Atlantic Herring in 6aS/7b, Industry Acoustic Survey Cruise Report, November-December 2020 and January 2021.

Abstract

An acoustic survey of Atlantic herring Clupea harengus was conducted in ICES areas 6aS/7b in November-December 2020 and January 2021. The 2020 survey was conducted using five vessels; MFVs Crystal Dawn WD201, Ros Ard SO745, Johnny G S653, Abigail S SO354, and St. Catherine D299. The 6aS/7b survey design changed in 2020 compared with previous years in that only 6 core areas with prior knowledge of herring distribution from the monitoring fishery were targeted for surveying. This was largely based on the results from ICES WKHASS (ICES 2020) and from lessons learned in the previous surveys in this area from 2016-2019. This design resulted in a much reduced survey area compared to previous years, but with better coverage of most of the important inshore bays where the monitoring fishery takes place. The survey design objective remained the same; to capture the distribution of winter spawning herring in the 6aS/7b area, but this design was not expected to achieve overall stock containment. The timing of surveys in the core areas was flexible from the outset by design. It was decided that greater flexibility would allow for a targeted spatial and temporal approach which avoided the inevitable poor weather that can happen in this area during this time of the year and which lead to reduced survey effort in previous years. Using smaller vessels allowed surveys to be conducted in shallow inshore areas where herring are known to aggregate during this time of the year. This survey is the fifth consecutive annual acoustic survey for herring in this area at this time of the year. A pole-mounted system with a combi 38 kHz (split) 200 kHz (single) transducer was used successfully for the survey on small vessels (<18m) in 2020. Herring were again distributed inshore in shallow areas, and the improved survey design and use of small vessels for the survey resulted in a good measure of uncertainty (CV). The stock was not overall contained in 2020, particularly in the Donegal Bay area (Bruckless, Inver Bays, etc.) and more effort is required to target survey effort later in December and January when herring appear to show up in these areas in greater numbers. The COVID pandemic affected the ability to conduct surveys in late December. Very strong herring marks were evident in Lough Foyle and Lough Swilly in the channel in marks that extended for many miles. This was in areas where smaller boats in the fishery were concentrating effort. There was also a series of herring marks in Bruckless Bay, Fintra Bay and Inver Bay in discreet areas. There were a few small herring marks in the Achill strata. The monitoring fishery was being conducted on smaller boats in the same areas and close to the same time as the survey and biological samples from some of these vessels were used. There was a wide distribution of length classes in all hauls, with most hauls dominated by larger (> 22 cm) mature fish. The 2- and 3-wr age class of herring accounted for 54% of the overall numbers in 2020. The total stock biomass (TSB) estimate of 45,046 tonnes is considered to be a minimum estimate of herring in the 6aS/7b survey area at the time of the survey. The flexible survey design and focusing on discreet areas was generally successful and should provide a template for future survey designs.