Physiological changes in off-season smolts induced by photoperiod manipulation in masu salmon (Oncorhynchus masou)

Abstract

We induced season independent smolts in masu salmon (Oncorhynchus masou) by rearing juveniles under an advanced photoperiod (AP) in which LD15:9 was interrupted by LD9:15 from June for 4 months. AP fish showed higher degree of skin slivering from November than fish reared under a simulated natural photoperiod (SNP) and became larger in length and lower in condition factor in January. Gill Na+,K+-ATPase in AP fish started increasing in November and peaked in December to typical spring smolt levels while it remained low in SNP fish. In addition, AP fish had higher hypo-osmoregulatory ability than SNP fish, as judged by the lower serum sodium and chloride concentrations 48 h after seawater transfer. Serum levels of insulin-like growth factor (IGF)-I and IGF-binding protein (IGFBP)-1b, components of the somatotropic axis, in AP fish peaked in November and January, respectively. Serum IGFBP-1b levels were increased 48 h after transfer to 70% seawater in both groups, suggesting that it is not a quantitative index of hypo-osmoregulatory ability in the short-term but a marker of osmotic stress. The present study showed that off-season smolts could be induced in underyearling masu salmon by a square-wave photoperiod, and off-season smolts showed morphological and physiological changes comparable to yearling spring smolts. Increased levels of circulating IGF-I and IGFBP-1b in smolting fish suggest that these endocrine parameters are involved in the smoltification process.