Understanding how winter conditions in the North Atlantic affect the physiology and behaviour of Atlantic Salmon in sea-cages

In recent years there have been a number of instances where declines in water temperature are suspected to have contributed to winter mortalities at salmon cage-sites in Atlantic Canada. In addition, it is expected that temperature variability due to more intense and frequent winter storms will increase with climate change, and this may further threaten Atlantic salmon health and welfare in sea-cages. To better understand cage-site conditions and aspects of salmon behaviour and physiology during the winter, we fitted Atlantic salmon in commercial size sea-cages in Newfoundland (Canada) for two consecutive years with internal and external data storage tags (DSTs) that recorded depth, temperature, heart rate ( f H ) and 3D-acceleration (swimming activity). Tags were surgically attached / implanted in late October / early November when water temperatures were 8–10 °C, and the DSTs were collected at the time of harvest. Although tag retrieval was limited due to some early mortality, challenges due to Covid-19 and with fish identification during harvest, the data collected for both years were comparable. Water temperature decreased at a rate of 0.35–0.57 °C week −1 from October / November to March, and the coolest average weekly temperature experienced by the fish was 1.10 °C in early March of both years. Average f H decreased with temperature starting in the fall, and the fish primarily occupied the upper 5 m of the cage for the duration of this study, despite the fact that temperatures were generally homogeneous with depth. Nonetheless, vertical migrations to deeper depths were observed during some periods (e.g., at night during the coldest months), and thus, the average depth of the fish was greater during these times. These data reveal that temperatures in sea-cages in Newfoundland remain below 5 °C for ∼5 months and are below 2 °C for ∼1.5 months. Thus, understanding how cold temperatures affect various aspects of salmon biology, and monitoring these fish during this period, is critical to the sustainability and future of this industry. • Sea-caged salmon were monitored during the fall/winter using Star-Oddi data loggers. • Water temperatures fell by 0.35–0.57 °C week −1 and were below 5 °C for 5 months. • Heart rate closely followed the temperature decline. • Fish mainly occupied the upper 5 m of the cage with excursions to deeper depths. • Fish activity was higher during the day, but reduced at temperatures below 5 °C.