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

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 (6L:18D) followed by exposure to 8L:16D, 12L:12D, 16L:8D, 20L:4D and 24L:0D in experiment 1 or to 6L:18D followed by maintenance 6L:18D or exposure to 12L:12D and 24L: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 20L:4D and 24L:0D in experiment 1 and 24L:0D in experiment 2. In experiment 2, gill Na+ - K+ -ATPase activity increased in the group exposed to 24L:0D, but not in the groups exposed to 12L:12D and 6L:18D. The groups exposed to 20L:4D and 24L:0D in experiment 1 and 24L:0D in experiment 2 also grew better than fish exposed to shorter photoperiods. In experiment 2 only the group exposed to 24L: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 16L: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. This article is protected by copyright. All rights reserved.