Low temperatures typical of winter cage-site conditions in Atlantic Canada impact the growth, physiology, health and welfare of Atlantic salmon (Salmo salar)

Limited research has been conducted on the physiology of Atlantic salmon (Salmo salar) at cold temperatures despite significant mortalities during the winter at sea-cages in Atlantic Canada. Thus, in this thesis, I exposed post-smolt cultured Atlantic salmon to a seasonal decline in temperature from 8 to 1°C (at 1°C week-1), and a ‘cold-shock’ from 3 to 0°C for 4 or 24 hours. During the seasonal decline in temperature, feeding decreased starting at 6°C (and ceased by 1-2°C), osmoregulatory changes and increases in heat shock protein expression began at 4-5°C, and at 1°C elevated plasma cortisol levels indicative of mild stress were measured. The ‘cold-shocks’ resulted in a relatively small stress response (i.e., increases in plasma cortisol and glucose), but no other adverse effects or mortalities. Nonetheless, a number of mortalities/moribund fish were noted when various groups of Atlantic salmon were held for long periods at < 8°C. Moribund fish were lethargic and swam erratically, had enlarged livers and plasma enzymes suggestive of liver damage, and developed ulcers to the head/jaw. The former symptoms are indicative of ‘Winter Syndrome’ described in gilthead sea bream (Sparus aurata) aquaculture, and these ulcers have previously been observed in Norway and associated with tenacibaculosis