Journal Article

Tilt angle and target strength: target tracking of Atlantic cod (<i>Gadus morhua</i>) during trawling

Ian H McQuinn and Paul D Winger

in ICES Journal of Marine Science

Published on behalf of ICES/CIEM

Volume 60, issue 3, pages 575-583
Published in print January 2003 | ISSN: 1054-3139
Published online January 2003 | e-ISSN: 1095-9289 | DOI:
Tilt angle and target strength: target tracking of Atlantic cod (Gadus morhua) during trawling

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  • Environmental Science
  • Marine and Estuarine Biology


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Vertical orientation (tilt angle) is known to affect the target strength (TS) of ensonified fish and is a large component of the variability inherent in acoustic-biomass estimates. To measure the effects of changes in tilt angle on TS during diel vertical migrations, a concentration of migrating Atlantic cod (Gadus morhua) was observed acoustically from a research vessel over several days. Single-target data were collected from a split-beam echosounder and were subsequently tracked, corrected for vessel orientation and movement, and analysed for 3-dimensional displacement (speed and direction). The results revealed a large variability in TS and several patterns of swimming behaviour from random to directed orientation and movement, with changes in both vertical and horizontal displacements and inferred orientation. These behavioural patterns and their affects on TS were analysed as a function of “time-since-sunset”. Regular diel orientation patterns were observed as cod rose from the ocean bottom in the evening, increasing their tilt angle, and descended at sunrise to regain the ocean floor. Standardized TS (B20) was found to be highly correlated with tilt angle. This relationship can be used to correct for the diel changes in the TS of these migrating cod as a function of the in situ-measured tilt angle and thus to improve the accuracy of acoustic-biomass estimation.

Keywords: tilt angle; Atlantic cod; trawling; swimbladder volume

Journal Article.  4540 words.  Illustrated.

Subjects: Environmental Science ; Marine and Estuarine Biology

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