Sea lice are ectoparasitic copepods that cause significant welfare problems in farmed Atlantic salmon. The treatments used to control them also cause welfare impacts that must be weighed against the harm caused by the lice themselves.
Sea lice infestations cause chronic pain, immune suppression, and osmotic stress in heavily infested fish. Chemical treatments involve abrupt environmental changes that cause acute stress and, in some cases, significant mortality. Thermal treatments using hot water cause thermal shock with high welfare costs. Mechanical treatment including pumps and water jets causes physical injury and haemorrhaging. The welfare costs of all available treatments require careful consideration against the welfare cost of untreated infestation.