osmoregulation ability in different sizes of caspian trout (salmo trutta caspius)parrs, with the same age, following direct transfer from fresh water to the caspian sea water

in order to determine the effects of fish size and weight on its salinity tolerance, the chloride cells (ccs) immunolocalization changes were examined in caspian salmon (salmo trutta caspius) parrs, with the same age (about 2 years old) but different types (type a: 4.88 g, 8.36 cm; type b: 14 g, 11.84 cm; type c: 24.05 g, 14.08 cm). fish survival rate, blood osmolality, gills ccs histological and immunohistochemical changes were investigated following their transfer from freshwater (fw) to the caspian sea water (csw). the survival rate increased in larger sizes and blood osmolality showed a tendency to increase in parallel with salinity. after 10 days in csw, some abnormalities were observed in gill structure such as: lamellae cohesion, lamellae rupture and separated lamella from filament epithelium that were shown in all types. these abnormalities in type b were less than the other types. gill ccs were observed on the gill filament and lamellae. in direct transfer of salmon parrs to the csw, changes in number, sectional area, and the surface occupied by ccs in the gill tissue were observed in all the three types. in the type b, the number of ccs did not change, however, in the type c, they decreased, while in the type a, there was significant increase. but in the csw, the occupied surface by gill ccs in the type c, was reduced significantly compared to other types. according to the present results, among the salmon parrs with the same age, the fish with type a lacked osmoregulation, while the type b had better compatibility with the csw. however, after reaching the size of the type c and considering osmultification, it is probable that the type b fish would become compatible with the fresh water environment and they will not have osmoregulation ability in saline water for the long term.