Atlantic salmon are farmed at densities that significantly exceed their social and spatial preferences. Understanding crowding stress is central to improving welfare across a major global industry.
Crowding stress in farmed Atlantic salmon represents one of the most thoroughly documented welfare problems in aquaculture. Research consistently shows that salmon prefer lower densities when given choice, that cortisol levels rise with increasing density, and that fin and scale damage — indicators of aggression and physical crowding — increase at commercial densities. Norwegian regulations at 25 kg/m³ represent a reasonable baseline, but even these densities exceed salmon behavioral preferences. Seasonal variation in pen crowding — as fish grow between harvests — means welfare impact varies across the production cycle. The economic incentive is to stock densely; the welfare incentive is to stock less densely. Certification standards like ASC and RSPCA Assured set density limits that provide some welfare protection, though enforcement and verification remain challenges.