Atlantic salmon in commercial net pen aquaculture are held at densities far exceeding their natural distribution, causing chronic stress, aggression, and disease vulnerability that represent major welfare challenges in a globally significant industry.
Crowding stress in farmed salmon creates a cascade of welfare harms. Dominant individuals establish access to oxygenated surface water, while subordinate fish are forced into poorer-quality water layers. Chronic stress from social subordinance suppresses immune function, accelerating disease spread. Sea lice infestations cause skin damage and can reach intensities where they consume significant tissue. Chemical treatments for sea lice stress all fish in the net and have ecosystem impacts. The use of cleaner fish — wrasse and lumpfish — adds a complex welfare dimension, as these species are often caught wild, transported in poor conditions, and experience high mortality. Improved welfare standards are gaining ground through the ASC certification scheme, but significant gaps remain between certification requirements and daily farm practices.