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The effects of fish weight on salinity tolerance were studied in Caspian salmon (Salmo trutta caspius) parr. 180 fish (all with 2 years old but with three weights; 5, 15, 25g) were selected and they reared in freshwater (FW) and brackish water (BW; 13ppt salinity) for 10 days. The mRNA expression of two α-subunit isoforms of Na⁺, K⁺-ATPase (α1a and α1b) and NKCC co-transporters were studied in their gill tissue. In all three weight groups, the mRNA levels for the α1a isoforms decreased following BW exposure, whereas α1b levels significantly increased in 15g and 25g groups. In addition, NKCC gene expression were significantly higher in the groups of BW than FW in 15g and 25g weights (P<0.05). The reciprocal expression of Na⁺, K⁺-ATPase isoforms (α1a and α1b) during salinity acclimation suggests that they may have different roles in the gill of FW and BW fishes; ion uptake in FW and ion secretion in BW. In conclusion, in the Caspian salmon, between parrs with the same age, the group with the weight of 15g possesses better compatibility with BW than to other groups.  After reaching to 25g, fish passed smoltification and they became more compatible with the FW environment and maybe lost its osmoregulation ability in saline or brackish water.

Keywords: Caspian Salmon, Na+-K+ pump, Osmoregulation

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