Detection of a stress related acoustic signature by passive acoustic monitoring in Atlantic salmon farming

Crowding is a necessary operation for moving fish out of or between sea cages in Atlantic salmon farming. These operations consist of gradually reducing available volume in the nets to increase the biomass density. This is known to stress the fish, increase risk of injury, and can in extreme cases increase mortality rates. To reduce the negative impacts of crowding, the operation must be adjusted based on real-time monitoring of fish welfare. Today, this is mainly done by manually monitoring surface activity, as the high fish density limits the use of available tools. In this paper a new approach for monitoring was applied to crowding of meso-scale cages containing 40 fish each, using automatic detection of passive acoustic signatures. Four synchronised hydrophones were deployed and the acoustic signature was defined from the recordings. The automatic detection algorithm recorded an extreme increase in registered acoustic events during crowding, as high as 1000 events per 10 min period, compared to only 20 events per 10 min period on the day following crowding. Correlating the number of acoustic events to recorded heart rate and activity measures from implants in the salmon indicate that automatically registering acoustic events is a promising method for monitoring farmed fish.