Current Atlantic salmon farming practice induces early smoltification with artificial photoperiod regimes, however the importance of these photoperiods on parr maturation and interactions with smoltification are poorly understood. These questions were addressed in the present investigation, which examined the effects of photoperiod manipulation on the development, maturation and smoltification of individually tagged parr. Approximately 9000 salmon parr from a high grilsing stock were exposed to continuous light (LL) from first feeding. Three sub-groups of 2400 parr, each sub-group in triplicate tanks, were then exposed to an 8 week “winter photoperiod” (LD 10:14) starting on either the 18th May, the 9th August or the 20th September (defined respectively as the May, August and September groups). Following the artificial winter each group was returned to LL. A fourth group of 1600 fish was maintained in replicate tanks on LL throughout. The highest levels of maturation (approx. 20%) were recorded in the May group. August and September groups showed low levels of maturity
