Twenty-six cetacean and seven seal species occur in the ecoregion, many only as vagrants or occasional visitors. Two species of seal are common in the ecoregion: grey seal and harbour seal. Four cetacean species also occur commonly or are resident: minke whale (
Balaenoptera acutorostrata), harbour porpoise, white-beaked dolphin (
Lagenorhynchus albirostris), and bottlenose dolphin (
Tursiops truncatus). A further six species are considered regular but less common: common dolphin, Atlantic white-sided dolphin (
Lagenorhynchus acutus), long-finned pilot whale (
Globicephala melas), killer whale (
Orcinus orca), Risso's dolphin (
Grampus griseus), and humpback whale (
Megaptera novaeangliae)
.
Both seal species have experienced large population changes over the past century. The abundance of harbour seals reached an all-time low in the 1970s but subsequently increased at an annual rate of 4%; however, this increase was then affected by two major outbreaks of the phocine distemper virus in 1988 and 2002. Over the last 15 years, local declines in the harbour seal population have occurred in the northwestern North Sea, for unknown reasons. Grey seals occur predominantly along the British coasts, where they have been increasing at an annual rate of up to 16% in some areas.
Trends in the abundance of cetaceans are less known. The spatial distribution of harbour porpoises was observed to shift southwards following changes in the availability of prey, such as sandeel. Minke whales and white-beaked dolphins are found mainly in the central and northern North Sea, without substantial changes in abundance. The population of bottlenose dolphins off the eastern British coast has been increasing since the 2000s and over this period has extended its range southwards. Killer whales regularly occur in the northern North Sea, with at least some individuals inhabiting the waters around northern Scotland being part of the Icelandic population. In recent years, humpback whales have increasingly been recorded in the North Sea, including in the southern part where they were previously vagrant.
The main threats for marine mammals in the ecoregion are from human activities and their pressures, including fisheries (i.e. bycatch), contaminating compounds, underwater noise, and shipping (i.e. vessel strikes). Fisheries can also indirectly affect marine mammals through reduction in prey. Climate change may also be causing some distributional shifts in marine mammals for example through changes in food availability.
