Sideways exit during crowding utilises natural salmon behaviour for easier transfer

Efficient crowding of fish in sea cages for the purpose of transferring them to fasting, treatments or slaughter, is critical for insuring fish are moved quickly with minimal stress. With the development of larger offshore cages holding millions of fish and/or submerged cage salmon aquaculture, extraction of fish will potentially be more difficult. Particularly if farmers seek to remove portions of the biomass at a time without lifting cages to the surface. Using salmon innate swimming behaviours may aid to develop innovative, simple, welfare-friendly removal methods. Here we investigate whether the direction of crowding in a submerged cage influences the exit behaviour of salmon when extracted at depth for the purpose of pumping to a well-boat or otherwise. Using replicates of 46 large (∼4.3 kg) or 128 small (∼1.3 kg) Atlantic salmon and a prototype submerged cube cage (27 m³) fitted with a movable wall, we test to see if crowding salmon towards a 50 cm diameter circular opening in either the top, side, or bottom of the cage influences fish exit rate. Our results show that when crowding fish by incrementally reducing the cage volume by a factor of 12 over 25 mins, for both fish sizes ∼80 % of fish exited the cage via sideways crowding, whereas only 20 to 50 % exited by top up or bottom down crowding directions. Furthermore, maximum relative fish densities reached during crowding tests were almost halved for sidewards crowding (37–43 kg m⁻³) compared to downwards or upwards crowding directions (59–73 kg m⁻³). We conclude that fish visualization of the exit hole and their natural circular swimming behaviour favoured sideways extraction. Hence, with a sea cage design that enables sideways crowding, it may be possible to extract fish quickly with minimal stress, and without needing to raise cages to the surface.