Sublethal effects of organophosphate, diazinon on gill tissue and growth performance of Caspian roach (Rutilus rutilus) fingerling kept in fresh water and brackish water

&nbsp; The stocks of Caspian roach ( Rutilus rutilus ), an economically important species in the Caspian Sea, is depleting. Annually, millions of fingerlings of these species are produced artificially and restock in the mouth of Qare Soo River in the Southern Caspian Sea, where they exposed to the pesticides coming from rice and orchard farms in the region. The fate of this exposure in hypo-osmoregulatory ability of juvenile fish is important. Therefore, this study initially investigates fingerlings of Caspian roach which was exposed to different concentrations of diazinon (close to estuaries concentration) for 96 h, then they were transferred to the brackish water for 240 h where the growth and survival rate were examined. The major changes in the exposed fish gills to pesticides were collapsed secondary lamellae, oedema, epithelial lifting, and shortening of secondary lamellae. The findings of this study showed decrease of tissue damage in the gill tissue in fingerlings at brackish water, while, a lower growth rate was seen compared to the control group (P < 0.05). This study suggests that the exposure of Caspian roach fingerlings to diazinon in fresh water may jeopardize their physiological capabilities and subsequent survive in brackish water conditions which may cause failure in fish stocks rebuild project in the Caspian Sea. &nbsp; Caspian roach, histopathology, gill, growth, diazinon.