Sei Whale sightings and whaling catches in Europe primarily occur over deep water, either along, or seaward of, the shelf edge (Weir et al. 2001, Waring et al. 2008, Hammond et al. 2011, Prieto et al. 2012, Pike et al. 2019). Modelling in the central and eastern Atlantic indicated a preference for deep waters exceeding 1,400 m and increasing up to 4,000 m (Houghton et al. 2020). In the mid-Atlantic, Sei Whales were associated with topographic features including seamounts (Waring et al. 2008). They are also often associated with frontal systems and other mesoscale oceanographic features such as eddies that may favour upwelling (Skov et al. 2008, Houghton et al. 2020).

They generally inhabit temperate waters, with core global distribution concentrated within the 8–18 °C water temperature range (Horwood 1987, Murase et al. 2014). Temperature is a good predictor of Sei Whale occurrence in European waters; SSTs of 3–11 ºC during May had positive effects on density observed during July surveys, presumably due to impacts on productivity and prey (Houghton et al. 2020). A recently revealed multi-year presence of Sei Whales in the Fram Strait was suggested to be the result of ocean warming (Nieukirk et al. 2020).

Sei Whales both skim-feed and lunge-feed, providing ecological flexibility (Segre et al. 2021). Within the European Atlantic, their diet predominantly comprises crustaceans (Jonsgård and Darling 1977). They may target copepods (e.g. Calanus finmarchius in Norway: Ingebrigsten 1929) or euphausiids (e.g. northern krill, Meganyctiphanes norvegica in Iceland: Víkingsson et al. 2010, García-Vernet et al. 2021), depending what is available regionally and seasonally. Stomach contents in Iceland also comprised 2% (weight) of fish (Sigurjónsson and Víkingsson 1997), including sandeel (Ammodytes tobianus), lumpfish (Cyclopterus lumpus) and capelin (Mallotus villosus). Observations of faecal plumes and analysis of stable isotopes in the Azores indicate that feeding may also occur in lower latitude areas of Europe (Silva et al. 2019). Recent tagging work has shown that the species can dive to 580 m in search of food (Baracho Neto et al. 2019).

Sei Whales undertake seasonal latitudinal movements in the European Atlantic, with a northward migration in spring and a southwards movement during autumn (Horwood 1987, Christiansen et al. 1992, Prieto et al. 2014, Romagosa et al. 2020). They have the highest relative abundance in the Azores during April and May (Silva et al. 2014), were caught off the Hebrides mostly in June (Evan 2020), and peak in Iceland during August and September (Sigurjónsson and Víkingsson 1997, Víkingsson et al. 2010). Satellite-tagging work indicates that the species also makes extensive longitudinal movements; eight whales tagged in the Azores during spring moved north-west to the Labrador Sea and West Greenland, while one animal tagged in September moved south-east from the Azores towards West Africa (Olsen et al. 2009, Prieto et al. 2014).

Little is known of Sei Whale social structure in European waters. They are usually sighted alone or in small groups of 2 to 5 animals in foraging areas (Cattanach et al. 1993, Waring et al. 2008, Pike et al. 2019), with some larger aggregations of up to 25 animals reported (Houghton et al. 2020). The group size of migrating Sei Whales in the Azores is usually 2 or 3 animals (Silva et al. 2014).

Globally, the Sei Whale is widely distributed from the tropics to the polar regions, being most concentrated in temperate mid-latitude (20 to 55°) regions (Horwood 1987, Prieto and Weir 2022). The species migrates seasonally between feeding areas located across temperate and subpolar latitudes, and wintering grounds at subtropical and tropical latitudes (Horwood 1987). Its occurrence is well documented across the Southern Hemisphere, in the North Atlantic, and in the North Pacific, but there are no confirmed records from the Northern Indian Ocean (Rice 1998, Cooke 2018). It is also apparently rare in, or absent from, some semi-enclosed water bodies such as the Mediterranean, the Red Sea, and the Gulf of Mexico (Prieto and Weir 2022). Although its global range is extensive, its finer-scale distribution and abundance within many parts of that range remain poorly documented. This is particularly the case in (sub)tropical regions, due to confusion with Bryde’s whales (Balaenoptera brydei/B. edeni).

In the North Atlantic, the core area of Sei Whale occurrence extends across temperate waters from the European Atlantic seaboard of Ireland north to Norway (Horwood 1987), between western Iceland and south-east Greenland (Pike et al. 2019), in the central North Atlantic around the Charlie Gibbs Fracture Zone (Skov et al. 2008, Waring et al. 2008), and in the offshore waters of the Labrador Sea, Nova Scotia, and the southern Georges Bank, with episodic incursions into the Great South Channel, Stellwagen Bank and the southern Gulf of Maine (Schilling et al. 1992, Baumgartner and Fratantoni 2008, Davis et al. 2020). The northernmost limits of distribution include the Davis Strait (67.5°N) in the western Atlantic (Davis et al. 2020), the Fram Strait between Greenland and Svalbard in the central Atlantic (79°N: Nieukirk et al. 2020), and the Barents Sea in the eastern Atlantic, including occasional records as far north as 80°N off the west coast of Svalbard (Ingebrigtsen 1929, Jonsgård 1977, Christensen et al. 1992, Storrie et al. 2018). While it is presumed to winter in the warmer southern regions of the North Atlantic (Ingebrigtsen 1929, Horwood 1987, Christensen et al. 1992), its occurrence in, and use of, those areas are poorly understood. In the eastern North Atlantic, Sei Whales were recently reported feeding off Mauritania (20°N) during mid-winter (Baines and Reichelt 2014).

In Europe, Sei Whales are primarily distributed along the Atlantic seaboard, occurring over, and seaward of, the continental slope (Prieto and Weir 2022). However, there have been occasional reports from shallower shelf habitats (Evans et al. 2003).

