Complex gill disease (CGD) affects a large and growing proportion of farmed Atlantic salmon, causing chronic respiratory distress and significant welfare harm.
Gill disease causes persistent respiratory distress in affected salmon. The gill is the primary gas exchange organ; compromised gill function forces fish to compensate through increased ventilation rate (faster opercular movements), reduced activity, and surface-seeking behavior in search of higher oxygen concentrations. Chronic hypoxia — oxygen deficit from impaired gill function — is deeply aversive in air-breathing and water-breathing vertebrates alike. Fish with severe gill disease cannot exercise, cannot compete effectively for food, and may experience what is functionally equivalent to chronic difficulty breathing. The multi-pathogen nature of CGD makes treatment difficult; management primarily involves prevention through better biosecurity, stocking density reduction, and water quality management.