Overcrowding in salmon aquaculture causes chronic stress that impairs immune function, increases disease susceptibility, and causes behavioral abnormalities. Reducing stocking densities is one of the most impactful welfare improvements available.
Crowding stress is a pervasive background welfare harm in salmon farming that compounds other issues. Chronically elevated cortisol impairs immune function, making fish more vulnerable to the pathogens that already pose major production challenges. Fin damage from contact injuries is near-universal in high-density pens. Behavioral abnormalities including abnormal swimming patterns are observed at high densities. Because lower stocking density typically improves both welfare and biological performance metrics, the economic argument against density reduction is weaker than often assumed. Regulatory density limits combined with welfare certification pressure represent complementary approaches to driving industry improvement.