Atlantic salmon ( Salmo salar ) is the most important farmed species globally. In Norway for example, exports generated USD 11 billion in 2020. The government predicted a doubling of production by 2030, however this may be restricted due to the lack of suitable farming sites and limitations to the number of licenses issued. Consequently, alternative sites are increasingly being considered by industries in Norway and other countries. One popular alternative includes the attractive ‘offshore’ locations in areas further from the coastline, exposed to high current speeds and strong wave action.
Yet, t he effect that heavy wave action will have on fish reared within cages in these novel environments remain unknown, raising questions about the Atlantic salmon’s coping ability and welfare.
The purpose of this original study was to investigate, in a fully controlled environment, the response of Atlantic salmon exposed to chronic water turbulence by measuring growth performance, behavioral conditioning, welfare scores, stress physiology, skeletal deformities, and swimming cap ability.
Materials and Methods
For 8 weeks, post-smolt Atlantic salmon were grown in three control and three treatment tanks at relevant farm stocking densities. Each treatment tank consisted of two tailor-made 100L buckets attached to a water pump, that when filled (approx. every 23 s), tipped over back into the tank creating water turbulence. Appetite was monitored daily and various behavior traits were scored throughout the trial. At weeks 2 , 5 and 8, a sub-sample of 48 fish were obtained to measure size, blood parameters and score welfare. At the completion of the trial, all fish were re-measured. Another sample of fish underwent a swim trial where the critical swimming speed (Ucrit) was determined. A final sub-sample was analyzed with radiology to detect skeletal deformities.
Atlantic salmon were able to cope and grow in constant turbulent conditions. A small 40g difference was observed between the groups (Fig.1A) . The condition factor was similar; 1.27 in the turbulence treatment compared to 1.28 in the controls.
A ppetite in the treatment tanks was 14% to 7% lower during the first 4 weeks of the trial , before reaching similar levels as the controls (Fig.2B) . This variation coincides with a behavioral change observed. During the first 4 weeks of the trial, individuals in the treatment tanks swam less coherently (<90% standing in the current). However, after this period , the treatment and control groups were both swimming consistently (> 90% standing in the current ). No large difference in the total welfare score was observed.
Ongoing analysis of stress physiology, swimming performance and skeletal deformities will also be presented. These will provide further important insights into how well Atlantic salmon adapt, cope and develop in turbulent farm environments.
With the already analyzed data, both groups appear to have had similar coping, despite the slight differences observed during the first weeks of the trial. This suggests that Atlantic salmon have the capability to acclimatize to an environment with constant turbulence without compromising welfare or growth to a large extent . The difference in final size remains unexplained. I t may have been caused by the difference in appetite at the start of the trial or by increased energy expenditure due to the efforts needed to swim in waves . Alternatively, i f the trial had been running for longer, the weight differences might have disappeared due to compensatory growth.
This work is the first attempt to experimentally study the effect of chronic turbulent environments on a cultured fish species which provides valuable novel knowledge in the industry-wide efforts to establish responsible offshore aquaculture practices. However, as a land-based experiment, it does not fully mimic the true offshore conditions at a commercial production scale. Nevertheless, this work clearly demonstrate that Atlantic salmon are a robust species and have the capacity to adapt well in novel-high energy environments without suffering substantial reductions in welfare or long-term production performance.