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.

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The readaptation of the crucian carp after some generations of successful adaptation in the reservoir contaminated by inorganic nitrogen compounds and its return to favorable environmental conditions was investigated. After the 3-month adaptation to standard conditions, all considered physiological parameters (organ indices and Fulton fatness) reached the control values. At the same time, biochemical parameters did not return to the initial physiological state. According to the hormones (cortisol, T3 and T4) and glucose content in blood plasma, it can be stated that the experimental specimens had the energy-saving metabolism. The experimental specimens developed somewhat other way of the excess ammonium nitrogen excretion from the organism. Thus, content of protein as the basis of ammonia transport, in the blood plasma was lower than in control. Over the readaptation period, the experimental specimens had the increased hemoglobin content in blood. Thus, within three months complete readaptation of carp did not take place, the biochemical state was the most revealing.

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.