Body temperature and seawater adaptation in farmed Atlantic salmon and rainbow trout during prolonged chilling

Abstract

The core temperature of the rainbow trout Oncorhynchus mykiss (3·5 kg) dropped to 1·0° C during the first 6 h of chilling at 0·5° C, remained stable until 24 h, and dropped significantly to 0·7° C after 39 h. Blood plasma osmolality increased and muscle moisture content decreased gradually with increasing chilling time. After 39 h of chilling, the rainbow trout experienced 40 mosmol l‐1 higher blood plasma osmolality and 2·8% less muscle moisture content compared with initial values. In the Atlantic salmon Salmo salar (5·3 kg), core temperature dropped to 1·3° C and blood plasma osmolality increased significantly during the first 6 h of chilling at 0·5° C, but remained relatively stable throughout the rest of the experimental period. After 39 h of chilling, the salmon experienced 20 mosmol l‐1 higher blood plasma osmolality and 0·5% less muscle moisture content compared with initial values. In rainbow trout muscle moisture content was inversely related to blood plasma osmolality indicating reduced seawater adaptation with increasing hours of chilling. No such relationship was observed in the Atlantic salmon. Hence, changes in plasma osmolality and muscle moisture in the Atlantic salmon do not indicate osmoregulatory failure since the new levels, once established, were maintained throughout the chilling time.

Species‐specific welfare aspects of the main systems of stunning and killing of farmed Atlantic Salmon

“…In such cases, it may be necessary to examine the effect of the process on changes in stress hormone secretion or heart function. For example, cooling fish rapidly (as occurs in 'live chilling') leads to elevated plasma cortisol levels (Donaldson, 1981;Skjervold et al, 2001), indicating that it may be stressful, and over time to a disturbance of plasma osmolarity (Rorvik et al, 2001). Rapid live chilling results in a marked decrease in the muscle pH, that indicates increased muscle activity, and could be a sign of aversive activity (Skjervold et al, 2001).…”

“…For example, cooling fish rapidly (as occurs in 'live chilling') leads to elevated plasma cortisol levels (Donaldson, 1981;Skjervold et al, 2001), indicating that it may be stressful, and over time to a disturbance of plasma osmolarity (Rorvik et al, 2001). Rapid live chilling results in a marked decrease in the muscle pH, that indicates increased muscle activity, and could be a sign of aversive activity (Skjervold et al, 2001). Considering the methodology used in these studies it is difficult to separate between the effect of cooling and the effect of low dose CO 2 .…”

“…However, the mobilisation of glucose due to glycogenolysis (enzymic breakdown of glycogen) is slower than the cortisol release in most fish and it is likely that this process is further delayed by a rapid decrease in temperature in fish. Experiments with Atlantic salmon (Skjervold et al, 2001) revealed that exposure to a temperature drop was stressful.…”

“…Crowding of salmon prior to live chilling and slaughter has been shown to significantly increase cortisol, glucose, lactate and osmolarity in blood plasma (Skjervold et al, 2001) and a loss of scales (probably also involving loss of mucus) is often seen.…”

“…Taken together, this clearly shows that the live chilling method is basically equivalent to the traditional carbon dioxide stunning method with similar fish welfare concerns. Also, live chilling of salmon is known to produce elevated levels of plasma cortisol, glucose, lactate and increased osmolality (Skjervold et al, 2001). …”