Workshop on scrutinizing of acoustic data from the IESSNS survey (WKSCRUT2)

This report describes the results of the Workshop on acoustic scrutinising procedures for the International Ecosystem Summer Survey in the Nordic Seas (IESSNS) recommended by the Working Group of International Pelagic Surveys (WGIPS). The IESSNS targets mackerel, herring and blue whiting during their summer feeding migration in the Nordic Seas. However, mackerel is estimated by standardized swept-area trawl method as it currently cannot be estimated by acoustic methods, while herring and blue whiting are estimated using standard acoustic methods. In internationally coordinated acoustic surveys the scrutinisation procedures may differ slightly among participating nations. It is therefore very important that all scientists responsible for the scrutinisation are following the same general procedures, and that the procedures are revisited on a regular basis to ensure a standardised categorization and allocation into species or species groups during pelagic ecosystem surveys.
The group defined three areas with typical and common acoustic backscatter features within the total survey area covered by IESSNS: the Irminger Sea including East Greenland and West Ice-land , the Norwegian Sea and adjacent areas including Iceland Sea and the area around the Faroes, and shelf areas. Further, two general procedures were presented; the first was how to separate herring from plankton (the "200 kHz" method), and the second how to deal with the acoustic backscatter in the upper layers in years when strong year-classes of blue whiting occur in the survey area (that resemble small herring schools). Analyses of several examples showed that all participants used the same general procedure during the scrutinising process. However, some minor adjustments were made by individual participants to ensure that an identical procedure was followed by the whole group, as shown by the examples in this report.
An important lesson learnt from this workshop was to urge cruise leaders to take frequent trawl samples of the acoustic recordings during the survey, to be able to ground-truth (= identification by trawling) the echograms ('always trawl when in doubt'). This is especially necessary to iden-tify all observed acoustic layers/scatters in the area at the beginning of the survey, to ensure consistent and good quality scrutinising during the survey.
The 'threshold method' (i.e. adjusting the lower threshold) is described in the report and advice is given on how to use the method to identify schools and fish targets among masking plankton layers. A standardised threshold method reduces the need for human 'expert' judgement during the scrutinising process.The group recommends that all participants store the data at maximum -72 dB and preferably at -82 dB, but are aware of possible problems for some vessels with noise at that low level.