Journal Article

Counting capelin: a comparison of acoustic density and trawl catchability

R. L. O'Driscoll, G. A. Rose and J. T. Anderson

in ICES Journal of Marine Science

Published on behalf of ICES/CIEM

Volume 59, issue 5, pages 1062-1071
Published in print January 2002 | ISSN: 1054-3139
Published online January 2002 | e-ISSN: 1095-9289 | DOI: http://dx.doi.org/10.1006/jmsc.2002.1262
Counting capelin: a comparison of acoustic density and trawl catchability

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Acoustic estimates of capelin, Mallotus villosus (Müller), density were compared with catches in 97 Campelen 1800 bottom and 113 IYGPT midwater fishing sets to assess catching efficiency (qe) relative to acoustic estimates, and the ability of trawl and acoustic surveys to index capelin abundance. Catches in experimental IYGPT sets targeted at capelin and towed at constant depth or undulated over 40 m did not differ from acoustic predictions over a range of densities from 0.00001 to 0.2 fish per m2 (qe=1; p<0.05). Catch and qe did not differ between fishing methods (p>0.05, paired t-test). Campelen catches were typically bigger than acoustic estimates in the trawl zone at low or medium densities, likely a consequence of dead-zone non-detectability. Campelen qes were strongly density-dependent (>1 at densities >0.05 fish per m3; <1 at higher densities), and ranged over several orders of magnitude, making catches representative only of presence/absence. For the IYGPT trawl undulated at fixed survey stations, not targeted at capelin, mean qe averaged 11.6, and high variability, in part related to vertical distribution (s.e.=3.76), limited usefulness as an abundance index. A recognition threshold caused acoustic integrations based on a priori allocation of backscatter to underestimate capelin distributed at low densities (<0.05 fish per m3), as did dead-zone bias. A posteriori allocation reduced bias. It is concluded that acoustic integration supported by directed trawling is the most reliable method of counting capelin.

Keywords: acoustics; capelin; q; trawl catchability

Journal Article.  0 words. 

Subjects: Environmental Science ; Marine and Estuarine Biology

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