Exposure to a winter photoperiod to produce large Atlantic salmon smolts (Salmo salar) increases energetic costs and reduces hypoxia tolerance during seawater transfer

A goal of salmon aquaculture is to produce larger, physiologically robust smolts to improve performance at time of seawater transfer and reduce time spent in marine net pens. However, little is known about whether standard processes to induce smoltification provide benefits in the production of large smolts. In Atlantic salmon (Salmo salar) reared under continuous light (24 L:0D) we applied an artificial photoperiod treatment (8 weeks of 12 L:12D followed by 4 weeks of 24 L:0D before seawater transfer) at three sizes to produce fish of similar to 150-200 g, similar to 400-600 g, and similar to 1200-1300 g at seawater transfer. We then measured O-2 consumption (a proxy for metabolic rate) of photoperiod treated and control salmon (kept in constant 24 L:0D until time of seawater transfer) at three time points (in freshwater prior to seawater entry, 24 h after seawater entry, and 1 month after seawater entry) to assess impacts of photoperiod treatment and seawater transfer on performance. Consistent with known higher metabolic rates in smolts vs parr, photoperiod treatment (12 L:12D) increased minimum O-2 consumption ((M) over dot O-2min) by similar to 13% across all three sizes and at all time points measured. Increased (M) over dot O-2min was not solely a result of increased growth rates and was associated with lower hypoxia tolerance at time of seawater transfer in -500 g and - 1200 g salmon. Transfer to seawater reduced maximum O2 consumption ((M) over dotO(2max)) acutely in -200 g and - 500 g salmon regardless of the photoperiod treatment (12 L:12D or 24 L:0D), consistent with potential ion and osmotic disturbances leading to an osmo-respiratory compromise 24 h after seawater transfer. For both sizes of -200 g and - 500 g, (M) over dotO(2max) recovered to freshwater levels 1 month after seawater transfer. In contrast, -1200 g salmon showed no significant changes in (M) over dotO(2max) following seawater transfer. Overall, we showed little benefits of traditional photoperiod treatments (12 L:12D) on performance of large salmon following seawater transfer. In fact, photoperiod treatment (12 L:12D) may lead to negative impacts on large salmon by increasing (M) over dot(2min) and sensitivity to environmental stress such as hypoxia. Our results, combined with other recent studies, suggest that traditional photoperiod manipulations may be unnecessary for the development of larger Atlantic salmon smolts. However, further research is needed to fully understand impacts of photoperiod manipulations on physiological performance of larger salmon at time of seawater transfer.