Stocking density is a fundamental determinant of fish welfare in aquaculture. Overcrowding causes stress, injury, disease, and reduced growth that affect billions of farmed fish.
Fish under high stocking density conditions experience chronic stress responses that reduce immune function and increase disease susceptibility. Physical crowding prevents normal schooling behaviour, increases fin-nipping and scale damage, and impairs access to food by subordinate fish. Ammonia builds up faster in high-density systems, requiring more water treatment. The economic pressure to maximise density is in direct conflict with welfare optima for most farmed fish species. Welfare-based density limits enforced through certification schemes are an important tool.