Effects of weight on osmoregulatory ability of Salmo trutta caspius juveniles

Salmo trutta caspius is a commercial migratory fish species in the Caspian Sea. Over fishing and deterioration of natural spawning grounds of the species are two major causes depressing stocks of the fish in the Sea. Thus, artificial breeding and release of salmon juveniles into the Caspian Sea is now considered an urgent need towards the rehabilitation of the stocks of the fish. This study was conducted in early 2004 to determine the ideal weight for the release of salmon fry in order to increase fishery return coefficients in the species. For this research, we used facilities of the Marine Fisheries Breeding and Research Center in Ghazian, along the coastal areas of the Bandar Anzali. Salmon juveniles used in this study belonged to the same generation. Fishes belonging to different weight classes (5, 10, 15 and 20g) were selected in a random design and stocked in two groups of sea water at 7% salinity and freshwater. Osmosis was studied using blood samples collected at 0, 3, 6, 12, 24, 72, 168 and 240 hours. Heparin tubes were used for sampling blood which was centrifuged to separate blood plasma. Osmotic pressure was determined using osmometer. Statistical analysis of the results on variations in osmotic pressure showed that after 10 days of stocking fish in seawater (7%), fishes in all weight classes were capable of osmoregulation. Also fishes in weight classes 10, 15 and 20g were capable of osmore-gulation in the Caspian Sea water