The objectives of the Working Group on Surveys on Ichthyoplankton in the North Sea and adja-cent Seas (WGSINS) was to review ichthyoplankton surveys undertaken for assessment pur-poses, coordinate the surveys in 2021, 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) revealed small numbers of newly hatched larvae around the Orkneys in autumn 2019, while the other areas were more in line with preceding years.
The Midwater Ring Net (MIK) sampling during the first quarter international bottom trawl sur-vey in 2020 (Q1 IBTS) reported higher abundance of foraging herring larvae in the potential nurseries in the southeast of the North Sea, compared to 2019. However, the overall abundance has increased only slightly and the MIK index is still on a low level. Sardine larvae were recorded for the first time in the Kattegat and in the vicinity of the Orkney/Shetland area. The latter may indicate at another intrusion path into the North Sea, apart from the know one through the Eng-lish Channel.
A new molecular method to identify fish eggs, based on MIK samples, has recently been pub-lished. This method is not only less costly than DNA barcoding, it also delivers results much faster. It will be further developed by expanding the spectrogram database to a wider species scope.
The Rügen herring larvae survey (RHLS) considers the major spawning areas of western Baltic spring-spawning herring. There is still no substantial herring recruitment in the area.
The Northern Irish Herring Larvae survey monitored the vast majority of herring larvae in the eastern Irish Sea in the vicinity of the Douglas bank spawning ground and to the north of the Isla of Man. No larvae were detected on the Mourne ground.
A pilot study used samples from the Q3 IBTS to establish a sprat recruitment index of the North Sea. Results from 2018 to 2020 indicate very promising potential, but broader area coverage is necessary. Participants in the Q3 IBTS are encouraged to contribute in 2021.
The Downs recruitment survey (DRS) and the Northern Irish MIK survey (NIMIK) could not be conducted due to Covid-19 measures.
While progressing with the original survey objectives, WGSINS will continue to summarize in-formation on co-occurring fish larvae, and establish time-series as a basis for further analyses of species distribution, abundance and, if possible, trends in recruitment.