Producing a better smolt: Can ‘winter’ treatments combining low temperature and short daylength photoperiods enhance the smolt quality of Atlantic salmon?

The parr to smolt transition (PST) is a critical life stage for Atlantic salmon ( Salmo salar ) aquaculture. Aquaculture commonly uses artificial square wave photoperiod treatments to initiate the PST but faces problems related to poor smolt quality, including elevated mortality after seawater transfer and increased disease prevalence. We investigated whether the use of an additional seasonal cue, temperature, in combination with short daylength photoperiod treatments improved smolt quality. We exposed ∼35 g parr to an experimental treatment involving a 3 × 2 factorial design including ‘winter’ photoperiod treatments of 24L:00D, 12L:12D, and 08L:16D combined with a ‘winter’ temperature treatment of 14 °C or 8 °C. Short daylength ‘winter’ photoperiods (12L:12D and 08L:16D) led to improved measures of smolt status such as gill Na + /K + -ATPase activity, seawater Na + /K + -ATPase isoform expression in the gills, and plasma growth hormone levels when compared to salmon kept in constant light while reduced ‘winter’ temperature (8 °C vs 14 °C) had few impacts on smolt status measures. Despite this, there were no apparent benefits of exposure to short ‘winter’ photoperiods (12L:12D or 08L:16D) compared to constant light on whole organism osmoregulation (plasma osmolality and ion content, white muscle water content) after seawater transfer. Both short daylength ‘winter’ photoperiods (12L:12D or 08L:16D) and reduced ‘winter’ temperatures (8 °C) decreased growth rates in freshwater compared to salmon kept in constant light and temperature, but only salmon exposed to short daylength ‘winter’ photoperiods had enhanced growth in seawater. Our results indicate that the use of a reduced ‘winter’ temperature treatment in combination with short daylength ‘winter’ photoperiods does not enhance smolt quality in aquaculture production of Atlantic salmon when compared to the use of short daylength ‘winter’ photoperiods alone. This highlights a continued need for research into alternative strategies to improve the robustness of salmon smolts. • A ‘winter’ temperature treatment of 8 °C did not improve smolt quality regardless of ‘winter’ photoperiod treatment • Short daylength ‘winter’ photoperiods increased gill Na + /K + -ATPase activity and the ratio of seawater to freshwater iosoform expression. • Short ‘winter’ photoperiods reduced SGR and FCR in freshwater but improved them during the 1st month after SW transfer. • Short ‘winter’ photoperiod led to elevated ṀO 2min and ṀO 2max but no overall effect on aerobic scope or hypoxia tolerance • Further research is still required to optimize smolt production and improve smolt quality at the time of seawater transfer.