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ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on November 8, 2006
ICES Journal of Marine Science: Journal du Conseil 2007 64(1):39-48; doi:10.1093/icesjms/fsl010
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© 2006 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Improving acoustic estimates of krill: experience from repeat sampling of northern krill (Meganyctiphanes norvegica) in Gullmarsfjord, Sweden

Inigo Everson1, Geraint A. Tarling2 and Bo Bergström3

1 Environmental Science Research Centre, Anglia Ruskin University, Cambridge CB1 1PT, UK
2 Natural Environment Research Council, British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
3 formerly Kristineberg Marine Research Station, Kristineberg 566 450 54, Fisksebackskil, Sweden; now Greenland Institute of Natural Resources, P.O Box 570, 3900 Nuuk, Greenland

Correspondence to I. Everson: tel: +44 1223 363271 ext. 2805; fax: +44 1223 417712; e-mail: i.everson{at}anglia.ac.uk

Everson, I., Tarling, G. A., and Bergström, B. 2007. Improving acoustic estimates of krill: experience from repeat sampling of northern krill (Meganyctiphanes norvegica) in Gullmarsfjord, Sweden – ICES Journal of Marine Research, 64, 39–48.

A series of eight replicated acoustic surveys, four by day and four by night, was undertaken in Gullmarsfjord on the Swedish west coast during two 24-h periods on 8 and 10 September 2003, using a calibrated echosounder operating at 120 and 38 kHz. The difference in signal strength ({Delta}Sv) was used to distinguish northern krill (Meganyctiphanes norvegica) from other acoustic scatterers. The approach is concluded to be very effective, but it can be improved greatly by applying the following series of simple extensions to current protocols: first, set a very low threshold on both frequencies to minimize sampling bias; second, undertake tests to confirm that the data extracted from each acoustic frequency apply to the same scatterers; third, ensure that the range of {Delta}Sv is not greater than the TSrange at either frequency; and finally, when abundance estimation is the primary aim, arrange for sampling at the time of day and using the acoustic frequency that together provide the least variance.

Keywords: acoustic surveys, dB difference, Meganyctiphanes norvegica, northern krill


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