The physiological stress response to crowding in Atlantic salmon provides welfare indicators that guide management decisions about density and handling.
The cortisol stress response to crowding provides a validated physiological welfare indicator in Atlantic salmon. Acute crowding causes rapid cortisol elevation followed by immune suppression and growth reduction. Chronic high-density conditions maintain elevated baseline cortisol indicating persistent welfare impairment. The non-invasive measurement of cortisol from tank water is an emerging welfare monitoring tool that allows repeated assessment without additional handling stress. Welfare-informed density management uses physiological indicators alongside behavioural ones to maintain fish below chronic stress thresholds. This evidence-based approach is increasingly being incorporated into welfare certification requirements.