Crowding of Atlantic salmon before slaughter is a standard procedure that concentrates fish for efficient transfer to the kill system. The welfare impacts of crowding are significant and depend on density, duration, and water quality management.
Pre-slaughter crowding is a significant welfare event at the end of a salmon's life and represents a critical control point for welfare improvement. The physiology is straightforward: high density reduces dissolved oxygen, increases carbon dioxide, and elevates ammonia from respiration and excretion. Fish experiencing hypoxia show gasping, altered swimming patterns, and increased stress hormone release. Scale loss during crowding is both a welfare indicator (contact trauma) and a product quality issue, providing commercial incentives for improved practices. The combination of feed withdrawal, transport stress, and crowding creates a compounded welfare burden in the 48-72 hours before slaughter. Improved crowding management - supplemental oxygenation, careful temperature monitoring, reduced crowding density, and minimised crowding duration - significantly improves welfare outcomes at minimal additional cost.