Assessing the Impacts of Water Salinity and Pollution on the Physiological Responses of the Common Carp (Cyprinus carpio, L.) in the Tigris River of Iraq: A Comparative Study with Emphasis on Osmoregulation and Immune Function

The Tigris River is increasingly affected by salinity changes and pollution from industrial and agricultural sources. These environmental stressors may disrupt key physiological functions in the Common Carp (Cyprinus carpio, Linnaeus 1758), such as osmoregulation and immune response. This study investigated how elevated pollutants and salinity levels impact fish health, using cortisol levels and tissue metal accumulation as indicators of chronic stress. Water samples were collected monthly for three months from five sites representing different human influences. Water quality parameters such as temperature, pH, dissolved oxygen, and salinity were measured on-site. Concentrations of heavy metals and pesticides were determined in the laboratory. The common carp were also sampled monthly, and measurements of length, weight, and a stress indicator (K-factor) were recorded. Blood samples were analyzed to measure cortisol levels using ELISA. Tissue samples were digested and analyzed using ICP-MS to determine heavy metal levels. Correlation and regression analyses examined relationships between water quality, pollutants, and fish indicators. The results showed that temperature and pH were within normal ranges, but dissolved oxygen was low. Salinity varied significantly among sites. Heavy metal concentrations, particularly for lead and copper, exceeded safe levels. The pesticide DDT was detected at all sites. Cortisol levels and tissue metal concentrations were elevated, indicating chronic stress in the fish. Significant positive correlations were found between pollutant levels in water and fish tissues. Regression analysis showed water quality parameters and pollutants explained 52% of the variation in cortisol levels. The findings proved evidence that pollution and salinity changes are harming the common carp in the Tigris River.