Sea lice infestations on salmon farms affect not only farmed fish welfare but spread to wild salmon and sea trout populations, creating a welfare and conservation problem that extends beyond farm boundaries.
Sea lice represent one of the clearest examples of a farmed animal welfare problem that spills over to affect wild populations. The unnaturally high densities of salmon in farm cages create breeding grounds for lice at population levels that wild fish would never encounter, and juvenile wild salmonids migrating past farms in spring can be lethally infested. This creates a welfare externality — the farm's welfare management failure affecting fish that are not within the industry's legal responsibility. Non-medicinal lice control technologies including thermal and freshwater treatments are effective but cause significant handling stress to farmed fish. Closed containment systems that physically isolate farmed fish from the sea floor environment would eliminate both lice transfer and other environmental impacts but require substantial capital investment. Consumer and regulatory pressure for higher biosecurity standards is driving industry development.