Atlantic salmon in net pen aquaculture are constrained to a tiny fraction of their natural ranging behavior, with behavioral restriction contributing to stress, aggression, and reduced immune function.
The behavioral restriction of net pen salmon is substantial but largely invisible to consumers. Salmon that evolved to migrate thousands of miles are confined to circular cages where the repetitive swimming pattern becomes compulsive. Crowding prevents the spatial distribution that normally reduces competition and aggression. The welfare significance of this restriction depends on how we weigh behavioral freedom against the absence of predation and food security that captivity also provides. However, the chronic stress from crowding and behavioral restriction is measurable in cortisol levels and immune function, with direct health consequences.