Mismatching host and parasite environments reduces encounter rates and prevents infestation of farmed salmon by sea lice

Farming animals can fundamentally alter host-parasite dynamics, tipping the balance in favour of parasites and driving outbreaks. High-density fish farming creates ideal conditions for outbreaks, as hosts cannot typically... [ view full abstract ]

Farming animals can fundamentally alter host-parasite dynamics, tipping the balance in favour of parasites and driving outbreaks. High-density fish farming creates ideal conditions for outbreaks, as hosts cannot typically avoid parasite encounters. Based on knowledge of behaviours of farmed Atlantic salmon (Salmo salar) and the ecto-parasitic salmon louse (Lepeophtheirus salmonis), we hypothesized that areas of high infection risk existed on farms and could be avoided through technological modifications to farming structures. The free-swimming louse stage is phototactic and concentrates at the surface, which maximizes encounters with salmon, which often swim, feed and jump in surface waters. By placing a barrier in fish farm cages to hold salmon away from the surface layer, we reduced louse infestations by 76% compared to control cages without manipulation. Salmon accessed surface waters and exhibited their full behavioral repertoire through a central chamber that was impermeable to parasites. Our results demonstrate that farm design can create spatial mismatches within farms that limit host-parasite encounters, without compromising the behavioural requirements of the fish. Our results should drive technological innovation of current fish farming systems to increase fish welfare, improve production efficiency, reduce chemical biocide use and limit the population-level impacts of parasites on wild fish.