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Estimating tail-beat frequency using split-beam echosounders

Normally the echosounder is installed on the vessel hull. When lowered into a herring schoolar, the echosounder can monitor the frequency of the herring tail-beat. Among other things, the monotoring tells us how fat the herring is. This indicates the amount of food available to the herring. 

Scientists from the Institute of Marine Research and The Michelsen Centre for Industrial Measurement Science and Technology have developped the methode further. The method allows us to observe herring behaviour aspects previously obtained only from electroinc tagging .

The potential use is:

  • resolve schooling behaviour
  • improve target-strength estimates
  • estimate fish condition

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Data from a standard split-beam echosounder are used to estimate tail-beat frequency within a dense herring (Clupea harengus L.) layer. The data are collected by lowering a horizontally projecting 38 kHz split-beam transducer into a herring layer at 245-m depth. Individual targets are concatenated into tracks, and a fast Fourier transform is used to estimate the periodogram of the backscattering strengths along each track.

A simple model, assumed accurate under certain conditions, is used to relate the periodogram to tail-beat frequency. The requisite conditions are discussed in the report. Examples of accurate tail-beat estimates from single high-quality tracks, which are consistent with statistics on all tracks in the test dataset, are also presented.

The study is published in ”ICES Journal of Marine Science”.
Handegard, N. O.; Pedersen, G. & Brix, O. Estimating tail-beat frequency using split-beam echosounders. ICES Journal of Marine Science, 2009, 66, online version, doi:10.1093/icesjms/fsp003