Interaction between body size and commencement of smoltification in masu salmon Oncorhynchus masou

This study analysed the relationship between body size and the commencement of smoltification in masu salmon Oncorhynchus masou. Juveniles of a strain of O. masou were reared under a simulated natural photoperiod (SNP) and an advanced photoperiod (AP), in which LD15:9 was interrupted by LD9:15 from June to October to induce spring smoltification. Regardless of the photoperiod, the underyearlings were just shorter than 12 cm in fork length (L_F) in October. When the photoperiod in the AP group was switched from short to long days in late October, juvenile O. masou exceeded 12 cm and exhibited increased gill Na⁺/K⁺-ATPase (NKA) activity in December. These changes were accompanied by an increase in circulating levels of insulin-like growth factor (IGF)-1 and IGF-binding protein-2b. In contrast, fish under SNP remained below 12 cm and had low gill NKA activity and hormone/protein levels from October to January. Underyearlings of the same strain reared in a larger tank under a natural photoperiod grew well and exhibited increased gill NKA activity in July when they exceeded 12 cm. These results suggest that an L_F of 12 cm is the threshold size above which juvenile O. masou increase gill NKA activity throughout the year. This implies that they may cease their growth during autumn and winter to avoid exceeding the threshold size required to commence smoltification during spring. Although the threshold size may be related to rearing density or tank size, the findings of the present study suggest that increases in gill NKA activity during summer and the following spring are tightly linked to body size in O. masou.