed the detection limits.
Thus, for 2-3-years-old fish, caught in the summer period, the assumption about the differences in the growth rates and the content of radionuclides in the tissues of residential and migratory individuals was confirmed. To explain the absence of differences between individuals of the two strategies in other months, a temperature model of the distribution of grayling in the tributaries of the Yenisei was proposed. According to our assumption, when the water temperature rises to the physiological optimum (17-18°C) in the lower and middle reaches of the tributaries in July-August, part of the local fish rises upstream, and part rolls down into the colder Yenisei (summer temperatures no more than 12°C). It is during this period that the most dramatic differences between the Yenisei fishes and migrants can be diagnosed. By mid-late autumn, such differences are leveled, and the migrants themselves are scattered among the Yenisei fish (Fig. 5).
The reason for the increased content of 137Cs in sedentary grayling in August is not clear to us, but, possibly, it is due to the dynamics of permitted radioactive discharges into the Yenisei, the frequency of which during the year is not known to the authors, since it is not published in the annual radioecological reports. Cesium, being a chemical analogue of potassium, behaves in the body of fish similarly to this biogenic element. The main deposits of cesium are found in fish muscles. When balancing the intensity of consumption and excretion of radiocesium, its content in the tissues of sedentary individuals reaches a stationary level, which we observed in September-November.
The results of the work show that fish isolated from the general sample on the basis of a smaller number of sclerites in the second annual ring can indeed be considered migrants from tributaries. The approach we used reveals predominantly immature and first maturing individuals settling from tributaries, but is unsuitable for identifying sexually mature fish that carry out short-term intra-river migrations. In the thermally altered section of the Yenisei in the downstream of the HPP, tributaries can still make a significant contribution to the grayling population. In the area of the mouth of the Kan River, one of the largest tributaries of the middle Yenisei, the percent of migrants in the summer-autumn period of 2019 was about 20% of the total grayling population. The need to separate sedentary and migratory individuals is important for obtaining homogeneous samples of grayling, which is traditionally used as a model object in analyzing the distribution of technogenic pollutants in the river, as well as for studying the nutritional value of grayling and environmental risks for the population associated with its consumption.