Workshop on Herring Acoustic Spawning Surveys (WKHASS)

The purpose of the Workshop on Herring Acoustic Spawning Surveys (WKHASS) was to review the methodology (survey design, timing, identification of the sources of uncertainties and how to address them, among other issues), and abundance estimates (including CV) provided from surveys carried out in 6a and 7a on commercial vessels. The results from this workshop will be analysed by the Working Group of International Pelagic Surveys (WGIPS) with the aim to establish a protocol to be included in a future update of the Manual for International Pelagic Surveys (SISP 9). This will ensure repeatability of the survey as reviewed, allow appropriate data quality checks and provide transparency of methodologies, as a prior step for using it as a fishery-independent index in assessment models. Workshop participants presented state of the art of each acoustic-trawl survey carried out on board commercial vessels in 6aN, 6aS/7bc and 7a targeted on herring at the spawning season.

Special emphasis was placed on examining survey strategies to increase the precision of the abundance estimates. The main issues with precision are related to both the aggregation and spatial patterns observed in the herring shoafils before and during the spawning period. These issues can be addressed by accounting for both the survey design and timing. For this purpose, a series of statistical analysis (e.g. geostatistics, bootstrap) were carried out not only for the acoustic records but also in the number of fishing hauls for biological (length and age among other biological parameters).

The three surveys analysed (6aN, 6aS/7b,c and 7a) give robust estimates of abundance although there is still room for increasing their precision. Herring behaviour is an important issue as fish change both the aggregation and spatial distribution patterns during the spawning season. In some areas, particularly where trawling is difficult or few marks are seen, it is difficult to confidently allocate echotraces to fish species, and the lack of enough biological data leads to an increase of the associated CV abundance estimates by age group. An increase in the number of tracks or even a reduction in Elementary Sampling Distance Unit, ESDU distance, both allowing for a better spatial resolution of the herring spawning aggregations, may help address some of these problems, and such tests have been underway. The use of cameras could be used to identify fish aggregations on non-trawlable areas to identify acoustic marks and make appropriate allocations to species. Prior scouting surveys would help to address other of the issues regarding with survey timing.

The analysis carried out during this workshop showed that both the 6aN and 6aS/7bc industry-led surveys are not yet sufficiently developed for them to be included in the SISP 9 survey man-ual because they are still undergoing regular changes as they learn from testing different designs regarding the issues and the solutions proposed to address them. It was recommended that the 7a survey in the Irish Sea is included in the SISP 9 manual for pelagic acoustic surveys. This survey is already used as a biomass index in the Irish Sea herring assessment.