Beyond disease and lice, farmed Atlantic salmon face welfare harms from social aggression in crowded conditions and the absence of environmental complexity that would allow natural behavioral expression.
The behavioral welfare of farmed salmon has received increasing attention as fish sentience research has progressed. Atlantic salmon are active, exploratory fish that naturally navigate complex three-dimensional environments — rapid river currents, diverse substrates, and varied light conditions. Net pens are featureless cylinders of water with uniform light and no structural complexity. Under these conditions, dominance hierarchies form and persist, with subordinate fish experiencing chronic social stress. Fin erosion from repeated aggressive encounters is measurable and indicates sustained conflict. The use of light manipulation to control sexual maturation alters circadian and seasonal physiological rhythms in ways not fully understood from a welfare perspective. Enrichment research — adding structures, current variation, or substrate to pens — is nascent but promising, showing behavioral improvements with modest infrastructure additions.