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

Abstract

Smolting 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 constantlong photoperiod (LD 17:7), or LD17:7 interrupted by LD7:17 for one (1mo-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%). Smolt development in autumn was highest in the 2mo-short group, but was inferior to normal spring smolts, 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 smolts.