Morpho-histological evaluation of young-of-the-year channel catfish (Ictalurus punctatus Raf., 1818) in extremely wintering conditions

Purpose. To investigate the effect of extreme low water temperatures on morphological and histological parameters of young-of-the-year (YOY) channel catfish (Clarias gariepinus (Burchell, 1822) during wintering at a warm water fish farm. Methodologu. The study was carried out during fish wintering (October-March) in 2016-2017 in the Pridniprovs'k thermal water tank fish farm (city of Dnipro). Water temperature in tanks was monitored twice a day, the hydrochemical parameters – once a week. Following parameters were determined in YOY channel catfish: individual weight, body length, Fulton condition factor, survival rate (%). Ichthyopathological studies were performed once a week: color, presence of mucus, any skin damages and the state of gills. For the parasitological analysis, scrubs were taken from the body surface and gills, followed by microscopy. During postmortem examination of fish, special attention was paid to the pathological changes of internal organs. To evaluate the physiological state of YOY channel catfish during wintering, an analysis of tissue and organ microstructures was carried out with the use of histological sections according to generally accepted methods. The analysis of histological preparations was carried out with the light microscope with 8x and 40x magnification and a photographic digital camera "Sciencelab T500 5.17M" at the DNU Department of General Biology and Aquatic Bioresources. The digital material was processed by statistical methods with the determination of the standard error and Student's test. Findings. A pecularity of fish wintering at modern warm water fish farms is a long period of too low water temperatures (3-5oC). The study showed that under these conditions, YOY channel catfish lost up to 76% of their weight. Fulton's condition factor was reduced on average by 40%. The parameter of fish survival during the winter period varied from 51 to 64% depending on the individual YOY weight before wintering. Long-term winter starvation of YOY channel catfish caused damage to the the skin epidermis, limiting the number of epidermal glandular cells that perform the protective function; the muscle fibers destruction and their replacement with the fibrous tissue, disorders of myometers and miosept topography. The exhausted YOY channel catfish had following pathological changes in the internal organs: hepatocyte dystrophy, vacuolation of the bile duct epithelium, exfoliation of the intestinal epithelium, death or fusion of intestinal villi. Originalitu. The obtained data on microstructural analysis of YOY channel catfish tissues and organs in the conditions of extreme wintering allows identifying the adaptive possibilities of this species to low water temperatures and can be used for scientific adjustment of keeping the channel catfish during winter at warm water fish farms. Practical volue. The presented results of the scientific work are intended for warm water fish farms, which experience problems with the water temperature regime associated with irregular amounts of warm water discharges from energetic objects. Information on histological changes in the YOY channel catfish body in the conditions of low water temperatures under prolonged action could be used to improve fish feeding before unfavorable wintering.