Working Group on Surveys on Ichthyoplankton in the North Sea and adjacent Seas (WGSINS; outputs from 2021 meeting)

The Working Group on Surveys on Ichthyoplankton in the North Sea and adjacent Seas (WGSINS) carries out surveys on early life stages of North Sea, Baltic and Irish Sea fishes. The objectives of WGSINS described in this report include review and coordination of ichthyoplankton surveys undertaken for assessment purposes, preparation of data for archival, documentation to provide quality assurance on species identification, and identification of additional objectives that can be achieved within the existing survey designs.

The international herring larvae surveys in the North Sea (IHLS) revealed low numbers of newly hatched larvae around the Orkneys in autumn 2020. In the Buchan and Banks, larvae hatched in larger quantities, but concentrated in only two dense areas. As in recent years, large numbers of herring larvae were found at the Downs herring spawning sites.

The Midwater Ring Net (MIK) sampling during the first quarter international bottom trawl survey in 2021 (Q1 IBTS) reported higher abundance of foraging herring larvae, compared to 2020. However, at the potential nurseries in the eastern part of the North Sea, abundances of larger herring larvae were lower than last year.

The Rügen herring larvae survey (RHLS) considers the major spawning areas of western Baltic spring spawning herring, but there is still no substantial recruitment in the area.

The Northern Irish Herring Larvae survey monitored the vast majority of larvae in the eastern Irish Sea, in the vicinity of the Douglas bank and north of the Isla of Man. In contrast to the previous year, larvae were also detected in the vicinity of the Mourne spawning ground, along the Irish coast.

The Northern Irish MIK survey resumed in 2021 after missing 2020 due to Covid-19 restrictions. Target species juvenile gadoids cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) were located predominantly in the western Irish Sea. Additional MIK hauls indicated higher abundances of juvenile gadoids north of the study area, which will be monitored in future years to ensure that the main distribution of juvenile gadoids is surveyed.

The Downs recruitment survey (DRS) resumed after a pause in 2020 due to Covid-19 measures. In 2021, highest numbers of herring larvae were observed at the southern-most stations, just above the Wadden Sea isles. Results were comparable to 2018. In 2019, the numbers were much lower and the larvae more westerly distributed.

The individual Baltic Ichthyoplankton Surveys (BIS) are distributed over the spawning season of the main target species Eastern Baltic cod. Sample analyses are still ongoing. 2021 preliminary data indicate that the egg-based stock biomass remains at a low level as in 2018-2020.

A four-year pilot study tries to establish a sprat recruitment index for the North Sea. Survey results are promising, but broader area coverage is necessary. Another pilot study investigated the distribution and abundance of small, recently hatched sandeel larvae. High larval abundances were found at Dogger Bank and Horns Reef, indicating that these are major spawning areas.

Results of marine litter in the North Sea MIK surveys are available for 2017-2020 and sampling was continued in 2021.

WGSINS will continue to collect and archive information on co-occurring fish larvae to establish time-series as a basis for further analyses of species distribution.