Journal Article

Fish behaviour and orientation-dependent backscatter in acoustic Doppler profiler data

Len Zedel, Ranjan Patro and Tor Knutsen

in ICES Journal of Marine Science

Published on behalf of ICES/CIEM

Volume 62, issue 6, pages 1191-1201
Published in print January 2005 | ISSN: 1054-3139
Published online January 2005 | e-ISSN: 1095-9289 | DOI: http://dx.doi.org/10.1016/j.icesjms.2005.04.022
Fish behaviour and orientation-dependent backscatter in acoustic Doppler profiler data

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It is well known that fish target strength can be affected by fish behaviour. In particular, the specific orientation between a target (fish) and an echosounder system is critical in interpreting the apparent target strength. This situation is further complicated for Doppler current-profiler systems that have an array of diverging beams and where each beam will interact with targets with a different orientation. When targets themselves have random orientations, each beam will see the same range of target strengths and the average values would be the same. However, if targets such as schooling fish have a preferred orientation, then an asymmetry is present and different target strengths can be expected. This hypothesis is explored using observations of Norwegian spring-spawning herring (Clupea harengus) under a variety of schooling conditions. When schooling fish are migrating and have well-defined, coherent orientations, differences in backscatter strength of about 5 dB can be seen depending on beam orientation. This difference disappears when fish are not actively migrating and so do not have coherent orientation. These differences can be used to infer behaviour of schooling fish but they also indicate that caution must be exercised when averaging backscatter data from separate Acoustic Doppler Current Profiler (ADCP) beams.

Keywords: ADCP; fish behaviour; Norwegian herring; target strength

Journal Article.  5126 words.  Illustrated.

Subjects: Environmental Science ; Marine and Estuarine Biology

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