Dissolved oxygen variability in a commercial sea-cage exposes farmed Atlantic salmon to growth limiting conditions

Understanding dissolved O₂ flux in marine cages, and how individual fish respond to and experience such variation, is critical to optimizing growth and production performance of farmed salmon. We used a high resolution environmental monitoring system to create a 3-dimensional map of a commercial marine cage with respect to salinity, temperature and dissolved O₂ through time, while also tracking the oxygen experience of 4 individually tagged Atlantic salmon. Despite all of the dissolved O₂ measurements at the reference site being physiologically suitable for maximum growth, 1 in 4 of the recordings collected within the cage were below dissolved O₂ levels known to reduce feed intake and growth. Recorded dissolved O₂ in the cage ranged from 26 to 90% saturation with a high degree of vertical, horizontal and temporal variation. Poorest dissolved O₂ conditions consistently occurred at night in the central and down-current cage positions. Dissolved O₂ levels experienced by individual fish ranged from 30 to 90% saturation, with variation within 5 minute intervals as large as 32 percentage points. These results expand the current body of knowledge on environmental variability in marine cages, and provide valuable insights to aid farm managers in focusing mitigation and monitoring efforts when and where they are most needed.