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

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 (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 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.

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