Sea lice infestations are a major welfare and production problem in Atlantic salmon net pen farming. Lice cause skin erosion, stress, and secondary infections, and treatment options each carry their own welfare costs.
Sea lice represent a cascading welfare problem: the infestation itself causes chronic suffering, while the treatments used to control it cause acute distress. Chemical bath treatments involve crowding fish into confined spaces and exposing them to irritant chemicals — a process shown to elevate stress hormones significantly. Cleaner fish such as wrasse, introduced to eat lice, suffer high mortality rates in net pen environments where they lack appropriate shelter and food variety. Laser treatments that target individual lice show promise as a lower-stress alternative. The fundamental welfare solution lies in preventing high lice loads through lower stocking densities and strategic fallowing of sites.