Welfare indicators of Atlantic Salmon (Salmo salar) observed with broadband acoustic in a submerged cage

Farming of Atlantic salmon is Norway’s largest animal production, and with this follows welfare problems like diseases and parasites. It is desirable to improve welfare, and one possible solution to reduce lice infections is to submerge the cages. However, Atlantic salmon has a physostomous swim bladder and needs to re-fill it with air almost daily to regulate buoyancy. A key future challenge is thus to measure the welfare of Atlantic salmon when using new technology such as a submerged cage, where access to air is taken away or supplied through a small artificial surface in one or more air domes at depths. The aim of this thesis is to investigate whether broadband echosounders can be used as a device for obtaining data on abnormal swimming behaviour for salmon, specifically detect changes when the swim bladder is getting low on air. By submerging the cages, the swim bladder will almost be empty after about 2-3 weeks. our work show that broadband acoustics can be used to measure several welfare params. We found that swimming speed gradually increases when the salmon has no access to air and swim bladder emptied from 0.2 ml to 0.004 ml of air over the 15 days. In comparison, both target strength (TS) and volume backscattering (SV) showed similarities and clearly decreased after around day 10 without air access. TS gradually declined -13 dB down and increased 12 dB when roof was removed, and Sv decreased and increased 11dB. Further, we observed that at night salmon stays higher up in cages without light, compared to swimming behavior around the artificial light when light was on at night. With broadband acoustics it is easier to define salmon’s welfare with TS and SV, compared to swimming speed where there was a clearer pattern when taking swimming speed with camera than using broadband. Broadband acoustics may solve key challenges in measuring salmon welfare. Our results highlight that broadband acoustic can be used to observe and quantify normal and abnormal behaviour, when salmon is submerged, and can potentially be developed into an “alarm system” that will give indications on how the salmon’s welfare is.