A Case Study Documenting Growth and Welfare of Atlantic Salmon Reared to Market Size in a Novel Commercial Hybrid Flow-Through System in Norway: A Comparison With Fish Performance in Traditional Sea Sites

Production of harvest-size Atlantic salmon in land-based hybrid flow-through systems (HFS) has recently emerged as an alternative to overcome sustainability challenges linked to both the production of postsmolts in recirculation aquaculture systems (RAS) and the grow-out phase at sea. In the present case study, we assessed growth, stress and welfare, key performance indicators (KPIs), and harvest results in a commercial seawater HFS facility, where Atlantic salmon were reared from smolt (~92 g) to market size (~5 kg) between June 2023 and May/June 2024. This was compared with the performance of salmon transferred from the HFS to open net pens at 910 g (to Site A) and 1900 g (to Site B). Salmon in HFS displayed better growth (over 80% higher body weight in May 2024), welfare and stress indicators (lower incidence of skin ulcers, fin damage, and lower plasma cortisol), better KPIs of growth, feeding efficiency and mortality, and better harvest results than in traditional sea Sites A and B. This positive effect could be linked to the absence of severe disturbances and stressful operations in HFS (i.e., well-boat transport or de-lousings) and to the provision of a high-quality rearing environment with large amounts of new fresh seawater, but isolated from external stressors like parasites or pathogens. HFS, thus, appears as a plausible alternative to produce fast-growing harvest-size salmon with high welfare standards, which may raise the interest of more salmon producers.