How caged salmon respond to waves depends on time of day and currents

Abstract

Disease, pest control, and environmental factors such as water quality and carrying capacity limit growth of salmon production in existing farm areas. One way to circumvent such problems is to move production into more exposed locations with greater water exchange. Farming in exposed locations is better for the environment, but may carry unforeseen costs for the fish in those farms. Currents may be too strong, and waves may be too large with a negative impact on growth and profit for farmers and on fish welfare. This study employed two major fish monitoring methods to determine the ability of Atlantic Salmon (Salmo salar) to cope with wavy conditions in exposed farms. Echosounders were used to determine vertical distribution and horizontal preference of fish during different wave and current conditions as well as times of day. Video cameras were used to monitor shoal cohesion, swimming effort, and fish prevalence in locations of interest. The results indicate complex interacting effects of wave parameters, currents, and time of day on fish behaviour and vertical distribution. During the day, hydrodynamic conditions had stronger effects on vertical distribution than during the night. In weak currents, fish generally moved further down in taller waves, but stronger currents generally caused fish to move upwards regardless of wave conditions. Long period waves had unpredictable effects on vertical distribution with fish sometimes seeking deeper water and other times moving up to shallower water. It is unclear how much the cage bottom restricted vertical distribution and whether movement upwards in the water columns was related to cage deformation. In extreme cases, waves can reach below the bottom of a salmon cage, preventing fish from moving below the waves and cage deformation could exacerbate this situation. Farmers ought to take into consideration the many interacting effects on salmon behaviour within a cage as well as the potential for cage deformation when they design their farms for highly exposed locations. This will ensure that salmon are able to cope when storms and strong currents hit at the same time.

Fish welfare in offshore salmon aquaculture

“…To our knowledge, there are presently only two available studies on this topic, where both were conducted at wave‐exposed sites on the Faroe Islands (Dam 2015; Johannesen et al . 2020). In the first study, the group structure was disestablished by powerful waves with significant wave heights of 2–2.5 m, periods of ~14 s and vertical movements of 0.3–0.8 m. Furthermore, it seemed that during daytime, the fish would avoid the surface layers (Dam 2015).…”

“…Similar observations were made by Johannesen et al . (2020) at a farm site with significant wave heights of up to 2.9 m, but with mostly wind driven short period waves of less than 14 s and low current speeds of less than 20 cm s −1 . In addition, fish tended to move away from the side of the cage during large waves, orienting their swimming direction along the wave rather than maintaining a position against the current (Johannesen et al .…”

“…In addition, fish tended to move away from the side of the cage during large waves, orienting their swimming direction along the wave rather than maintaining a position against the current (Johannesen et al . 2020). These behavioural responses were also seen in a miniature wave and current tank study, where waves caused 16 cm long salmon to swim in individually independent patterns rather than displaying organized group behaviours as when only water currents are present (He et al .…”

“…Accordingly, observations of diurnal fish behaviour at a wave‐exposed site revealed a wider depth distribution of the fish at night where a part of the group occupied shallower waters (Johannesen et al . 2020). Whether such variation in diurnal depth preference can be an issue in exposed aquaculture is worthy of additional study.…”

“…Johannesen et al . 2020). Moreover, advances in bio‐logging technologies may provide additional information about the physiology and behaviour on the individual level in the ambient farm environment (Brijs et al .…”