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

• Published in: ICES Journal of Marine Science
• Main Authors: Everson, Inigo, Tarling, Geraint A., Bergström, Bo
• Format: Text
• Language: English
• Published: Oxford University Press 2006
• Subjects: Article; Meganyctiphanes norvegica; Northern krill
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/fsl010v1; https://doi.org/10.1093/icesjms/fsl010

<qd> 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. </qd>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&emsp14;kHz. The difference in signal strength (Δ S v ) 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 Δ S v is not greater than the TS range 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.