Gill disease is the leading welfare and mortality cause in farmed Atlantic salmon, with complex amoebic, bacterial, and viral pathogens increasingly affecting the respiratory organs that are essential to survival.
Gill disease in farmed salmon creates chronic welfare compromise that is particularly difficult to address because it affects the organ most critical to survival. Damaged gills reduce oxygen extraction efficiency, forcing fish to increase their respiratory effort — a state analogous to chronic breathing difficulty in humans. The behavioural changes associated with gill disease — surface crowding, lethargy, reduced feeding, and abnormal swimming — are welfare indicators that accumulate over days to weeks before mortality occurs. Treatment via freshwater bathing, while effective against AGD, introduces its own acute welfare costs through the crowding, handling, and osmotic stress of transfer. The growing frequency of treatment cycles as water temperatures rise is increasing the cumulative welfare burden on fish through their production cycle. Investment in preventive strategies — site selection, reduced stocking density, cleaner fish (wrasse, lumpfish), and gill health monitoring — reduces both disease incidence and treatment frequency.