Advancing smolting to autumn in age 0+ Atlantic salmon by photoperiod, and long-term performance in sea water

Smelting in autumn in age 0 + Salmo salar was improved by 2 months of short photoperiod in an otherwise constant long photoperiod regime. Four groups of fry were reared under either constant-long photoperiod (LD 17:7), or LD17:7 interrupted by LD7:17 for one (Imo-short; July 27-September 1) or 2 months (2mo-short; June 30-September 1), or simulated natural day length (LDN). Bimodality in size-frequency developed in all groups by August. Probability of an individual entering the upper-modal group (UMG) was related positively to fork length in late June. The LDN regime had the highest proportion of both lower-modal group fish (46% vs. 25-27%), and 0 + sexually mature males of the four groups (26% vs. 10-12%). Smelt development in autumn was highest in the 2mo-short group, but was inferior to normal spring smelts, as judged by hypoosmoregulatory ability and gill Na+ K+ ATPase activity. Following transfer of UMG fish to a sea-cage in November, the 2mo-short group exhibited best survival and growth over winter. From spring onwards all groups grew well, reaching harvest size (3-4 kg) in August after 20 months in sea water. Mortality in sea water was 24% and 28% in the 2mo- and 1mo-short groups, and 54% and 67% in the constant-long and LDN groups, The study indicated the feasibility of stabilising the undesirable seasonal fluctuation in commercial salmon production by producing 'off-season' smolts 6 months out-of-phase from regular spring smelts.