Photoperiod control and influence of body size on off-season parr–smolt transformation and post-smolt growth

Abstract

To examine the combined effects of photoperiod and body size on off-season parr–smolt transformation and post-smolt growth, groups of small and large graded potential Atlantic salmon smolts (initial size 10 and 23 g) were raised under four different photoperiods between 26 July and 14 November. Photoperiod treatments included a 10 months phase-advance of simulated natural photoperiod (SNPa, from LD12:12 to LD24:0 over 16 weeks); two square wave photoperiod regimes; which were continuously light interrupted by LD12:12 for 9 (9) or 12 (12) weeks, respectively; and continuous light (LD24:0). Photoperiod manipulation significantly influenced growth and hypo-osmoregulatory ability. Between July and November, smolts raised on SNPa and LD12:12 (9) showed an increase in seawater tolerance, concurrent with an increase in Na +, K +-ATPase activity and a decrease in condition factor (CF). No differences in plasma chloride levels were seen among the small and large graded smolts (SG and LG), whereas a higher Na +, K +-ATPase activity was recorded in the small smolts at weeks 12, 14, and 16. Higher growth rates were seen in the small smolts during the freshwater period. Continuous light increased growth in freshwater, but interfered with smolt development. Following transfer to seawater, both SG and LG previously held under SNPa and LD12:12 (9) grew better than the fish reared under LD24:0, which showed a decrease in growth. The high growth rates in seawater in the SNPa and LD12:12 (9) groups concur with the good smolt status prior to transfer. Our findings indicate that both SNPa and LD12:12 (9) can be recommended in the production of off-season salmon smolts and among the smolt parameters monitored, gill Na +, K +-ATPase activity appears to be the most reliable indicator of smolt status.