Differential Seawater Adaptability in Three Different Sizes of Under-yearling Steelhead Trout.

Seawater adaptability of steelhead trout increases along with the increase in the size of the fish, independent of parr-smolt transformation. Three 96 h seawater challenge tests were conducted to determine the size at which seawater adaptability of steelhead trout develops. Plasma Na and Cl levels, moisture content, gill Na/K ATPase activity, and mortality during the 96 h after direct transfer to seawater (32 ppt) were determined. Plasma Na and Cl levels in 50 g fish continuously increased during the 96 h after the transfer to seawater (<0.05), but the levels in 100 and 150 g fish leveled off after 24 h (<0.05). Both 100 and 150 g size steelhead trout maintained muscle moisture content (%) better than 50 g size fish (<0.05). Gill Na/K ATPase activity in the 100 g size group increased in a time-dependent manner after transfer to seawater (<0.05), whereas activity in the 50 and 150 g sizes did not increase (>0.05), for which a possible explanation was discussed. A mere 2.6% mortality in both the 50 and 150 g size groups was observed. In conclusion, the current results indicate that 50 g size steelhead trout did not show development of a high level of hypoosmoregulatory capacity, whereas fish in the 100 and 150 g size groups showed a high level in our experimental conditions. Therefore, the steelhead trout larger than a 100 g size is recommended for transfer to seawater culture.