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ICES Journal of Marine Science: Journal du Conseil 2004 61(6):1004-1013; doi:10.1016/j.icesjms.2004.04.004
© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
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Species discrimination of fish using frequency-dependent acoustic backscatter

Elizabeth A. Logerwell* and Christopher D. Wilson

National Marine Fisheries Service, Alaska Fisheries Science Center F/AKC2, PO Box 15700, 7600 Sand Point Way, Seattle, WA 98115, USA

*Correspondence to E. A. Logerwell: tel: +1 206 526 4231; fax: +1 206 526 6723. e-mail: libby.logerwell{at}noaa.gov.

The difference between mean volume-backscattering strength at 120 and 38 kHz ({Delta}MVBS) has been used to discriminate acoustically between macrozooplankton species, and between macrozooplankton and fish or small zooplankton. We examined whether {Delta}MVBS could be used to discriminate between juvenile pollock (Theragra chalcogramma) and capelin (Mallotus villosus). Acoustic data at 38 and 120 kHz were collected in the Gulf of Alaska during August 2000 and 2001. We selected scattering layers of juvenile pollock and capelin that were sampled directly by midwater trawls. Although we found statistically significant differences at minimum integration thresholds ranging from –85 dB to –69 dB, the greatest difference between {Delta}MVBS of juvenile pollock and capelin was observed at the highest integration threshold (–69 dB). We also found that, although there was substantial overlap between the frequency distributions of juvenile pollock and capelin {Delta}MVBS at the smallest scale of analysis (0.1 nautical mile x 5 m cells), there was virtually no overlap between the {Delta}MVBS distributions at the largest scale (~1 nautical mile x 20 m aggregations). We conclude that acoustic differencing at the scale of fish aggregations and at high integration thresholds can be used to distinguish between juvenile pollock and capelin.

Keywords: acoustic survey, capelin, dual frequency, Gulf of Alaska, Mallotus villosus, pollock, Theragra chalcogramma

Received 1 December 2003; accepted 6 April 2004.


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