Stocking density in salmon net pens significantly affects individual fish welfare through effects on competition, injury, disease, and behavioral expression.
Salmon crowding welfare harms operate through both physical and behavioral mechanisms. Physical crowding increases fin and eye damage from contact with pen structures and pen-mates. Behavioral crowding disrupts feeding hierarchies: in crowded conditions, dominant fish monopolize feed while subordinates are displaced, resulting in uneven growth and nutritional stress in lower-ranking fish. The social environment of a crowded net pen bears no relationship to natural salmon habitat and social structure. Reducing stocking density improves welfare across multiple metrics simultaneously: less physical injury, more equitable feed access, lower stress hormones, and higher survival. The welfare-density relationship is well-established and provides clear grounds for setting lower limits than those currently permitted in major producing countries.