4.5 Article

Effect of Photoperiod and Transfer Time on Atlantic Salmon Smolt Quality and Growth in Freshwater and Seawater Aquaculture Systems

Journal

FISHES
Volume 8, Issue 4, Pages -

Publisher

MDPI
DOI: 10.3390/fishes8040212

Keywords

smoltification; salmon aquaculture; gill NKA activity; compensatory growth; specific growth rate

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In this study, the effects of two freshwater photoperiod regimes and four transfer times to seawater on Atlantic salmon smolt development and growth were assessed. The results showed that smoltification and growth in freshwater were slightly affected by the photoperiod treatment but greatly influenced by low water temperature. The growth rate in seawater was highest in the groups that experienced delayed growth in freshwater, indicating the potential benefits of transferring larger smolts to seawater.
Smoltification is a key process in Atlantic salmon aquaculture, given it prepares the fish for a successful transit from fresh to seawater. However, industry players have not yet reached a consensus on the best protocols to produce high-quality smolts. In this study, we assessed how the combination of two photoperiod regimes in freshwater (continuous light or LL, and natural photoperiod or LDN) and four transfer times to seawater (February, March, April, and May) affected smolt development and their subsequent growth in seawater until slaughter during commercial production. The results demonstrated that smoltification and growth in freshwater were only slightly modulated by the photoperiod treatment and were instead much more affected by the limiting effect of the low water temperature during that period. In seawater, the growth rate was the highest in the same groups, which had, however, experienced a delay in growth when in freshwater, and consequently, no differences in the final body weight between the eight treatments were found. Such compensatory growth in the sea was probably enhanced by the increasing smolt quality, which could allow for better performance in seawater. A significant link between the weight at slaughter and weight at transfer was observed only in the groups with a lower smolt quality (LL-Feb, LDN-Feb and LDN-Mar), which suggests that larger individuals could cope better with a saline environment. In contrast, smaller smolts probably suffered greater osmotic stress that hindered their performance at sea. Afterwards, as smolt quality increased in the subsequent transfer groups, the relevance of this size effect decreased. This means that the industry may benefit from transferring larger smolts to seawater, especially if these are suspected of having developed suboptimal seawater tolerance. Those individuals are likely to cope better with saline conditions than smaller smolts. Future research should focus on the possible long-term effects of freshwater-rearing regimes on smolt performance in the seawater phase.

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