EFFECT OF 6-MONTH, 12-MONTH, AND 18-MONTH PHOTOPERIOD CYCLES ON SMOLTING AND SEXUAL-MATURATION IN JUVENILE ATLANTIC SALMON (SALMO-SALAR)

The results support the hypothesis that the commencement of smolting can occur during the decreasing phase of the annual photoperiod cycle, and sexual maturation during the increasing phase of the cycle, provided individuals have attained certain (unknown) growth thresholds. Thereafter the completion of smolting is entrained by the increasing phase of the photoperiod cycle, and sexual maturation by the decreasing phase. Three groups of Atlantic salmon (Salmo salar) were raised in freshwater for over 2 yr from the eyed egg stage under photoperiod cycles of either 6-, 12- (control), or 18-mo periodicity and an ambient 12-mo temperature cycle. Smolting was judged by changes in salinity tolerance and condition factor. All groups developed bimodal length-frequency distributions by December following hatch. Fish continued to be recruited into the upper modal group (UMG) beyond the shortest day of the photoperiod cycle, providing temperature was not limiting growth. The 6-, 1 2-, and 18-mo photoperiod cycles resulted in approximately 50, 60, and 100% of the populations being recruited into the UMG. Sexually mature male parr (1 + yr old) occurred only in the lower modal group of the 6- and 12-mo groups.