Photoperiod revisited: is there a critical day length for triggering a complete parr–smolt transformation in Atlantic salmon Salmo salar?

• Published in: Journal of Fish Biology
• Main Authors: Strand, Jo E. T., Hazlerigg, David, Jørgensen, Even H.
• Other Authors: UiT the Arctic University of Norway, Tromsø, Norway
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2018
• Subjects: Atlantic salmon; Salmo salar
• Online Access: http://dx.doi.org/10.1111/jfb.13760; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fjfb.13760; https://onlinelibrary.wiley.com/doi/pdf/10.1111/jfb.13760

The present study investigated whether there is a critical length of photoperiod needed to stimulate a completed parr–smolt transformation (PST) in Atlantic salmon Salmo salar . In two experiments, S. salar parr of the Norwegian aquaculture strain held on continuous light were exposed to a short photoperiod (6 L:18D) followed by exposure to 8 L:16D, 12 L:12D, 16 L:8D, 20 L:4D and 24 L:0D in experiment 1 or to 6 L:18D followed by maintenance on 6 L:18D or exposure to 12 L:12D and 24 L:0D photoperiods in experiment 2. All groups, irrespective of photoperiod treatment, developed improved hypo‐osmoregulatory ability. However, the development was greatest in the groups exposed to 20 L:4D and 24 L:0D in experiment 1 and 24 L:0D in experiment 2. In experiment 2, gill Na + – K + ‐ATPase activity increased in the group exposed to 24 L:0D, but not in the groups exposed to 12 L:12D and 6 L:18D. The groups exposed to 20 L:4D and 24 L:0D in experiment 1 and 24 L:0D in experiment 2 also grew better than fish exposed to shorter photoperiods. In experiment 2 only the group exposed to 24 L:0D showed a decrease in condition factor and increases in plasma growth hormone and brain type 2 deiodinase mRNA abundance. Hence, only the groups exposed to photoperiods above 16 L:8D developed classical smolt indices in the present experiment, leading us to conclude that the photoperiod increase needs to exceed 16 h daylight for stimulating a complete PST in the S. salar used in the present study.