Relationship between weight and osmoregulation ability in Salmo trutta caspius fries, after introduced to brackish water (13ppt)

Fish age and weight, are important factors in osmoltification and adaptation to sea water. In this study, mortality rate, gill morphological and chloride cell changes were examined in different weights (5, 15, 25 g) but same age in Salmo trutta caspius after introduced to Caspian Sea water for 10 days. Gill histology was observed through light microscopy using Hematoxylin-Eosin staining and immunolocalization was performed by using mouse monoclonal antibody (IgGα5) raised against Na+K+ATPase α-subunit. Immuno micrographs were analyzed by using Image Tools Software. Mortality rate was 25%, 8% and 0% in 5, 15 and 25g weights, respectively after were transferred to 13ppt salinity and some structural damages were observed in gill structure. Chloride cell number in 5g weight, were significantly increased but sectional area were decreased, and in 15g weight, were showed no significant difference in chloride cell number, while in 25g weight were decreased. With regard to the present results, In the same age fishes, fish with 5g weight is not able to have osmoregulation but 15g weight it has better compatibility with the salinity and after acclimated to freshwater until 25g weight, considering the pass of osmultification level. It is probable that the fish became compatible with the fresh water environment and did not lose its osmoregulation ability in saline water in the long term.