Stress responses in Atlantic salmon (Salmo salar L.) smolts during commercial well boat transports, and effects on survival after transfer to sea

Disease costs the Norwegian aquaculture industry approximately NOK 1 billion ($ 150 000 000) every year, and NOK 250 000 000 ($36 000 000) is spent on vaccines each year by the industry. Many of the disease outbreaks take place during the first months of transfer to the sea sites. Thus, the aim of this project was to monitor physiological stress parameters as plasma cortisol, glucose and lactate in salmon smolts before and after regular well boat transports. The loading process was observed to be a more severe stressor than the transport itself. Only minor plasma cortisol increases were observed during unloading. In four out of five transports, plasma cortisol had returned to resting level at arrival. However in one, which showed an unusually high mortality rate during the first month after transfer to the sea, the plasma cortisol level remained high during unloading. The study showed that well boat transports seemed to have an important recovery function. Without this ability to recover between the stressors, as on- and unloading, the ability for salmon smolts to handle multiple stressors was reduced. Further, the present study seems to strengthen the fact that some of the increased mortality experienced at sea sites in Norway may be explained by handling and transport prior to deliverance of smolts. Care should be taken during commercial boat transports. Planning of well boat routes, and avoidance of high seas could contribute to minimise mortality experienced after transports.