The Norwegian salmon sector often uses Recirculation Aquaculture Systems (RAS ) to produce 500-700 g post-smolts, which are transferred to sea. This strategy shortens the seawater grow out phase in open net pens , thus helping to reduce the severity of problems like sea lice infestations , escapees or organic discharge in the environment. However , RAS are expensive and present a high biological complexity. In addition, RAS post-smolts can face difficulties adapting to sea water pens , potentially impacting their growth performance and mortality. As a result, some industry players are considering the use of Hybrid Flow-Through Systems (HFS) to produce post-smolts or market-size salmon . Using HFS aims to improve challenges resulting from the intense RAS conditions, yet still maintaining high control of the rearing environment . However, HFS are new systems, thus salmon growth and welfare in them must be assessed.
Our study aimed to document growth and welfare in a commercial batch reared at Bue AS HFS facility (Vestland, Norway), from smolt (90 g) in June 2023 to slaughter size (5 kg) in June 2024 . In addition, this performance was compared with groups of salmon transferred from the HFS to two traditional sea water sites, at 1 kg on 16 October 2023 ( to SW pens A) or at 2 kg on 10 December 2023 ( to SW pens B). Five samplings were carried out , 3 at Bue HFS only, and 2 more ( only one by abstract submission) in the three sites (n=15-20 fish per tank/sampling).
On 12 March 2024 (4th sampling), body w eight was over 50% higher at the HFS than at the two sea water sites (Fig. 1) , and SGR was almost double (Table I) . Weight and SGR in sites A and B were similar . Welfare-wise, s almon in site B displayed more winter ulcers, fin damage and mortality than the in HFS and site A , which had similar values. F eed utilization (FCR) was also worse in site B. Mean temperature was similar in the three sites, while stocking density was highest in the HFS, medium in site B and lowest in site A (Table I).
The better fish growth in HFS might be explained by the absence of handling and disturbance , plus the controlled environment in a land-based system . B oth groups A and B experienced transfer events that can induce stress , reduced growth, susceptibility to disease and mortality. In the case of site B, the transfer caused scale loss, more skin ulcers and mortality . In the case of site A, fish had the extra disturbance of one de-lousing treatment. In conclusion, lack of handling and a high-quality controlled environment (like in HFS) can boost salmon growth.