LAUSR

Abstract Broadband echosounder systems provide very high range resolution that could resolve different parts of fish and hence provide estimates of fish size. The potential for, and accuracy of, direct acoustic sizing of fish was tested on fish-like targets of known dimensions and orientations. Prolate spheroids, 91–477 mm long, with and without an air-filled inclusion (simulating a fish swim-bladder), made from polyvinyl-alcohol-cryogel, were suspended in an acoustic beam and rotated while the backscattered signal was recorded. The echoes from linear frequency modulated pulses of 45−90 and 160−260 kHz with a fast and a slow amplitude modulation were pulse-compressed to provide data with a range resolution of 6.6 mm at its highest. Echoes from the target boundaries were manually identified, tracked and used to directly estimate the thickness and length of the targets. An accuracy of ±11−17 mm was achieved for targets longer than about 200 mm and thicker than about 20 mm when using the 160−260 kHz pulse. The length of the air-filled inclusion could also be estimated for the larger targets. Echoes from the slow amplitude modulation were easier to interpret than the fast amplitude modulation despite the lower range resolution, due to the lower pulse compression temporal sidelobes. The results show that remote acoustic sizing of fish is feasible and of sufficient accuracy for practical use.