'Cold shock' has few physiological effects on cultured Atlantic Salmon (Salmo salar) acclimated to low temperatures

With climate change, winter storms are predicted to increase in frequency and intensity, and result in rapid declines in water temperatures at sea-cages due to the mixing of deeper waters with cold surface waters. Given that winter storms have been associated with mortalities of Atlantic salmon in sea-cages, we acclimated fish for 5 weeks to 3 degrees C and: 1) maintained one group of fish at 3 degrees C (control group); and 2) exposed two other groups to a decline in temperature from 3 to 0 degrees C over 3 h, and then held them at this temperature for 4 or 24 h (i.e., to provide a 'cold shock'). At the end of the experiment, the temperature for these latter groups was returned to 3 degrees C, and the fish were recovered for 5 days. Blood samples were taken before and at the end of the 'cold shock' (i. e., while still at 0 degrees C), and at 6, 24, 72 and 120 h after the fish were returned to 3 degrees C. A moderate stress response was measured after the 'cold shock' with: plasma cortisol levels transiently (at 24 h) increasing to similar to 30 and similar to 25 ng ml(-1) in fish exposed to 0 degrees C for 4 and 24 h, respectively; and glucose increasing from similar to 3.8 mM to 5.5-6.0 mM. There were no changes in plasma lactate, indices of tissue damage, or in plasma ion concentrations. However, gill Na+-K+-ATPase activity was similar to 5-fold higher in fish given a 'cold shock' compared with those maintained at 3.C. Finally, there were no mortalities within 120 h of the end of the 'cold shock'. Collectively, these data indicate that Atlantic salmon have the capacity to withstand rapid declines in seawater temperature to close to 0 degrees C, and suggest that such phenomena alone are unlikely to be responsible for winter-related mortalities at salmon cage-sites in Iceland and Canada.