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 day-lengths. Seawater challenge (24 h, 28 ppt) tests showed these fish to develop strong hypo-osmoregulatory 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.