Cadmium is a heavy metal commonly introduced into the Caspian Sea and its rivers. The aim of this study was to investigate the effects of sublethal concentrations of cadmium on some hematological parameters and gill histopathology in sterlet ( Asipenser ruthenus ). A total of 60 fish with average weight 41.69 ± 0.95 g, and the mean length 23.98 ± 0.14 cm were divided into 4 groups with 3 replicates. Fish were exposed at zero (control), 16, 32 and 64 ?g/L Cd concentration for 21 days in 95-L tanks under laboratory condition in semi-static flow system. Physicochemical factors of water were measured during the experiment including dissolved oxygen, temperature, pH, EC, nitrate, phosphate, alkalinity and hardness. Blood samples were taken of caudal region of the fish using five percent EDTA as an anticoagulant and gill tissues were fixed in Bouin's solution for histopathological studies using hematoxylin-eosin method. Gill tissues showed several histological changes such as hyperplasia, hypertrophy of epithelial cells and epithelial edema as well as necrosis, rings and fusion of gill filaments and secondary lamellae. There were a significant relationship between the intensity of histological changes of the gills and Cd concentration, where the highest adverse effects on the gills were observed in fish treated with 64 µg/L of Cd in comparison with control. Hematology parameters were been measured including red and white blood cell counting, hematocrit, hemoglobin and secondary indices such as mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) and the ratio of white to red cells. As a whole, results showed that only hematocrit and hemoglobin in groups treated with 32 and 64µg/L of Cd increased significantly compared to control ( p 0.05). Other hematological factors didn’t show any significant changes among groups ( p 0.05). In conclusion, the results of this study showed that subletal concentration of cadmium can cause deleterious effects on gill as well as blood in sterlet. Key words: Sterlet, Acipenser ruthenus, Cadmium, Sublethal, Hematological parameters, Histopathological changes, Gill.
