The use of photoperiod in the production of out-of-season Atlantic salmon ( Salmo salar) smolts

Accelerated seasonally-changing and “square-wave” (direct change from short to long day) photoperiods advanced the timing of potential S1 (1+) and S2 (2+) smolts by a maximum of 3 and 7 months respectively. Pre-smolts underwent body silvering, fin darkening and a decrease in condition factor in response to changes from short to long daylengths. Seawater challenge (24 h, 28 ppt) tests showed these fish to develop strong hypoosmoregulatory ability. Smolts were transferred to full strength seawater in small land-based facilities and to commercial on-growing sites. S1 groups, which were advanced by up to 3 months, gained a growth advantage from early seawater transfer that was maintained throughout their first year in seawater. Potential S2 smolts transferred in October–November grew well over their first winter. Advanced mixed sex S1s did not show any post-smolt maturation, but did show increased rates of maturation (up to 19%) after one winter at sea. Advanced S2 smolts showed an increased post-smolt maturation.