Effects of crowding intensity on stress responses in Atlantic salmon (Salmo salar)

The increased interest in ensuring good fish welfare in Atlantic salmon (Salmo salar) production have led to the development of several welfare indicators that can be used to assess the condition of the fish. Crowding is the gathering of fish into high densities, usually conducted in relation to fish treatment or transport. This procedure can induce a stress response in the fish and be an issue of welfare. In the present study, Atlantic salmon were exposed to three different crowding intensity levels (Level 2, 3 and 4) from a crowding intensity scale based upon fish surface activity. This study aimed to evaluate the effect of crowding intensity on salmon stress responses, by using digital monitoring and different welfare indicators. Two field experiments were conducted, in October and January, respectively. There were three replicate sea cages at each crowding level, and the salmon were crowded for two hours. Swimming behaviour, swimming speed, respiration rates, blood chemistry (plasma cortisol, glucose and lactate) and environmental parameters (temperature, salinity, and oxygen saturation), were assessed to investigate the state of the fish during and post-crowding. Chaotic swimming and a high level of activity were observed during crowding at the higher levels (level 3 and 4). Post-release, the fish returned to schooling behaviour. The swimming speed of the fish crowded at level 4 were statistically significantly lower compared to the other levels post-release (p