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Modern dynamics of the main abiotic environmental factors responsible for fish fauna effective reproduction in the Kremenchuk Reservoir are discussed in the paper. Annual dynamics of water level and temperature were analyzed. It has been found that the main factor responsible for the rate of breeding stock maturation in the Kremenchuk Reservoir is water temperature dynamics rather than spawning temperatures. It is underlined that the operation of hydroelectric facilities and the spawning campaign must be synchronized to ensure sustainable fisheries. The drainage of spawning grounds in low-water years results in the decrease in their area and, as a consequence, in the formation of low-yielding generations. It is recommended to optimize the annual dynamics of water level according to the needs of fisheries.

Climate change inevitably leads to increased water temperatures, which is especially important in natural spawning grounds during mass fish spawning. Studies were performed to determine the effects of increased water temperature with decreased water oxygen concentrations on the activity of Na+/K+ ATPase, lactate dehydrogenase (LDH), and succinate dehydrogenase in developing carp embryos. The activities of ATP and LDH decreased several times when water temperature increased from 24 to 31°C. LDH activity increased up to 1.9 times. Thus, the fish embryos used glycolysis to a greater extent, the level of their aerobic respiration decreased, and mineral exchange with the environment was weakened. These phenomena could have disrupted embryonic development and decreased fish embryo viability.

Biogen elements that enter the water in large quantities are one of the main pollutants in the aquatic environment. They lead both to eutrophication of the reservoir and a decrease in the viability of young fish, acting as toxin. The energy enzyme activity (lactate dehydrogenase and succinate dehydrogenase), nitrogen (glutamate dehydrogenase) metabolism, dephosphorylation processes, and immune protection (by alkaline phosphatase activity) under the long-term action of ammonium nitrogen and phosphorus phosphates was studied. The biological object of research were 30–45 day common carp juveniles (Cyprinus carpio L.). According to our results, the peculiarities of the adaptive reactions of young carp to the action of these compounds have been established. We revealed, that under the loading of ammonium nitrogen, the activity of the processes of anaerobic digestion of energy-intensive complex increases directly in the organs of neutralization and excretion of nitrogenous compounds, namely in the gills. In general, the body of the carp due to these actions reduces the total energy consumption for maximum survival in these environmental conditions. We registered, that under the influence of elevated concentrations of phosphorus phosphates, oxidation of biogen elements to the body's energy needs in the presence of oxygen was observed. The processes of immune defense and dephosphorylation, for which LPh is responsible, also the detoxification, the excretion of nitrogenous substances from the body of carp (GDG) occurred differently depending on the chemical nature of the toxin. Thus, under the influence of ammonium nitrogen there was an increase in the activity of LPh in the highest concentrations of the toxin, and the activity of GDG decreased to reduce the production of ammonia, to avoid endogenous intoxication. Through these load of potassium monophosphate, the rise in the activity of immune defense processes and the involvement of ammonia in the cycle of tricarboxylic acids due to reamination to the energy supply of the processes of counteracting the weak but chronic toxic effects of orthophosphate ion. Under the load of ammonium chloride was observed not only the use of “traditional” energy substrates - glycogen and lipids, as well as proteins, which indicates the high toxicity of this compound for the body of young carp. Through the action of orthophosphate ion, the active use of lipids and even the activation of gluconeogenesis at maximum concentrations, as a reaction to weak long-term chemical stress. Keywords: juvenile common carp; ammonium nitrogen; orthophosphate ion; toxicity; enzymatic activity; energy substrates

Changes in the activity of Na+/K+-ATPase, lactate dehydrogenase, and succinate dehydrogenase in the white muscles and gill petals of roach, Rutilus rutilus (L), were studied for the combined actions of water mineralization and water temperature. It was established that decreased mineralization to 260.0 mg dm-3 and increased water temperature to 32°C increased aerobic processes in roach tissues. A sharp increase in water temperature and changes in water mineralization significantly affected the tolerance and physiological state of fish and could impact their abundance in waterbodies.

Recently, the water temperature significantly exceeded the climatic norms for the studied region of Ukraine. Water is heated to 30ºC with a decrease in the concentration of dissolved oxygen in shallow water. In this connection, the change in the hormone content in the blood plasma of carp, roach, rudd, perch and ruff in response to these factors has been studied. Most of the fish species studied in June respond by increasing the cortisol content in the blood by 1.2–3.3 times to an increase in water temperature. Fish quite successfully adapts after a month's stay in conditions of high water temperature to 31ºC, and the level of cortisol is reduced. The content of thyroid hormones in the blood of the rudd, roach and ruff is quite high and after a month of adaptation to the existing factors. This indicates an increased activity in their metabolic processes. The content of somatotropin and prolactin in the blood of the studied fish species decreases 2.4–5.0 and 1.9–7.7 times, respectively, in response to an increase in temperature and a decrease in the oxygen concentration in water. This should lead to a decrease in the rate of fish growth and change in the activity of osmotic exchange. However, hormonal reactions are species-specific in nature to the existing factors. Perch and roach are less plastic to increase water temperature. Carp most favorably tolerates an increase in water temperature and a decrease in oxygen concentration.