Reduced lice infection pressure at offshore salmon aquaculture sites

This is the authors' abstract. We don't add key points for this paper.

Salmon lice is a persistent challenge for the aquaculture industry and represents a key limiting factor for further growth in many regions. Offshore farming, and submerged cage technologies in particular, are expected to reduce the risk of salmon lice infections due to greater distances from coastal source regions and enhanced dilution of lice larvae. Thus, the establishment of offshore Atlantic salmon cages may enable more sustainable aquaculture production and increased biomass capacity. However, the extent to which strong oceanic currents and vertical mixing may facilitate lice transmission to and from emerging submerged offshore sites has not yet been assessed. Here we quantify salmon lice infection risks at prospective offshore aquaculture sites in Norway using state-of-the-art hydrodynamic and salmon-lice dispersal models. Overall, lice infection pressures decrease with distance to the coast, and only 7%–13% of the total infection pressure is found within an area from the Norwegian baseline and 60 nautical miles offshore, with the distribution varying interannually. Vertical mixing capable of transporting lice to submerged cage depths occurs mainly during winter, whereas infection pressure peaks during summer and autumn, limiting the temporal overlap between high lice abundance and strong mixing. Southern sites exhibit less vertical mixing than northern sites, pointing to more favourable submerged effects in the south. Overall, the results indicate that submerged offshore aquaculture may substantially reduce lice exposure compared with traditional coastal farming, although infection risk is not entirely eliminated.