Lowering temperature to 1 °C results in physiological changes, stress and mortality in cultured Atlantic Salmon (Salmo salar)

Abstract

Atlantic salmon (Salmo salar) reared in sea-cages in Atlantic Canada and Iceland can be exposed to temperatures as low as 0–1 °C during the winter months, and this can result in large-scale mortalities. Despite this threat to salmon aquaculture, most of our knowledge of this species' thermal biology is at temperatures above 10 °C. In this laboratory experiment, Atlantic salmon post-smolts (175 ± 24 g) were exposed to a temperature decline of 1 °C/week (from 8 to 1 °C) with an additional week at 1 °C (to simulate temperature changes in sea-cages in Atlantic Canada during the winter months), or a constant temperature of 8 °C (control), and sampled weekly. Growth was reduced by 58% in the cold-exposed group and was associated with a decrease in appetite beginning at 6 °C. Starting at 4–5 °C, an osmoregulatory disturbance ensued characterized by higher plasma [Cl−], lower plasma [K+] and increased osmolality. Hsp70 and hsp90 transcript expression also increased as temperature was reduced from 5 to 1 °C. However, plasma cortisol levels did not increase (to ∼18 ng/ml) until the fish reached 1 °C, and serpinh1 and cirbp expression were not good biomarkers of decreasing temperature. Symptoms suggestive of liver dysfunction (changes in plasma [aspartate aminotransferase] and an increase in liver size) were also evident at 1 °C. Finally, at 1 °C, 5% of the fish died, and these mortalities were typically associated with snout ulceration/erosion and fin rot. In summary, Atlantic salmon held at temperatures <5 °C suffer from decreased growth performance, osmoregulatory disturbance, liver dysfunction, stress, opportunistic infections, and increased mortality.