Oceanic currents in the southern Norwegian trench - A candidate site for offshore aquaculture

The potential for offshore salmon aquaculture has been investigated for several years in Norway. Finding appropriate locations requires evaluations of local oceanography to ensure that fish can thrive in these new and more extreme environments. Concerns for animal welfare pertains to strong currents exceeding swimming capacities, but also calm periods limiting water exchange through sea cages which causes environmental hypoxia. The Southern Norwegian Trench was recently chosen by the Norwegian Directorate of Fisheries as a potential location for offshore aquaculture. The purpose of this study was therefore to provide a thorough report of relevant oceanography in this area to evaluate its suitability for salmon aquaculture. This was achieved by installing three oceanographic rigs along a transect of the Southern Norwegian Trench that recorded current profiles down the water column over a full year. Additionally, salinity, temperature and oxygen levels were measured with a CTD-O. We found that temperature and oxygen levels were suitable for salmon aquaculture across seasons. Periods of strong currents exceeding 1 m s −1 occurred occasionally. However, when considering current damping through farm structures, peak currents should not exceed swimming limits of farmed salmon. Meanwhile, extended periods of calm conditions (< 5 cm/s) makes hypoxia a concern. This problem needs to be solved by careful management of biomasses and cage designs to facilitate sufficient oxygenation. In conclusion, the Southern Norwegian Trench can be a suitable location for salmon aquaculture if cage designs and management strategies are able to balance biomass with adequate water exchange to mitigate hypoxia. • The Southern Norwegian Trench is a candidate site for offshore salmon aquaculture. • Comprehensive hydrographic data were collected along a transect for an entire year. • Temperature, ambient oxygen, and water currents remained within suitable ranges. • Periods of low current speeds may cause hypoxia risks in fully stocked sea cages. • Salmon should be able to thrive in offshore cages in the Southern Norwegian Trench.