© 2005 International Council for the Exploration of the Sea
Fish behaviour and orientation-dependent backscatter in acoustic Doppler profiler data
a Department of Physics and Physical Oceanography, Memorial University of Newfoundland St. John's NF A1B 3X7, Canada
b Department of Physics, College of the North Atlantic Labrador City NL A2V 2Y1, Canada
c Institute for Marine Research Bergen, Norway
*Correspondence to L. Zedel: tel: +1 709 737 3106; fax: +1 709 737 8739. e-mail: zedel{at}physics.mun.ca; ranjan.patro{at}northatlantic.nf.ca; tor{at}imr.no.
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
Received 1 November 2004; accepted 28 April 2005.
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