Southern Europe
Its documented range includes the archipelagos of the Canary Islands (Spain), Madeira (Portugal), and the Azores (Portugal), the Strait of Gibraltar, the Iberian Peninsula of mainland Spain and Portugal, and the Bay of Biscay. Only low numbers are recorded around Madeira and the Canary Islands (Carrillo et al. 2010, Freitas et al. 2012, Prieto et al. 2014). The species is observed regularly in the Azores, particularly during its spring and autumn migrations but also in summer and winter (Silva et al. 2014, Romagosa et al. 2020, Prieto and Weir 2022). Small numbers were landed at whaling stations in the Strait of Gibraltar, southern Portugal and northwest Spain between 1925 and 1980 (Sanpera and Aguilar 1992). Modern-day sightings and strandings in this area are scarce (Sequeira et al. 1992, Prieto et al. 2012, Bencatel et al. 2019), although 12 sightings were recorded off north-west Spain during the CODA survey in 2007 (Hammond et al. 2013) and a group of four animals was seen north of Spain in the Bay of Biscay during the SCANS-III survey in July 2016 (data provided by Phil Hammond, Sea Mammal Research Unit). Two strandings were recorded on the Biscay coast of France in the 1990s (French Stranding Network data), and the species is occasionally observed in the Bay of Biscay (Prieto et al. 2012).

Ireland to Norway
A total of 91 Sei Whales were caught off the west coast of Ireland between 1908 and 1920 (Prieto and Weir 2022). The waters off Scotland (UK), the Faroes, and Norway produced some of the highest reported North Atlantic whaling catches. Over 2,200 animals were landed in Scotland (Hebrides and Shetland Islands) and in the Danish Faroe Islands respectively, predominantly between 1900 and 1960 (Prieto and Weir 2022). The highest European Sei Whale catches were in Norway, where over 4,100 animals were landed between 1884 and 1900 and an additional 3,400 animals from 1901 to the 1960s. Catches of the species fluctuated markedly between years in Norwegian waters (Ingebrigtsen 1929, Christensen et al. 1992). About 50 Sei Whales were taken off the Murman coast in the 1880s (Jonsgård 1977), but in view of the lack of sightings in the Barents Sea in recent times despite extensive surveys, it is now considered to be at most an occasional visitor there (Мишин 2021).

In recent decades, Sei Whale sightings across this region have been relatively scarce. Few recent sightings are reported from Irish waters (Borchers and Burt 1997; Hammond et al. 2011, 2013; Rogan et al. 2018), and only small numbers have been recorded during surveys in suitable habitat off north and west Scotland including the Faroe-Shetland Channel (Weir et al. 2001, MacLeod et al. 2009, Hammond et al. 2011). Only fifty-nine animals were recorded during 20 years of surveys around the Faroe Islands (Skov et al. 2002), while large-scale surveys of Norwegian and Russian waters since 1987 have yielded only rare sightings in Norwegian waters and none in Russian waters (Christensen et al. 1992; Leonard and Øien 2020a,b; Мишин 2021).

Iceland and the central Atlantic
In Iceland, over 2,800 whales were captured over a period extending from the late 1800s to the late 1980s, with an average of 48 animals taken per year between 1948 and 1985 (Víkingsson et al. 2010). The species is most numerous to the south and south-west of Iceland, where the highest abundances of Sei Whales in the European region have been recorded (Cattanach et al. 1993, Borchers and Burt 1997, Davis et al. 2020, Pike et al. 2020). Concentrations have also been recorded during summer surveys of the mid-Atlantic ridge, just north of the Charlie Gibbs Fracture Zone (53ºN: Cattanach et al. 1993, Skov et al. 2008, Waring et al. 2008). Acoustic monitoring in the Fram Strait at 79ºN detected Sei Whale calls between spring and autumn, peaking during summer (Nieukirk et al. 2020).

Semi-enclosed European seas
Although the Sei Whale type specimen originates from the Baltic coast of Germany (Rudolphi 1822), the shallow Baltic Sea is not part of the usual distribution range. A small number of strandings have been reported on the North Sea coasts of Denmark, the Netherlands, Belgium and the United Kingdom (Camphuysen et al. 2008, Evans 2020), but are similarly considered extralimital.

The species is not resident in the Mediterranean, although rare sightings and confirmed strandings have been reported in the western region (off Spain and France) and support its occurrence as a vagrant (ACCOBAMS 2021). It has not been recorded in the Black Sea.

The first European catch limit for Sei Whales was issued in 1977, applying to the Iceland-Denmark management stock (Donovan 1991). In 1986, the International Whaling Commission’s global moratorium on commercial whaling came into force. However, Iceland landed 70 Sei Whales between 1986 and 1988, under Special Permit.

It is listed in Appendix I (endangered migratory species) and Appendix II (migratory species conserved through international agreements) of the Convention on the Conservation of Migratory Species of Wild Animals (Bonn Convention, 1979), and on Appendix II (strictly protected species) of the Convention on the Conservation of European Wildlife and Natural Habitats (Bern Convention). It is included on Annex IV Council Directive 92/43/EEC on the Conservation of Natural Habitats and of Wild Fauna and Flora (the Habitats Directive) as a species requiring strict protection across its entire natural range within the European Union. It is listed in Appendix I (species threatened with extinction) of the Convention on International Trade in Endangered Species (CITES), which prohibits international trade.

Sei Whales are included in Annex 1 of the Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and contiguous Atlantic area (ACCOBAMS), which was concluded under the auspices of the Bonn Convention. ACCOBAMS came into force in 2001 to conserve cetaceans in the Mediterranean and Strait of Gibraltar and was extended in 2010 to include the Spanish and Portuguese Exclusive Economic Zones.