INFLUENCE OF PHOTOPERIOD ON GROWTH CHANGES IN JUVENILE ATLANTIC SALMON, SALMO-SALAR L

During their first 6 months sibling Atlantic salmon parr, Salmo salar L., grew larger under constant light than under natural photoperiod or simulated natural photoperiod (control). When rate of change of photoperiod was accelerated after midsummer, .times. 2, .times. 3 and .times. 4, there were no growth differences between the three groups, but all were smaller than the control population. Under constant autumn photoperiod of 8 h light: 16 h dark growth was less than under all other experimental photoperiod conditions. Mean length was directly correlated with total hours of daylight experienced, excluding those fish kept under constant light. Fish reared from first feeding under photoperiod regimes delayed 6 and 9 months out of phase with the natural light cycle were smaller than the controls, whereas those under a regime 3 months out of phase did not differ from the controls. The clear segregation of modal length groups within the 3, 6 and 9 months out-of-phase populations occurred 1, 4 and 4 months, respectively, after the segregation in the control group. Under constant light, and under constant 12 h light:12 h dark (12 LD), the segregation was delayed 3 and 4 months, respectively. The proportion of the population which maintained growth (upper model group) was significantly less in the 9 and 6 months out-of-phase and 12 LD groups (39, 40 and 42%, respectively) than in the other three groups (82.5 - 85%). Acceleration of photoperiod change also resulted in decreased growth. The results support a model of salmon development in which, 2-3 months after first feeding, growth is maintained if feeding opportunities at that time are above a threshold level, and in which this critical timing is influenced by photoperiod. It is suggested that the delays reflect a synchronizing effect of photoperiod on an endogenous rhythm of appetite and growth. The differences in upper modal group proportions observed in the present experiments, reflect the relative feeding opportunities available at the critical period in July-August.