Cleaner fish including wrasse and lumpfish are used in salmon sea pens to remove sea lice biologically, reducing reliance on chemical treatments. These cleaner fish have their own welfare needs that require specific management attention.
Cleaner fish welfare represents an emerging area of aquaculture welfare science. Wrasse and lumpfish deployed in salmon pens face significant welfare challenges: they are small fish in a large pen environment dominated by much larger salmon, exposed to predation risk and competition for food. Wrasse require invertebrate-based diets not naturally available in sea pens and need shelter structures to refugia from salmon harassment. Without adequate shelter and supplementary feeding, cleaner fish rapidly lose condition and die. Lumpfish, which are more cold-tolerant and can be hatchery-produced, are increasingly preferred over wild wrasse but also require appropriate environmental provision. The welfare irony of using sentient fish to control parasites on other sentient fish requires honest assessment: the welfare cost of cleaner fish mortality and poor condition must be factored into the overall welfare accounting of salmon aquaculture.