The Sei Whale, Balaenoptera borealis is a recognized species but catch and sightings records of Sei Whales prior to 1972 tended to include Bryde's Whales (B. edeni or B. brydei) in areas where both species occur. The Society for Marine Mammalogy’s Committee on Taxonomy (Committee on Taxonomy 2017) recognizes two subspecies, B. b. borealis in the Northern Hemisphere and B. b. schlegelii in the Southern Hemisphere, as proposed by Tomilin (1957). The Sei Whales found in the North Pacific and North Atlantic have not been formally compared and so are still placed in the same nominate subspecies by default, although Huijser et al. (2018) found a high degree of genetic divergence between North Atlantic and North Pacific Sei Whales.

European regional assessment: Least Concern (LC)

The cause of the population reduction in Sei Whales (commercial whaling) that occurred in the 20^th century is reversible and is understood and has been brought under control. For this reason, the species is assessed under IUCN Red List criterion A1, not under A2, A3, or A4. The main whaling reductions for North Atlantic Sei Whales occurred prior to the three-generation window (1953–2023) that would trigger the population reduction (A1) criterion. No recent estimate of population trend is available. Given that most catches from the North Atlantic subpopulation occurred prior to 1952, a net increase since that time would be expected unless counteracted by other factors. In the absence of observations or inferences of recent decline, the population does not qualify for criteria A or C.

Sei Whales in the Pan-European region are considered to belong to a single subpopulation, in view of genetic data and movements of tagged animals between areas. The population is widely distributed across the eastern and central North Atlantic and therefore does not qualify for criterion B.

Although sightings of Sei Whales remain scarce along the Atlantic seaboard between the Canary Islands and Norway, the occurrence of summer and autumn concentrations south and southwest of Iceland mean that abundance surveys in the Pan-European region estimate a total of around 5,700 mature animals. Consequently, the Sei Whale does not qualify under criterion D, and is assessed as Least Concern.

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.

Population structure
Three ‘management stocks’ (i.e. population units designated for management purposes, rather than biological stocks based on genetic separation) were defined in the North Atlantic by the International Whaling Commission (Donovan 1991), comprising Nova Scotia and coastal waters of the USA (west of 42ºW), Iceland-Denmark Strait and the central Atlantic (42ºW to 18ºW), and the eastern Atlantic including the waters of Spain, Portugal, Ireland, the UK, the Faroe Islands, and Norway (east of 18ºW). However, the relevance of the IWC management stock boundaries to biological stocks in the North Atlantic has been questioned (Donovan 1991, Víkingsson et al. 2010, Prieto and Weir 2022). For example, movements of satellite-tagged whales from the Azores to the Labrador Sea and West Greenland (Prieto et al. 2014) confirm linkage between those areas.
North Atlantic Sei Whales are genetically differentiated from those in the North Pacific, but significant heterogeneity has not been identified within the North Atlantic (Huijser et al. 2018). Genetic sampling across the entire North Atlantic is required to clarify whether distinct biological stocks occur. In the absence of evidence to the contrary, Sei Whales within the Pan-European region are considered a single population.

Historical population size
No direct pre-whaling population size information exists for Sei Whales in European waters or the wider North Atlantic. However, modelling based on catch data and assuming that the population was at carrying capacity during its first year of exploitation in 1884, estimates a pre-whaling North Atlantic population size of ~17,700 aged 1+ animals including ~10,300 mature animals (Cooke 2018).

Abundance
North Atlantic Sightings Surveys (NASS) have been carried out in 1987, 1989, 1995, 2001, and 2015, covering the waters of the Faroe Islands, Iceland and Norway, and extending to adjacent regions in different years including south to north-west Spain, the northern North Sea and the Irminger Sea towards the Greenland Coast. The 2007 Trans-Atlantic NASS (T-NASS) survey also included West Greenland and Canada. Because the NASS geographical coverage has varied between years and did not always include the entire summer distribution of Sei Whales, and the timing of surveys was often earlier in the summer than peak Sei Whale season, most resulting abundances were only considered partial estimates. Too few (or zero) sightings of Sei Whales were recorded in most regions to estimate abundance. During NASS-1987, an abundance of 1,293 (CV=0.60) animals was generated for Icelandic waters (Cattanach et al. 1993). The remaining available NASS estimates for Sei Whales relate to combined Icelandic and Faroese waters, comprising 10,300 (CV=0.27) animals in 1989 (uncorrected: Cattanach et al. 1993), 9,249 (CV=0.49) animals in 1995 (uncorrected: Borchers and Burt 1997), 2,716 (CV=0.61) animals in 2001 (corrected: Pike et al. 2011), 5,159 (CV=0.47) in 2007 (uncorrected: Pike et al. 2020), and 3,767 (CV=0.54) in 2015 (corrected: Pike et al. 2019). During the T-NASS survey in 2007, an estimate of 9,737 (CV=0.38) animals was generated for combined Icelandic and Faroese waters and including an extension to the south-west of the core NASS area (uncorrected: Pike et al. 2020).

