the effect of size on osmoregulation ability, chloride cells and na+, k+-atpase activity in salmo caspius kessler, 1877 juveniles

this study was carried out to determine the appropriate size of caspian trout ( salmo caspius kessler, 1877) juveniles for releasing into the rivers that enter south of the caspian sea for stock enhancement, or possibility of cage culture in the caspian sea. a total of 1611 specimens in 4 weight groups of 5, 10, 15 and 20g were exposed to 3 salinity trials which include caspian sea water (11- 11.5ppt), water of 7ppt salinity and freshwater (as control). each trial was done in 3 replicates. the blood samples and tissue fixations were taken from juveniles of control group (in freshwater) and 3, 6, 12, 24, 72, 168 and 240 hrs after exposure of fish in different salinities to determine the dynamics of some parameters that can give characteristic of osmoregulatory process and morphological changes of organs that take part in osmoregulation. the results of osmolality and ions measuring concurrently showed that all weight groups can live in salinity of 7ppt and they maintain the osmolality and ion concentrations. in the caspian sea water, weight groups excluding 5g juveniles showed the same result. the number and size of gill chloride cells were not changed significantly ( p >0.05) after 7 days of fish exposure in the caspian sea water and in water of 7ppt salinity for 5g juveniles, whereas within weight groups of 10, 15 and 20g in caspian sea water and groups of 15 and 20g in water of 7ppt salinity, the increase of chloride cells were observed ( p <0.05). na + , k + -atpase activity in juveniles of 5g in the caspian sea water was low (4.3-6.1μmol pi /mg protein/h) and this group was in parr stage. exposure of 15 and 20g fish to the caspian sea water increased activity of na + , k + -atpase after 3hrs. the results show that under the salinity condition of the caspian sea, juvenile fish (≈10g) are able to survive and adjust functions of their organs. increase in body size shows positive feedbacks thus release of caspian trout juvenile at this weight for stock rehabilitation or future cage culture is feasible and exerts no negative impact on fish.