Environmental and biological requirements of post-smolt Atlantic salmon (Salmo salar L.) in closed-containment aquaculture systems

Production of Atlantic salmon (Salmo salar L.), exceeds 2 million tonnes globally, and accounts for 90% of the salmon on the market. Presently, the predominant production of post-smolt Atlantic salmon occurs in open sea cages. Environmental concerns, during the production phase at sea are limiting further growth of the industry. In closed-containment aquaculture systems (CCS), the cultured fish are separated from the natural environment by a physical barrier; these can be land-based systems or closed units in the sea. Using CCS to shorten the time fish are reared in open sea cages has been highlighted as key to solving important challenges the industry is facing today. However, there is a lack of knowledge on the biological and environmental requirements of post-smolt Atlantic salmon in CCS. This thesis aims to provide insight on the effects of some key husbandry conditions on post-smolt performance and welfare in both seaand land-based CCS. Commercial feasibility of farming post-smolt Atlantic salmon in CCS in the sea relies on maximizing fish density. To assess stocking density limits, five different densities (25, 50, 75, 100 and 125 kg m-3) were maintained in flow-through seawater systems for eight weeks. Increased stocking density had a negative effect on growth and feed utilization, and increasing density from 100 kg m-3 to 125 kg m-3 lead to a 42 % decrease in growth rate. After eight weeks, primary (elevated plasma cortisol) and secondary (hydro-mineral and acid-base) stress responses were observed in the highest density treatment compared to other treatments. Densities of 100 kg m-3 or more also increased pectoral fin damage and cataracts. Fish stocked at the medium (75 kg m-3) density displayed more robust telencephalic activation of both stress and neural plasticity responses, compared to fish in the lowest (25 kg m-3) and highest (125 kg m-3) densities. Overall, the results suggest that stocking density can be maximized up 75 kg m-3 without compromising performance and welfare in Atlantic salmon post-smolts in CCS in sea. Given the peak expression of genes that are important for cognition and memory, densities around 75 kg m-3 may in fact be optimal for welfare. Determining the mass-specific water flow (SWF) required by post-smolts will largely influence the design and dimensioning of closed-containment systems in the sea. Prominent physiological regulatory responses to increased water carbon dioxide were observed in post-smolts kept in the lowest SWF (0.2 L kg fish-1 min-1) in sea water flow-through systems. At these conditions the fish were able to compensate and maintain growth within the entire eight week period studied. However, the responses observed have an energetic cost revealed by increased oxygen consumption. Overall, it can be recommended that without any in-tank water treatment, specific water flow should be maintained above 0.3 L kg fish-1 min-1 as physiological regulatory responses are energy costly and reduced SWF can have a negative effect on other factors, not studied in this thesis, such as skin quality. Furthermore, our results suggest that fish density affects the ability of fish to react to additional challenges, more than in the case of specific water flow. This thesis also identifies several markers that when combined with an acute challenge test consistently reveals effects of pre-existing environmental conditions, and can be used to predict the fish´s resilience and potential for adaptation to changes in their environment. The optimal strategy for rearing large post-smolts in land-based recirculating aquaculture systems (RAS), with respect to salinity, water velocity and timing of seawater transfer is not known. In a long-term study, from 70 g up to 800 g, post-smolts were reared in three separate RAS at different salinities (12, 22 and 32‰) and subjected to moderate (~1 bl s-1) or low (~0.3 bl s-1) water velocity. Results suggest that salinity isotonic to the fish (12‰) and moderate exercise training has a positive effect on post-smolt growth, feed efficiency, welfare and survival in RAS. At 250 and 800 g all treatments handled sea water transfer, at 450g handling and transfer caused high mortality in several treatments. Using water with salinity around 12‰ may therefore be an advantageous production strategy for large post-smolts in RAS provided that the post-smolts can handle the subsequent transfer to open sea cages, and this needs further investigation. In conclusion, the knowledge gained on key husbandry conditions described in this thesis will contribute towards optimizing post-smolt Atlantic salmon production in closed-containment aquaculture systems on land and in the sea