Lake Kronotskoye is located in the Far East, on the territory of the Kronotsky State Natural Biosphere Reserve (Kamchatka). The flow from the lake is carried out through the river of the same name with a length of about 40 km, which flows into the Kronotskoye Bay off the eastern coast of Kamchatka. Due to the peculiarities of the hydrological regime and the relief of the Kronotskaya River basin, anadromous migration along it is limited and anadromous fish do not return to the lake after the marine period of their life cycle. The lake basin is inhabited by a polymorphic resident form of the sockeye-kokanee Oncorhynchus nerka (Walbaum, 1792) and at least six reproductively isolated forms (species) of chars [21; 22]. All types (species) of chars in the Kronotskoye Lake basin differ from each other morphologically, in age structure, growth rates, type of nutrition and also the level of parasite infestation. White char Salvelinus albus Glubokovsky, 1977, at the early stages of ontogenesis feeds on invertebrates, and as it grows up, it goes on to a predatory lifestyle. It lives a migrant lifestyle, spawning takes place in the tributaries of the lake, and feeding occurs in the lake itself [1; 15; 21; 22].
Earlier, we determined the quantitative content of immune complexes, products of lipid peroxidation and antioxidant activity in blood serum and organs of white Salvelinusalbus Glubokovsky, 1977, long-headed S. kronocius Viktorovsky, 1978, and long-headed S. schmidti Viktorovsky, 1978, chars [3-5]. Based on the obtained data, it was suggested that the physiological parameters of chars depend on the lifestyle, type of nutrition and parasite infestation.
Since of all connective tissues, blood is one of the most sensitive and quickly reacting to changes in both the body's habitat and its physiological status, the composition of leukocyte cells reflects the species and ecological characteristics of fish, and the ratio of individual cell types reflects the functional state of the body and the nature of the influence of biotic and abiotic stress factors [13; 16; 23; 25; 26]. Currently, the composition of leukocytes in fish has been studied in representatives of many systematic and ecological groups living in freshwater, brackish and oceanic ecosystems, as well as in big aquaculture objects [7; 9; 19; 23]. However, except for our study, literary data on the composition of leukocytes in char inhabiting Lake Kronotskoye are practically absent.