Despite the availability of more recent survey data, the NASS 1989 estimate of 10,300 animals is currently considered the most representative estimate of Sei Whale abundance in the central North Atlantic, since that survey extended further south and west of Iceland, and occurred during July and August, thus overlapping spatially and temporally with peak Sei Whale aggregations (North Atlantic Marine Mammal Commission 2022).

A series of large-scale sighting surveys have been carried out in shelf and slope waters of central and southern Europe, including Small Cetaceans in the European Atlantic and North Sea (SCANS) surveys in 1994, 2005 and 2016 (Hammond et al. 2002, 2013, 2017, 2020), the Cetacean Offshore Distribution and Abundance in the European Atlantic (CODA) survey in 2007 (MacLeod et al. 2009, Hammond et al. 2011), and Ireland's Atlantic Margin and Irish Sea in 2015-2017 (Rogan et al. 2018). Combining 12 Sei Whale sightings recorded off north-west Spain during the 2007 CODA survey, and a single sighting recorded in the Celtic Sea during the 2005 SCANS-II survey, a total abundance of 619 animals (CV=0.34) was generated for the European Atlantic shelf and offshore waters west of Spain, France, Ireland and the UK (Hammond et al. 2011). Sei Whale sightings were too scarce during other surveys to generate abundance. No Sei Whale sightings were recorded during an extensive abundance survey of the Mediterranean in 2018 (ACCOBAMS 2021).

Summed estimates for the NASS and SCANS/CODA surveys indicate around 11,000 animals in the Pan-European region. Using the 52% mature value from Taylor et al. (2007), the number of mature animals is estimated at 5,720. While summing estimates from surveys carried out almost two decades apart is generally undesirable, the bulk of this estimate derives from the 1989 synoptic survey. Additionally, the summer distribution of Sei Whales in the central North Atlantic may extend south of the 50°N limit of the surveys, or into the far western (40°-45°W) part of the assessment area which has not been surveyed.

Population trend
Using global data, the generation length for Sei Whales was estimated as 23.3 years (Taylor et al. 2007). The three-generation time window for applying criterion A is therefore 70 years, spanning 1953 to 2023. No robust information is available on the current population trend in Pan-European waters. However, Cooke (2018) applied a conventional population assessment model to provide an illustration of the extent of possible reduction relative to criterion A. Given that European Sei Whale catches were most numerous in the period prior to 1940 (Prieto and Weir 2022), the three-generation time window for applying criterion A since 1953 includes several decades of potential population recovery. The model result indicated an expected net increase in population size since 1953.

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).

The exploitation of Sei Whales during the commercial whaling era is the most significant threat to have impacted European populations over the last century. European Sei whaling commenced with the onset of modern whaling techniques in Norway during the late 1800s, and catches peaked between 1884 and 1920 all but ceased in most areas by the 1960s, catches in Iceland peaked during the 1970s. In total, around 15,500 Sei Whales were landed during the commercial whaling era at European shore stations (Allison 2016, Prieto and Weir 2022). That value does not account for whales lost at sea, unknown proportions of Sei Whales in catches of ‘unspecified whale species,’ or potential confusion with Bryde’s Whales in southern Europe. Sei Whales have not been harvested in Europe since 1988, and this threat is therefore not current.

Little information is available on other threats in European waters. There is one example of a vessel strike in the Canary Islands in 2004 (Carrillo and Ritter 2010), and several other vessel strikes on the species have been reported globally (Brownell et al. 2009, Glass et al. 2010, Weir 2018). The offshore distribution of European Sei Whales may limit their exposure to ship strikes but also make it less likely that struck animals will be identified and reported.

The species is occasionally encountered during seismic surveys in UK waters, particularly in the Faroe-Shetland Channel (Stone 2015). There is no clear evidence of avoidance of the low-frequency impulsive sounds generated by airguns either in Europe (Stone et al. 2017) or in the wider northeast Atlantic (Baines and Reichelt 2014). However, repeated exposure to underwater noise such as airguns and shipping has the potential to displace baleen whales from favoured habitats, cause stress, and injure animals via hearing loss (Thomas et al. 2016).

Sei Whales use non-selective skimming and gulping to feed and may be prone to ingesting debris. A Sei Whale stranded in the UK was found to have ingested plastic debris, although that had not caused its death (Baulch and Perry 2014). This threat remains unquantified.

Other known causes of Sei Whale mortality, such as entanglement in fishing gear (Glass et al. 2010) and harmful algal blooms (Häussermann et al. 2017), are not known to represent significant threats in the offshore habitats that the species occupies within Europe. Similarly, population-level impacts from contaminants are less likely offshore, and pollutant concentrations in Icelandic Sei Whales were below harmful levels (Borrell 1993).

The potential effects of global climate change on North Atlantic Sei Whales are unknown, and require investigation.

The species is not currently utilised in European waters.

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.