The effect of photoperiod manipulation on Atlantic salmon growth, smoltification and sexual maturation: A case study of a commercial RAS

The effect of photoperiod manipulation on growth, smoltification and maturation was assessed in Atlantic salmon through an observational study performed in a commercial RAS facility from June (25.0 ± 11.0 g) to October 2018 (151.1 ± 25.5 g). Half of the commercial cohort 721 was raised in continuous light LD24:0 (LL) while the other received a 6-week LD12:12 winter signal (WS) for smoltification from 12 August to 21 September. Parameters related to growth (body weight, gene expression of pituitary gh1 and gh2, and liver ghr1, igf1 and igfbp1a), smoltification (condition factor, plasma sodium and cortisol, gill NKA activity and nka1a, nka1b and nkcc1a expression) and maturation (GSI and pituitary fshb and lhb expression) were analysed. Afterwards, a multivariate analysis was performed on production data from five commercial cohorts raised in the facility (including 721) to identify variables potentially linked to early maturation in RAS. Results from the observational study indicated weak compensatory growth and slightly better smoltification in WS, although signs of size-induced smoltification were present in LL. Smoltification indicators were poor in both treatments, suggesting that smolts may not be yet ready for seawater. No maturation was observed in any photoperiod treatment; however, the multivariate assessment suggested that such lack of maturation might be rather linked to the low mean temperature and SGR experienced by our cohort