Stocking density is one of the most significant determinants of fish welfare in aquaculture. Crowding causes physiological stress, impairs immune function, increases aggression, and reduces growth efficiency.
Crowding represents a chronic stressor for farmed fish that operates continuously throughout the production cycle. Unlike acute stressors, chronic crowding stress causes persistent cortisol elevation that suppresses the immune system, accelerates disease spread, and reduces the capacity for normal behaviour including foraging, social interaction, and rest. Fish show clear preferences for lower-density conditions in choice chambers, demonstrating that crowding is aversive. Regulatory density limits represent a compromise between welfare and productivity, but evidence suggests that reduced density is beneficial on both welfare and economic grounds through improved growth efficiency and reduced disease costs.