The salmon farming industry has invested in larger and more technologically sophisticated RAS facilities over the last decade. Current practices favour constant high temperatures, use of continuous light and increased salinity in RAS to accelerate the growth of Atlantic salmon smolts from 50-80 grams a few years ago to approximately 500-600 grams today. The anadromous salmon goes through parr-smolt transformation (PST), which entails a series of morphological and physiological changes preparing them for seawater. However, industry reports several biological traits, such as increased silvering, elevated gill NKA activity and ability to ion regulate when challenged with SW, associated with PST to occur independent of applying standard photoperiod protocols. This have made it increasingly difficult to accurately time the “smolt window” and evaluate quality of large smolts (>250 gram). Thus, there is a need to better understand the physiological responses of larger smolts produced under intensive conditions in RAS.
Material and method
Here we present the effects of rearing protocols using four different photoperiod regimes on three different sized (150g, 350g, 800g) in freshwater or brackish water (12 ‰). The photoperiodic regimes were: No winter signal (NW), 6 weeks LD 12:12 winter signal given early (EW), 6 weeks LD 12:12 winter signal given late (LW) and 20 weeks LD 12:12 winter signal given late (LLW). We analysed how these protocols affect smolt development, measured as Na+; K+ -ATPase (NKA) enzyme activity in the three major osmoregulatory organs, gills, intestine, and kidney. Smolt development was also evaluated with a short-term seawater performance challenge test.
Results and discussion
Our findings suggest that a more holistic approach to evaluate overall smolt quality in modern aquaculture would be beneficial, in particular when producing large smolts and post-smolts. Our findings were also analysed in the context of long-term seawater performance, health and welfare.