Out of the 23 species of marine mammals (7 pinnipeds, 1 polar bear, 6 baleen whales and 9 toothed whales) occurring regularly in the ecoregion, 9 are of particularly importance: minke whale, fin whale (
Balaenoptera physalus), humpback whale, and sperm whale dominate in biomass, but are mainly present in summer and autumn; hooded seal and bottlenose whale have a partial distribution in the Arctic; and harbour porpoise, grey seal, and harbour seal are resident on the continental shelf of Norway. Killer whale may occur all over the Norwegian Sea year-round, but are mainly associated with the herring and mackerel migrations. All these marine mammal species have been substantially affected by historic harvesting and bycatch levels, but only minke whales, and grey and harbour seals are currently hunted.
Relative abundance indicators suggest stable occurrence of the deep diving sperm whales over the 2002–2018 period. During the same period, abundance estimates for both harbour porpoises and killer whales have been highly variable in the Norwegian Sea ecoregion but show no clear trend. Abundance trends are not available for bottlenose whales, but primary observations of this deep diving species have doubled recently (survey observations during 2014–2018 compared to previous survey in 2002–2013).
Abundance of the Northeast Atlantic hooded seal population has continued to decline after protection in 2007. Over the past decade, declines in harbour and grey seals observed in Central Norway have led to their full protection in some areas. Fin and humpback whales have shown strong recoveries in the Northeast Atlantic over the past decades, but many appear to travel through the Norwegian Sea to the Barents Sea ecoregion. Northeast Atlantic minke whales have maintained healthy and stable population sizes under the recent harvesting regime, but distribution among these two ecoregions may vary between years.
