No abstract

Lysozyme acts as a non‐specific defence substance and is found in the peripheral blood, cutaneous mucus and certain tissues of marine and freshwater fishes. In the present study, we examined the effect of various environmental factors (water temperature, salinity, pH and suspended sediments) on plasma lysozyme activity in the Nile tilapia, Oreochromis niloticus L. When the fish were reared at different water temperatures (18.4, 23, 28 and 33°C), plasma lysozyme activity increased at 28°C after 2 and 4 weeks. A significant decrease in lysozyme activity was found in the fish reared at 33°C for 4 weeks. These results suggest that there is a water temperature range that affects the amount of plasma lysozyme activity that can be detected. Fish cultured at 24 g L−1 of salinity for 2 and 4 weeks and 12 g L−1 for 4 weeks resulted in significantly increased plasma lysozyme activity, suggesting that environmental salinity also affects the amount of plasma lysozyme that can be detected. Lysozyme activity also significantly increased when the fish were held in acidic water at pH 4.0 and in suspended sediments at 2000 mg L−1 for 2 weeks. It was concluded that changes in some aquatic environmental factors affect the non‐specific immune responses of Nile tilapia.