Cleaner fish including wrasse and lumpfish are used in salmon farming to eat sea lice. Their welfare as a tool of integrated pest management raises distinct ethical questions for both species' welfare.
Cleaner fish welfare represents a welfare trade-off internal to aquaculture: deploying fish of one species to improve the welfare of another. The welfare costs for cleaner fish are severe: high mortality rates, stress from salmon aggression, and temperature conditions optimised for salmon rather than the cleaner fish species. Wrasse require complex structural environments with refuges from larger salmon; barren pens without such structure result in welfare-inadequate conditions for the cleaner species. Lumpfish welfare is compromised by the thermal mismatch between their cold-water preference and salmon pen temperatures. The welfare calculation requires comparing the suffering of tens of millions of cleaner fish against the reduced sea lice burden for billions of farmed salmon. Alternative sea lice management methods including mechanical removal, laser systems, and chemical treatments have different welfare profiles but may cause less systemic welfare harm to additional species.