Verification of the acoustic techniques used to identify Antarctic krill

Acoustic surveys to estimate krill biomass require that the sound backscattered by krill can be identified and distinguished from all other types of backscatter. Sampling acoustic targets with nets to verify their identity have achieved this traditionally. More recently backscattered sound has been partitioned into krill or non-krill groups using differences in mean volume backscattering strength (MVBS) at two acoustic frequencies (DeltaMVBS = MVBS120 - MVBS38; where DeltaMVBS between 2 and 12 dB indicated krill). Here we compare net and acoustic data from two cruises around South Georgia in 1996 to assess the reliability of acoustic-based, target-identification techniques. MVBS data at 120 and 38 kHz were collected with a Simrad EK500 echosounder and net samples were collected with an RMT8. Around 80% of the echo integration cells from targets believed to be krill on the basis of their appearance on echo-charts, were also identified as krill from their difference in backscatter at 38 and 120 kHz. Krill biomass estimated from acoustic targets identified using echo-chart appearance or DeltaMVBS were broadly similar (regression: DeltaMVBS = 0.94 visual classification, r(2) = 0.99). Krill size was calculated from scattering models using the two frequency data and compared with that obtained in net hauls. This comparison revealed that a simplified bent-cylinder model was a better predictor of krill length than a fluid-filled, sphere model. We conclude that use of DeltaMVBS to identify Antarctic krill is advantageous because it is more objective than using echo-chart appearance. (C) 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.