HEMATOLOGICAL PARAMETERS OF EUROPEAN PERCH (<i>Perca fluviatilis</i> Linnaeus, 1758) ASSOCIATED WITH PARASITIC INVASIONS
European perch ( Perca fluviatilis Linnaeus, 1758) is widespread in water basins and has a commercial value. However, in the natural environment, fish are often infected with invasive diseases, which exclude its processing for food purposes. Being in the body of fish, parasites produce toxic substances that affect various organs of fish. Widespread in the Volga Delta pathogenic species, Eustrongylides excisus , in large quantities occurs in perch in larval state in the abdominal cavity of the body, in the walls of the intestine, in the liver, in the calf and in the spinal muscle tissue, reaching up to 30 or more parasites per fish. Sexual maturity of the parasite occurs in the stomach of the birds. This disease is peculiar to perch species; larvae of this nematode cause granulomatous inflammatory reaction. In addition, these worms, penetrating deep into the muscles of the perch, spoil the commercial quality of fish, which leads to the culling of fish raw materials and rejection of foodstuff. This work, for the first time, compares the hematological status of infected and healthy perch of natural populations. The work aimed to assess biological and physiological effects of various parasitic invasions on European perch Perca fluviatilis (Linnaeus, 1758). Fish of both sexes at the age from 0 to 4 years were caught in natural reservoirs of the Lower Volga (Bolshaya Bolda and Bely Ilmen rivers, Astrakhan region) in September 2016 to June 2019. The total body length, the length to the end of the scale cover, body weight and age were determined, and the Fulton’s fatness coefficient was calculated. The skin, fins, mouth, gills, eyes, heart, abdominal cavity, muscles, brain, and spinal cord of fish were examined for parasitological contamination. Blood for analysis was taken from the tail vein in vivo. Nematodes were found in more than 55 % of the caught perch. Nematodes were found in the liver (30 %), abdominal cavity (45 %), intestines (10 %), muscles (15 %), and gills (5 %). Parasites were found free within the body cavity, or encapsulated, with Eustrongylides excisus being the most abundant. In the infected fish, the average body weight and body condition coefficient decreased as compared to the healthy fish of the same age. However, the revealed growth retardation in the infected perch was statistically insignificant (p > 0.05), while the body condition coefficient was significantly lower than in uninfected fish (p < 0.01 for 3-year-old perch). The parasitic infestation of P. fluviatilis with Eustrongylides nematode caused symptoms of anemia, suppression of erythropoiesis (i.e., a decrease in the proportion of blast forms of the erythrocytic cell series), an increase in the proportion of neutrophils, monocytes, and lymphoblasts in the leucocyte count. In infected perch, the number of red blood cells significantly decreased (83.01½10<sup>4</sup>±4.17½10<sup>4 </sup>vs. 137.22½10<sup>4</sup>±5,26½10<sup>4</sup>/mm<sup>3</sup>), as well as the concentration of total protein (40.81±1.19 vs. 48.97±2.07 g/l) and blood cholesterol (5.17±0.28 vs. 6.81±0.30 mmol/l). MCH (mean corpuscular hemoglobin), erythrocyte sedimentation rate, the total number of leukocytes and platelets also increased compared to uninfected fish. The level of pathological red blood cells increased in the infected perch (9,17±0,23 % vs. 4,87±0,11 %). Changes in cell cytoplasm and nucleus, degenerative changes in the cell, changes associated with cell division were the main types of the discovered cell pathology of the infected perch.