Does size matter? The importance of size and winter signal for the smoltification of Atlantic salmon (Salmo salar, Linnaeus 1758)

Abstract

In Atlantic salmon (Salmo salar L.) aquaculture, there has been a trend toward producing larger smolt in freshwater (FW) as a way to reduce production time in open net-pens at sea (SW). Earlier, the common way to induce smoltification in commercial smolt facilities was via photoperiodic manipulation, by exposing them to a short-day (12h light:12h dark, LD) winter signal (WS) for 6-8 weeks before returning to continuous light (24L:0D, LL). As the smolt size is increased, the WS is introduced at a larger body size than before. This study investigates the effect of fish size at the initiation of WS and its significance for the smoltification process. Two different size groups, small (S – 60 g) and large (B – 300 g), were maintained under LL until the desired size was reached. Then, half of each group was either kept on LL as a control group (S-LL and B-LL), while the remaining fish were exposed to a 6 weeks WS before returning to LL for 570-degree days (dd) (S-LD and B-LD). Length and weight measurements were performed before initiation of WS and at the end of the experiment. Samplings were taken at 0-, 80-, 290-, and 515 dd post-WS, where weight, length, gill-, kidney-, and intestinal Na⁺/K⁺-ATPase (NKA) activity, plasma cortisol, and plasma chloride were measured. At 515 dd post-WS, a 24-hour SW challenge test was conducted.

The groups maintained under LL exhibited better growth in FW than those exposed to a WS, and S groups showed better growth than B groups. In addition, we observed characteristic signs of smoltification in the S-LD group, with a significant increase in gill NKA activity and simultaneous decrease in Fulton's condition factor (K) after return to LL. Concurrently, the S-LL group had continuously elevated gill NKA activity as expected for Atlantic salmon kept under LL. However, there was no observed change in gill NKA-activity and K associated with smoltification in the B-LD and B-LL group.

Our results suggest that large juvenile Atlantic salmon (~300 g) exposed to a 6-week WS does not exhibit a classic smoltification response, based on measured change in gill NKA-activity and K.