Sammendrag
Salmon lice and poor surface conditions are challenging for salmon aquaculture. A new innovative
preventative solution is to submerge the cages below the sea surface to avoid the surface-dwelling
infestation of lice larvae. However, the physostomous salmon require daily surface access, or the
swim bladder will deflate. The ensues negative buoyancy leads to increased swimming speed, and
over a prolonged time (2-3 weeks), the appetite and growth rate may be reduced, resulting in poor
welfare. For 28 days, 500 salmon were placed in a cage submerged to 1 m depth, preventing the
salmon from reaching the sea surface and refilling the swim bladder. Beneath the cage, three EK80
scientific wide-band echo sounders with 70, 120, and 200 kHz split-beam transducers were deployed
to study the acoustic backscatter over time. From the backscattered signals, parameters such as the
target strength (TS), volume backscattering strength (SV ), the frequency response of both TS and SV ,
and the swimming speed could be calculated. These parameters are studied to determine the first to
indicate an insufficient air level based on signal type and frequency. The results show an increase in
swimming speed and a decrease in both TS and SV values. From the measured daily T S values from
both FM and CW signals, the decrease appeared to begin from day 6 of submergence. As the air in the
bladder diminished, the ratio between the values from the three echo sounders increased, which was
evident from TS and SV measurements, and the frequency responses. The swimming speed acquired
from the acoustical recordings decreased from day 5, but the speeds were unexpectedly high. These
findings may be used to develop automatic alarms that detect low swim bladder inflations or poor
welfare and provide new fundamental knowledge about backscattering from salmon.
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