Due to increasing consumer demand for salmon, salmon farmers are constantly seeking to expand their production. However, environmental barriers such as parasite load and site carrying capacity are preventing expansion in established sites. Therefore, salmon farmers seek to move or expand their production to more exposed sites. In exposed sites, salmon are likely to experience stronger currents and larger waves than what they would normally do. Some previous work has established some baseline information on swimming capacity and behaviour of salmon in currents, but it is not well established how currents and waves interact, and how well salmon are able to cope in conditions where they are exposed to both.
In this study we investigate the effects of strong currents and waves on the behaviour of salmon and how they choose to use the space available to them. Using video cameras and echo sounders, we show that fish prefer to use the entire water column, narrowing their range only as a response to cage deformation (Figure 1), waves, or daylight.
Conversely, fish show strong horizontal preference, mostly occupying the portions of the cage exposed to currents. Additionally, waves cause salmon to disperse from the exposed side of the cage to the more sheltered side. Even when strong currents decrease the amount of available space, salmon choose to occupy the more exposed part of the cage.
This indicates that at least with good water exchange, the high density caused by limited vertical space is not so aversive that salmon choose to move to less desirable areas of the cage. However, the dispersal throughout the entire available water column indicates that making cages deep enough to compensate for deformation in strong currents would be beneficial to salmon welfare.