Effects of diazinon on adaptation to sea-water by the endangered Persian sturgeon, Acipenser persicus, fingerlings

To replenish the depleting populations of sturgeon fishes especially Persian sturgeon, Acipenser persicus in the Caspian Sea, millions of Persian sturgeon fingerlings are farmed through artificial propagation and released into the Iranian river estuaries annually. Fish osmoregulation is a vital physiological process that can be affected during the release. Many Iranian river estuaries are under the influence of pesticides originating from farming activities that may affect osmoregulation. In this study, Persian sturgeon fingerlings were exposed to sublethal concentrations (0, 0.18, 0.54, 0.9mg L_1) of diazinon for 96h (short-term trial) and 12 days (long-term trial) in fresh water (FW) and then fish were exposed in brackish water (BW) for 24h. After 96h and 12 days of exposure in FW, the lower levels of plasma triidothyronine (T3), thyroxine (T4), Na+, Cl−, K+, gill Na+/K+- ATPase activity and number of chloride cells were observed in exposed fish (0.54 and 0.9mg L_1 diazinon) compared to control group and 0.18mg L_1 diazinon treatment. Also, higher levels of plasma cortisol (except 0.18mg L_1 diazinon treatment in long-term trial) were observed in diazinon exposed fish compared to control group. However, gill Na+/K+-ATPase activity and the number of chloride cells were higher in fingerlings exposed to diazinon compared than control. When fish were exposed in BW for 24h, the following changes occurred: (a) in short-term trial: increases in cortisol and Cl− levels (0.54mg L_1 diazinon), Na+ (0.9mg L_1 diazinon) and gill Na+/K+-ATPase activity (0.18mg L_1 diazinon). In control group, cortisol, T4, Na+, gill Na+/K+-ATPase activity and the number of chloride cells increased significantly. (b) In long-term trial: increases in K+ levels in fish exposed to 0.9mg L_1 diazinon, Na+ in all diazinon concentrations and decreases in chloride cells number in fish exposed to 0.18mg L_1 diazinon. In control group, significant increases were observed in cortisol, T3, Na+ and chloride cells number. Finally, gill showed many histopathological damages during exposure in FW and BW. Our results suggest that the contamination of river estuaries with diazinon may alter the osmoregulation ability of released Persian sturgeon fingerlings, which could lead to a failure in their restocking program in the Caspian Sea.