Abstract
AbstractProducing a larger post‐smolt in recirculating aquaculture systems (RAS) could shorten the production time in sea cages and potentially reduce mortality. Knowledge of the biological requirements of post‐smolts in closed‐containment systems is however lacking. In the present study, the effects of salinity and water velocity on growth, survival, health, and welfare of Atlantic salmon reared in RAS were examined. Salmon smolts were stocked in three separate RAS with salinities of 12, 22, and 32‰ and subjected to high (1.0 body lengths per s−1) or low (0.3 body lengths second−1) water velocity. Growth performance, survival, welfare, and physiological stress responses were monitored until the fish reached a bodyweight of around 450 g. Growth rate was higher at lower salinity and higher water velocity generally had a positive effect on growth in all salinities. Feed conversion ratio was lower at 12‰ compared to the 22 and 32‰ when the fish were between 250 and 450 g. Higher mortality, elevated plasma cortisol levels, higher incidence of cataract, and a higher expression of stress‐induced genes in the skin (iNOS, Muc5ac‐like) indicated a negative effect of higher salinity on fish welfare. Male maturation was low (<1%), and not affected by salinity or water velocity.
Highlights
Of the Atlantic salmon (Salmo salar) smolts transferred to sea cages in Norway, an average of 16% is lost before they reach harvest size, and smolt quality and infections are the most important factors explaining the mortalities (Bleie & Skrudland, 2014)
The present study examined the effects and interactions of salinity and water velocity on performance and welfare of large post-smolts of Atlantic salmon reared in brackish (12 and 22 ppt) and seawater recirculating aquaculture systems (RAS) (32 ppt)
High water velocity only had a significant effect on the analyzed genes at 250 g (ANOVA p < .05), inducible nitric oxide synthase (iNOS) mRNA expression was lower in the skin of fish on high velocity compared to fish at low velocity
Summary
Of the Atlantic salmon (Salmo salar) smolts transferred to sea cages in Norway, an average of 16% is lost before they reach harvest size, and smolt quality and infections are the most important factors explaining the mortalities (Bleie & Skrudland, 2014). Most losses occur shortly after seawater transfer, so producing a larger and potentially more robust fish may improve survival and growth after sea transfer. Another major constraint for growth in the salmon farming industry is challenges related to parasites such as sea lice. Seawater RAS may have higher operating costs compared to freshwater (FW) or brackish water RAS, because of the lower efficiency of carbon dioxide (Moran, 2010) and ammonia removal in seawater (Bakke et al, 2017; Chen, Ling, & Blancheton, 2006; Nijhof & Bovendeur, 1990). The present study examined the effects and interactions of salinity and water velocity on performance and welfare of large post-smolts of Atlantic salmon reared in brackish (12 and 22 ppt) and seawater RAS (32 ppt). Because metals may accumulate in the water in RAS facilities (Davidson, Good, Welsh, Brazil, & Summerfelt, 2009), accumulation of metals on the gill epithelium was measured
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