LAUSR

The present study assessed whether non-anadromous masu salmon Oncorhynchus masou in Miyazaki, southern Japan, smoltify, and if so, at what time of the year. Yearling O. masou of Miyazaki and an anadromous population from Hokkaido, northern Japan, were reared in hatcheries in their respective regions and sampled monthly from February to June to examine the spring smoltification period. The Hokkaido population showed a peak of gill Na+ -K+ -ATPase (NKA) activity in May, which was accompanied with an increase in mRNA levels of the seawater (SW)-type NKA alpha subunit, nkaα1b. Increases in gill NKA activity and nkaa1b levels were not seen in Miyazaki populations. Transferring yearling Miyazaki population to 70% SW (salinity of 23) in mid-April resulted in an increased serum osmolality over 4 days. These results suggest that they do not smoltify in their second spring. Next, profiles of gill NKA activity and its subunit mRNA levels in under-yearling Miyazaki population in the autumn were examined. Two phenotypes differing in body color during this period were categorized as parr and smolt-like fish. Smolt-like fish had higher gill NKA activity than parr in December while there was no significant difference in gill nkaα1b levels. Smolt-like fish acclimated to 70% SW better than parr as judged by lower serum osmolality. However, serum osmolality in smolt-like fish did not return to the basal level 7 days after transfer to 70% SW, suggesting that their hypo-osmoregulatory ability was not fully developed to a level comparable to anadromous populations of this species. The present study suggests that, if O. masou in Miyazaki go though a smoltification process, it occurs in its first autumn instead of the second spring and is less pronounced compared with anadromous populations.