Abstract
The Hywind Tampen Offshore Floating Wind Park is a floating OWF (FOWF) situated in deep-water on the Norwegian Shelf in the Northern North Sea. On behalf of Equinor, in 2023-2024, NORCE Climate and Environment conducted an environmental DNA survey of surface (20 m) and bottom water samples from sampling stations upstream, inside, downstream and some distance away from the FOWF to assess fish (MiFish-U) and plankton (18S V1-V2) communities for potential impact (Hestetun et al., 2024). Study results revealed differences in community composition over time and with depth but could not detect impact – negative or positive – from the construction and operation of the FOWF itself. The study used fish capture and ROV data from IMR-conducted surveys in the area to ground truth completeness of the data. While the MiFish-U primer set was able to detect the vast majority of fish species reported in the fish capture and ROV studies, and detect some additional species, elasmobranchs (sharks and skates) were missing from the eDNA data. This technical note contains a re-sequencing of the Hestetun et al. (2024) samples using a combination of MiFish-U and MiFish-E primer sets, MiFish-E being a modification of the MiFish-U primer set specifically designed to detect elasmobranchs. The goal of this analysis was to assess the ability of this approach to get a more comprehensive overview of local fish communities also including elasmobranch species. In addition, new analyses of bottom water fish communities were made removing dominating and pelagic species to see if this revealed further information on differences in demersal fish composition between sampling stations. Re-sequencing of Hywind Tampen samples using the MiFish-U/E mixed primer approach yielded a dataset that retained 32 of 35 species from the previous MiFish-U only dataset of Hestetun et al. (2024). The approach was also successful in detecting several elasmobranchs not part of the MiFish-U dataset but reported from the fish capture and ROV surveys in the area, including the thorny skate (Amblyraja radiata), common skate (Dipturus sp.), blackmouth catshark (Galeus melastomus), and spurdog (Squalus sp.). In addition, a couple of previously unreported elasmobranchs, including velvet belly lanternshark (Etmopterus spinax) and porbeagle (Lamna nasus), were detected. Most elasmobranchs were detected with relatively low abundance, however. The results also highlight some ambiguities in taxonomic assignment where several species were equally similar in sequence identity, suggesting the need for taxonomist validation of taxonomy results based on knowledge of regional fish communities. In conclusion, the MiFish-U/E primer set approach was successfully able to recreate local fish communities with greater elasmobranch coverage with little reduction in nonelasmobranch coverage and represents a good alternative for maximum coverage in metabarcoding of fish communities. Concerning the reanalysis of the bottom water fish community datasets, both the previous MiFish-U and the newly sequenced MiFish-U/E datasets were analyzed removing pelagic and dominating species. This reanalysis reaffirmed the conclusions from the full dataset analysis in the original report: While there was a statistically significant support for N O R C E Norwegian Research Centre AS norceresearch.no Supplementary eDNA analyses at the Hywind Tampen FOWF 3 differences between sites and time, the size of this effect was small. The main impression is that demersal fish communities are stable and comparable between sampling stations and time points, with no detectable impact due to the FOWF. Importantly, bottom depth is similar across the sampling station, situated along a slope, in the Hywind Tampen study here in contrast to e.g. (de Jong et al., 2022), who did a transect perpendicular to the slope itself.