© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
A requiem for the use of 20 log10 Length for acoustic target strength with special reference to deep-sea fishes
National Institute of Water and Atmospheric Research Ltd PO Box 14901, Kilbirnie, Wellington, New Zealand
*Correspondence to S. McClatchie; tel: +64 4 386 0300; fax: +64 4 386 0574. e-mail: s.mcclatchie{at}niwa.cri.nz.
Although it is well known that the slopes of target strength (TS) and length relationships vary widely, it is common in fisheries acoustics to force the TS–length regression through a slope of 20. Is it time to abandon this practice? The theoretical justification was that TS should be proportional to cross-sectional area, and that area should scale as the square of the linear dimension (fish length). There are now many species other than gadoids that are the subject of acoustic surveys, and many of them do not have the same morphology as the gadoid fishes. The slope of the TS–length regressions deviates significantly from 20. The empirical slope should be used wherever it can be shown to be more appropriate than the 20 log10 L model. Using the data from swimbladder models, it is shown that Macrourids, a merluccid hake and Oreosomatidae have a different relationship should between swimbladder size and fish size compared with that of gadoids. It is demonstrated that the 20 log10 L model is not appropriate for these deep-water fish and that deviations from the model arise, to a considerable degree, from variation in fish morphotypes. The TS of deep-water Macrourids, a merluccid hake and Oreosomatidae are lower than that of gadoids. This is related to the swimbladder size–fish size relationship in different morphotypes, although not much evidence can be found to support the concept that swimbladder sizes are generally smaller in deep-sea fishes.
Keywords: fish target strength, acoustic deepwater size morphology
Received 26 June 2002; accepted 13 October 2002.
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