Environmentally responsive parasite prevention halves salmon louse burden in commercial marine cages

Though a naturally occurring species throughout the Atlantic, parasitic salmon louse (Lepeophtheirus salmonis) population dynamics are increasingly intertwined with salmonid aquaculture. In contrast to reactive louse management, tools and strategies which reduce louse infestation success provide an off-ramp from the 'more farming equals more lice' feedback loop. This experiment tested the efficacy of a dynamic, environmentally responsive louse prevention strategy using common, commercially available tools throughout a full production cycle at commercial scale. By strategically luring salmon away from the halocline where concentrations of infective louse copepodids are highest using feeding and lights, and minimizing surface water flow through the cage with a protective skirt barrier when no halocline was present, both new louse infestations and mobile louse numbers were cut by half compared to control cages. The reduced louse numbers resulted in 25% fewer delousing events and improved fish welfare in dynamic cages, with no differences in gill condition or growth between treatments. With farmed salmon driving the ecology of salmon lice, this dynamic, environmentally responsive prevention strategy offers a way to work with nature, rather than against it, to reduce the parasite burden on both farmed and wild salmonids.