The entrainment role of photoperiod on hypoosmoregulatory and growth-related aspects of smolting in Atlantic salmon (Salmo salar)

Potential yearling (1+) smolts were maintained under 8.25 h light: 15.75 h dark and constant temperature (10.0 °C) from late December. Groups were subjected to an abrupt increase to 16 h light: 8 h dark on December 31 (group A), February 1 (group B), March 1 (group C), or March 31 (group D). Group E was maintained under constant 8.25 h light: 15.75 h dark for the duration of the experiment and group LDN was maintained under a simulated natural photoperiod cycle (45°N). Plasma osmolality levels following 24-h, 29‰ salinity challenge tests indicated a photoperiod-independent development of hypoosmoregulatory mechanisms preceding completion of smoltification that was significantly correlated with fish body size. As judged by 96-h, 37.5‰ salinity tolerance tests and changes in condition factor, completion of smolting occurred in sequence; in groups A and B it was advanced to late February, while groups C and D completed smolting in mid-March and mid-April, respectively, compared with late May for group LDN. Group E, maintained under constant 8.25 h light: 15.75 h dark, developed salinity tolerance in late May, but unlike other groups exhibited no large reduction in condition factor. Following smolting, an increase in condition factor and a loss of salinity tolerance occurred in all groups. The results support the hypothesis that changes in photoperiod entrain an endogenous circannual rhythm involved in controlling the completion of smoltification and subsequent loss of some smolt characteristics.