Aeration Unexpectedly Increases Ventilation Rates of Farmed Atlantic Salmon ( Salmo salar ) in a River Environment During High Temperatures

Aeration is commonly used in commercial salmon aquaculture to improve water quality and fish welfare, but it also represents a significant operational cost. Its effects on fish behaviour and welfare, therefore, require evaluation. We used computer vision and a manipulative experiment to test whether salmon exposed to aeration reduced their ventilation effort (fewer open‐close mouth cycles) compared to non‐aerated conditions. Twenty‐eight underwater videos (14 pairs) were recorded across multiple pens with aeration on and off during the low slack water. A computer vision model detected, classified and tracked mouth opening and closing cycles per minute as a proxy for ventilation rate. Linear mixed‐effects analysis found a positive association between ventilation rates and aeration operations, likely reflecting a behavioural response to water turbulence and bubble flow rather than changes in dissolved oxygen (DO) or temperature. These findings highlight the utility of computer vision for monitoring ventilation responses under commercial conditions.