Abstract
This report describes baseline data and survey data from the two-year baseline survey program of marine mammals in the North Sea Energy Island pre-investigation area for seals and cetaceans, as well as from the tagging program in 2022. The survey program was conducted from November 2021 to November 2023. Data acquired during field surveys and tagging program were used combined with existing data to characterize the status of the Energy Island pre-investigation area for the most common marine mammal species in and around the North Sea Energy Island area. These species were harbour seals (Phoca vitulina), grey seals (Halichoerus grypus), harbour porpoises (Phocoena phocoena), white beaked dolphins (Lagenorhyncus albirostris), and less common: killer whales (Orcinus orca) and minke whales (Balaenoptera acutorostrata).
A one-year aerial survey program for seals was conducted at the two nearest seal haul-out areas in the western part of the Limfjord (Nissum Bredning) and the Danish part of the Wadden Sea, respectively. All eight planned surveys were conducted in 2022. Furthermore, data from a seal tagging program carried out in 2022 is included in the baseline analysis for seals in the Energy Island pre-investigation area.
Counts of harbour seals from previous surveys, as well as aerial counts conducted in 2022 during the current project, show that fewest harbour seals were counted in the winter and spring, and the counts peak in August (coinciding with their moulting period) with an average of 2506 harbour seals counted in the Danish Wadden Sea and 347 counted in Nissum Bredning. Data from satellite tag data from 27 tagged harbour seals indicated that the Energy Island pre-investigation area was not of high usage for male harbour seals (very few females were tagged and a conclusion is not possible for females).
Counts of greys seals showed less seasonal trend than the corresponding harbour seal counts with between 4-171 grey seal counted in the Danish Wadden Sea and between 1-51 at Nissum Bredning from January to December. Data from 15 tagged grey seals at Nissum Bredning and data from 33 grey seal pups tagged at Helgoland indicate that Energy Island pre-investigation area was not of high usage for tagged grey seals.
Harbour porpoises in and around the Energy Island pre-investigation area belong to the North Sea population. Passive acoustic monitoring (PAM) of harbour porpoises at fourteen stations, and from August 2022 nineteen stations, within and east of the pre-investigation area for the North Sea Energy Island showed, that harbour porpoises were present in the area year-round, and that the area was of statistically higher significance for harbour porpoises in the period June-August, which coincides with the calving, nursing and mating season, as compared to the rest of the year. The results of the PAM surveys were confirmed by the aerial marine mammal surveys, where the average density of harbour porpoises in the surveyed area were estimated to be 0.74 individuals/km2 for the April 2022 aerial survey and 1.96 individuals/km2 for the July 2022 aerial survey. The July 2023 aerial survey density was somewhat lower than the July 2022 survey 0.88 individuals/km2. It is not known where specific areas used for reproduction i.e. birth, nursing and mating for harbour porpoises are located in the North Sea, except for one identified breeding area near Sylt in the German Wadden Sea where the mother-calf ratio has been between 10-17% over many years. The mother-calf ratio observed in the middle of the breeding period in July 2022 and July 2023 in the Energy Island pre-investigation area was 16%. Due to the high density of harbour porpoises observed by aerial survey in 2022, the entire North Sea was covered by an aerial survey for harbour porpoises in 2023, and showed a relatively high density of harbour porpoises in the Energy Island extended survey area as compared to the rest of the Danish North Sea.
Wideband acoustic data showed that white-beaked dolphins and other delphinids were common in the Energy Island extended survey area especially during summer (June), which coincides with the calving season of white-beaked dolphins. They were present in all months in the two year survey period. White-beaked dolphins (also with calves) were observed during the aerial surveys, and groups of dolphins were observed from the boat during five of the eight service trips to the pre-investigation area. The results of the wideband PAM surveys are the first data on yearly pattern in presence of white-beaked dolphins in Danish Waters, as well as other parts of the North Sea.
Only one minke whale was observed in the aerial survey program, and none were detected in the acoustic wideband analyses. However, this should not be taken as an indication of minke whales not being present as they do not always use acoustic signals like the toothed whales do (i.e. porpoises, dolphins and killer whale).
No killer whales were observed during the aerial survey program. Killer whales are delphinids and as such included in the results of the acoustic wideband analyses. Delphinids were detected throughout the year and could as such also represent killer whales.