Atlantic salmon are farmed at high densities that restrict their natural behaviors and cause chronic stress. Understanding crowding effects helps assess the welfare impact of standard production practices.
Crowding is the central welfare problem in salmon aquaculture. The physiological stress response to crowding is well-documented and sustained: chronically elevated cortisol suppresses immune function, impairs growth, and alters behavior. Fish in crowded pens cannot perform characteristic behaviors including spatial avoidance of dominant conspecifics, causing chronic subordination stress. Fin and body damage from aggression is common. The sensory environment in crowded pens adds to chronic stress. The industry has economic incentives to maximize density; welfare improvements require regulatory pressure or consumer-driven certification standards that mandate lower densities.