Does the thermal component of warm water treatment inflict acute lesions on Atlantic salmon (Salmo salar)?

Abstract

Warm water treatment, i.e. exposure to sea water at a temperature of 28–34 °C for 20–30 s, has in recent years been widely used for delousing of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) in Norwegian aquaculture. High mortality and various lesions (e.g. injuries and/or bleedings in skin, fins, eyes, brain, and gills) have, however, been reported after industrial warm water treatments. The objective of this study was to reveal whether the thermal component of warm water treatment inflicts acute lesions on Atlantic salmon. The study was conducted by exposing individual, sedated Atlantic salmon post-smolts ( = 1117 ± 250 g) to sea water at a temperature of 34 °C (warm water treatment, n = 40) or 9 °C (control treatment, n = 20) for 30 s, and subsequently conducting welfare indicator scoring and histopathological examination of their skin, fins, eyes, snout, nasal pits/mucosa, palate, gills, thymus, pseudobranch, brain, heart, liver, kidney, pyloric caeca, pancreas, and spleen. The results showed that the prevalence and severity of acute lesions were not significantly different between the two treatment groups, except for higher prevalence of injuries on the caudal (p = 0.002), dorsal (p = 0.002), and right pelvic fins (p = 0.014) in the warm water treatment group. The main cause of these fin injuries may have been a strong behavioural reaction displayed by the fish when exposed to warm water. Possible consequences of fin injuries, the use of anaesthetic, and statistical limitations were discussed. It was concluded that exposure of Atlantic salmon to sea water at a temperature of 34 °C for 30 s did not lead to any statistically significant change in the prevalence of acute lesions except an increase in minor, possibly behaviour-related, fin injuries. Detection of a lower lesion prevalence than was possible in this study, but which may concern many individuals in an industrial setting, requires examination of a larger number of fish.