Assessment of the timing and degree of smolt development in southern populations of masu salmon Oncorhynchus masou

Abstract

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.

Anadromy, potamodromy and residency in brown trout Salmo trutta: the role of genes and the environment

“…Downregulation of the α1a subunit and upregulation of the α1b occur while the fish are still in fresh water and this occurs prior to the increase in NKA activity in S. trutta (Seidelin et al ., ), indicating that the migration destination is pre‐determined. Non‐anadromous Oncorhynchus masou (Brevoort 1856) were found to show an increase in NKA activity in smolt‐like individuals but, unlike anadromous individuals, this was not accompanied by an increase in the α1b isoform (Inatani et al ., ).…”

“…Although often seen only as a preparation for anadromy, there is increasing evidence that many similar changes occur for potamodromous migrants. Thus, increase in NKA activity, widely used as an indicator of anadromous smoltification, occurs also in potamodromous migrants (Boel et al ., ; Inatani et al ., ), as do changes in behaviour, skin pigmentation and body morphology (Table ). Whether this increase in NKA in potamodromous migrants has functional significance or simply reflects ancestral standing genetic variation is not known.…”

Advances in our understanding of the parr–smolt transformation of juvenile salmon: a summary of the 10th International Workshop on Salmon Smoltification

“…Inatani et al . () found that anadromous O. masou from the north of Japan had increased gill Na + –K + ‐ATPase activity and salinity tolerance in spring, whereas fish from landlocked populations in the south did not change in spring but showed modest increases in smolt appearance and salinity tolerance in autumn. It is less clear if populations in the same river catchment have evolved differential traits related to smolt development.…”