The objectives of the Working group on Surveys on Ichthyoplankton in the North Sea and adja-cent Seas (WGSINS) were to review Ichthyoplankton surveys undertaken for assessment pur-poses, coordinate the surveys in 2020, prepare data for archiving, provide quality assurance on species identification and identify additional objectives that can be achieved within the existing survey designs.
The international herring larvae surveys in the North Sea (IHLS) were in autumn 2018 and early 2019 affected by severe technical problems of one of the research vessels. As a consequence, spa-tial information on larvae abundance around the Orkney/Shetlands and in the English Channel was too limited to be included in the herring assessment 2019. Only abundance data of the Buchan and Banks components were used.
The MIK net (Midwater Ring Net) sampling during the first quarter international bottom trawl survey in 2019 (Q1 IBTS) reported foraging herring larvae in higher quantities in the western part of the North Sea, the Southern Bight and in the Skagerrak. In the eastern part of the North Sea, the potential nurseries, abundance of larger larvae was very low, and virtually no larvae occurred in the German Bight. Simulations on sampling effort reduction showed the MIK index to be relatively stable, even when sampling is reduced by 50%, but such a reduction could be disadvantageous for representative sampling of less abundant species.
As an addition to the conventional MIK sampling, a sampling programme on herring larvae recruiting from the Downs stock component has been carried out during 2018 and 2019. The Downs recruitment survey (DRS) revealed larger quantities of foraging larvae, which are so far not integrated in the recruitment index of North Sea autumn spawning herring.
The Rügen herring larvae survey (RHLS) considers the major spawning areas of western Baltic spring spawning herring. There is no substantial herring recruitment in the area.
Several studies tried to utilize surveys to gather additional information. The Northern Irish MIK survey (NIMIK) usually provides recruitment information on gadoids, but also collect infor-mation on the wider ecosystem. As one example, a two decade long time-series of gelatinous zooplankton abundance and distribution in the Irish Sea has been built on survey catches.
A pilot study tried to use samples from the Q3 IBTS to establish a sprat recruitment index. Results from 2018 and 2019 indicate very promising potential, but wider area coverage would be bene-ficial. Thus participating countries are welcome to contribute to the ongoing study.
Analyses of three years of litter sampling during MIK surveys revealed sources of litter in the water column, and uses the MIK net flowmeters to quantify the amount. The samples may also provide insights into possible pathways of drifted material.
While progressing with the original survey objectives, WGSINS will continue to invest efforts in using herring larval surveys to provide additional information. WGSINS will summarize infor-mation on co-occurring fish larvae, and establish time-series to form the basis for further analyses of species distribution, abundance and, if possible, trends in recruitment.