The sonar was a modified EdgeTech 3200 sub-bottom profiler comprising a towed body with four broadband sonar channels and on-board processing. It was deployed from MRV Scotia in the North Sea from 5-15 October 2015, during the annual mackerel survey carried out by Marine Scotland Science. The first activity was to calibrate the system in Loch Eriboll and St Magnus Bay. During the cruise, records were taken of schools of herring, pearlsides (Maurolicus muelleri) and mackerel. Analysis of these data included enhancement of the processing software to improve the estimation of volume backscattering strength spectra. Unfortunately, the mackerel were not present in the expected areas of the survey and only after searching extensively to the extreme north east of the North Sea were mackerel schools found. Only one school was sampled with the fishing trawl: the average fish size in this catch was 33 cm. The equivalent estimates of mackerel size as calculated from the sonar data was 21.5 cm. Although this is close to what was observed in the catch, it is not as close as is required. Clearly more data is required to tune the sizing algorithm using more catch data of various sizes. Enhancements to the algorithm and scattering model on which it is based are also required.