Abstract
This study investigated the influence of key environmental factors, including water temperature, ammonia concentration, and dissolved oxygen levels, on the immune status of common carp (Cyprinus carpio) and their susceptibility to bacterial and parasitic diseases. The research was based on experimental modeling of aquaculture conditions with controlled variation of environmental parameters, followed by assessment of immunological indicators such as lysozyme activity, immunoglobulin content, neutrophil phagocytic activity, complement activity, and cortisol levels. Evaluation of the physicochemical characteristics of water confirmed the stability of experimental conditions and allowed isolation of the effects of the studied stressors. The results showed that an increase in temperature to 30°C, ammonia concentration to 1.0 mg/L, and a decrease in dissolved oxygen to 3 mg/L led to a pronounced suppression of immune responses and a significant increase in susceptibility to Aeromonas hydrophila and Dactylogyrus spp. The most substantial impairment of immune status was observed under hypoxic conditions, accompanied by elevated cortisol levels and maximal intensity of the infectious process. These findings highlight the key role of oxygen regimes in forming carp resistance to pathogens and emphasize the necessity of optimizing water quality control in freshwater aquaculture systems. The results allow the conclusion that environmental stressors significantly increase carp susceptibility to infectious diseases and should be considered a critical factor in practical fish farming.