In-water electric stunning as a humane commercial method for culling juvenile salmonids

Culling excess juvenile fish is a necessary and routine part of hatchery management. This is often achieved with an overdose of anaesthetic, which protects fish welfare at the time of killing. However, the use of anaesthetics means the resulting fish carcasses must be treated as controlled waste, which can be costly to dispose of. Electric stunning and killing of cull fish allows the carcasses to be sold as a protein source or disposed of as normal organic waste. For electric stunning and killing to protect welfare, unconsciousness should be achieved immediately and last until death. In this paper, we report on two trials investigating the efficacy of a commercial stunner designed for culling juvenile salmonids. To test whether unconsciousness was achieved immediately, Atlantic salmon smolts and fry in a small tank were exposed to an electric field similar to that inside the stunner for 1 s. Subsequent behavioural observation determined that they had been rendered unconscious. In the second experiment, Atlantic salmon parr and smolts were observed for up to 17 min after transiting the commercial stunner to ensure that unconsciousness persisted long enough for death to supervene. Of the 640 fish observed (both in air and water) none showed any signs of recovery. The commercial system and the scaled-down tank used electric fields of 3.1 V/cm and 2.36 V/cm respectively. We conclude that in-water electric stunning can deliver instant and lasting unconsciousness for salmonid juveniles and is well suited for culling operations.