Biodiversity and distribution of gelatinous macrozooplankton in the North Sea and adjacent waters - dataset from winter 2022.

The diversity and distribution of gelatinous macrozooplankton is described by presenting qualitative and quantitative data of the jellyfish and comb jelly community encountered in the North Sea and Skagerrak/Kattegat during January/February 2022. Data were generated as part of the North Sea Midwater Ring Net (MIK) survey [1], an ichthyoplankton survey conducted at night-time during the quarter 1 (Q1) International Bottom Trawl Survey (IBTS), aboard the Danish R/V DANA (DTU Aqua) and the Swedish R/V Svea (SLU). A total of 100 stations were investigated using a 13 m long Midwater Ring Net (MIK net) with an opening diameter of 2 m and a mesh size of 1.6 mm, which is 0.5 mm meshed for the last meter of the net and the cod end [2]. Samples were collected by double oblique hauls from the surface to 5 m above the seafloor [2]. Twelve gelatinous macrozooplankton species were encountered during the Q1 2022 survey. Species encountered included the hydrozoan jellyfish i) , ii) , iii) spp., iv) , v) , vi) ; the two scyphozoan jellyfish i) and ii) as well as the comb jelly (ctenophora) species i) spp., ii) , iii) and iv) the non-indigenous . In total 4882 individual specimens from samples and sub-samples were analyzed and extrapolated to 71,888 records of gelatinous macrozooplankton in the investigation area. For rare species, the entire sample was analyzed, while for abundant taxa, sub-samples were used to assess abundances. The raw counts were converted to volume-specific densities (individuals m) and area-specific abundances (individuals m), based on calibrated flow meter recordings and recorded maximum depth of the MIK net during each haul. Further, size data for the different species were obtained from a total of 4775 individual gelatinous macrozooplankton organisms. Size data are presented in the accompanying database and was used to calculate species-specific wet weights, using published size-weight regressions [3]. In addition, we present spatial distribution patterns of the weight specific biomass for the total gelatinous macrozooplankton community as well as the sub-groups i) hydrozoa, ii) scyphozoa and iii) ctenophora across the investigation area. The presented data contribute to a baseline describing the gelatinous macrozooplankton diversity and distribution in the extended North Sea area during winter [3,4] and summer [5]. The data can contribute to address the question if gelatinous macrozooplankton densities increase due to global change pressures and will help to understand their interaction with commercially important fish species, which are assessed during the same surveys. As such, this data paper presents a valuable resource on biodiversity and non-indigenous species records and highlights the importance of monitoring gelatinous macrozooplankton to facilitate an ecosystem approach to assess if the ecosystem state meets a 'good environmental status (GES)', as demanded by the EU Marine Strategy Framework Directive (MSFD).