Environmental conditions and occurrence of hypoxia within production cages of Atlantic salmon on the south coast of Newfoundland

Dissolved oxygen (DO) levels were monitored in two marine Atlantic salmon (Salmo salar L.) cage sites on the south coast of Newfoundland (Canada) during the summer and fall of 2008 and 2009. In 2008, sites A and B had cages with stocking densities of approximately 9 kg m-3. In 2009, site A was being fallowed while stocking density at site B was of 10 kg m-3. During 2008 and 2009, few hypoxic events (defined as DO <6 mg L-1 O2) were recorded at site A; however, at site B intermittent hypoxic events (46 mg L-1 O2) were observed. Our study provides a description of intermittent hypoxia occurring in salmon cages. These hypoxic events occur mostly during the summer season and last for as long as 2.5 months. Fish swimming behaviour show that water layers exhibiting low DO did not cause avoidance behaviour. This confirms results found by other authors. Site B exhibited low water currents below 5 m suggesting that caution should be taken before considering an increase in stocking densities in cages. Questions remain about the effect of this intermittent hypoxia on fish health and growth, and will be the focus of future research.