Sei whale

Updated: September 2019

The sei whale is the third largest of the rorquals, and it closely resembles both the Bryde’s and fin whales. This species has a more southerly distribution than other large whales found in the North Atlantic. The sei whale has a dark grey colouration dorsally, with lighter markings on its underside. It is unusual from other baleen whales in its ability to utilise different feeding techniques, making it much more capable of switching between prey taxa when needed.

The sei whale is predominately found offshore, alone or in small groups. The knowledge about this whale is limited, as its range and distribution do not coincide with most other whales and have also been known to fluctuate between years.

© Peter Duley/NEFSC/NOAA Fisheries

ABUNDANCE

Recent surveys suggest in excess of 10,000 sei whales in the North Atlantic (Pike et al., 2019a).

DISTRIBUTION

In the North Atlantic, it is most abundant between West Iceland and Southeast Greenland, and rare along the Norwegian coast and in British waters.

RELATION TO HUMANS

The sei whale was exploited up until the 1970s, after which it became protected.

CONSERVATION AND MANAGEMENT

International management regime by NAMMCO and the International Whaling Commission.

Listed as Endangered but increasing, bordering on Vulnerable in the global IUCN Red List.

© Christin Khan, NEFSC/NOAA

Sei whale with white-sided dolphins © Peter Duley, NOAA/NEFSC

Scientific name:  Balaenoptera borealis

Faroese: Seihvalur
Greenlandic: Tikaagulliusaarnaq
Icelandic: Sandreyður
Norwegian: Seihval

Danish: Sejhval
English: Sei whale

There are two recognised sub-species of the sei whale:  the northern: B. borealis borealis and southern: B. b. schlegellii.

The name “sei whale” comes from Norwegian “sei”, pollock/coalfish, because this whale appeared along the Norwegian coast at the same time of year as the fish.

LIFESPAN

50-70 years

AVERAGE SIZE

15 m, 15-20 tonnes (larger in the Southern Hemisphere)

PRODUCTIVITY

1 calf every 2-3 years from an age of 8-10 years

MIGRATION AND MOVEMENTS

Seem to move north in spring and south in autumn

FEEDING

Copepods, euphausiids (krill), other crustaceans and fish. Area dependent.

General characteristics

© Christin Khan, NEFSC/NOAA

The sei whale is the fourth largest whale. Of the rorquals, only the fin and blue whales are larger, however, the sei whale’s dorsal fin is relatively taller than for these two larger whales. The sei whale closely resembles the Bryde’s whale (B. edeni/brydei), and the two were long thought to be the same species. However, the sei whale has only one distinct ridge on its head, whereas the Bryde’s whale has three (Horwood, 2018). It is also generally larger. At sea, it can be difficult to distinguish the sei from the Bryde’s whale, and also from the fin whale. To distinguish sei whales from fin whales, survey vessels try to pass these whales on their right-hand side. This is because the fin whale’s head has asymmetrical colouration, with the ventral white colouration extending up over the right lower lip and inside the mouth cavity and the baleen plate.

The sei whale is dark grey dorsally with irregular light grey to white markings on its underside, and its body is often scarred from lamprey bites. As is common with rorquals, females are the larger sex, and the sei whales occupying the Southern Hemisphere are generally larger than its northern conspecifics. Icelandic sei whales appear to have an overall greater body weight relative to length than North Pacific sei whales (Lockyer & Waters, 1986).

Sei whales have the same general mouth morphology as other rorquals, but very fine, silky baleen fringes, enabling them to switch between prey taxa by both being able to lunge feed and utilise a surface skim feeding technique (e.g. Ellis, 1991; Baumgartner & Fratantoni, 2008).

Did you know?

Sei whales don’t normally arch their backs and show their tail fluke when diving – they usually just sink below the surface.

Underwater sei whale footage

© Christin Khan, NEFSC/NOAA

Life History and Ecology

The sei whale is a species where there is limited knowledge. Compared to other large whales, research effort has increased less for sei whales, and most of the current knowledge on this species is based on data from whaling or research focusing on management. As it utilises a pelagic habitat and is most commonly found offshore, as well as often being mistaken for a Bryde’s or fin whale, research can be difficult and/or very expensive (Prieto et al., 2012; Cooke, 2018).

The sei whale seems to prefer more offshore and warmer waters than for instance the fin, minke and blue whales, staying in waters with temperatures between 8 and 18ºC (Cooke, 2018). During a survey along the Mid-Atlantic Ridge, sei whales were generally encountered at the slopes of seamounts and rises, in waters with depths between 1,160 and 4,500 m and coinciding with calanoid abundance (Waring et al., 2008).

Lifespan

Sei whales are thought to reach an age of 50 to 70 years.

Reproduction

Sei whale feeding alongside white-sided dolphins © Peter Duley, NOAA/NEFSC

The sei whale is sexually mature at an age of 8-10 years. The average age of maturity for sei whales in most seas declined by 2-3 years after populations were depleted due to whaling. Females usually have one calf every 2-3 years. Conception occurs around December, followed by a gestation period of almost a year. Calves are 4.5 m when born (Sigurjónsson, 1997) and are weaned when they are about 7 months old (Horwood, 2018).

Feeding

Large whales exhibit different feeding strategies, with some whales lunging for food, while others “cruise along” with their mouths open, skimming water as they go. Sei whales, on the other hand, seem to exhibit both these strategies, making them able to utilise a broader prey taxa than other baleen whales (e.g. Baumgartner & Fratantoni, 2008). The sei whale thus has a much more varied diet than for instance the blue whale, and it appears to feed on different prey items depending on both area and season.

Copepods, euphausiids (krill), various fish species, different life stages of several crab species, gastropods (snails) and even squat lobsters have been identified as sei whale prey (e.g. Christensen et al., 1992; Kapel, 1979; Leonardi et al., 2011; Sigurjónsson, 1997; Sigurjónsson & Vikingsson, 1997; Watanabe et al., 2012). Habitat selection based on prey selection has also been suggested, with sei whales utilising the distribution of prey items as a migration route and forage habitat (Watanabe et al., 2012).

In Norwegian, Icelandic and Greenlandic waters, either copepods (mainly Calanus finmarchicus) or euphausiids (krill) seem to be the preferred prey, with Icelandic sei whales also including some fish in their diet (e.g. Christensen et al., 1992; Kapel, 1979; Sigurjónsson, 1997; Sigurjónsson & Vikingsson, 1997). Sei whales off Nova Scotia also seem to prefer copepods and krill, but also showing monthly trends in prey items that include various fish species (Flinn et al., 2002; Sigurjónsson, 1997). In the southwestern Gulf of Maine, sei whales seem to prefer to feed on C. finmarchicus at or near the surface (Bumgartner et al., 2011).

Sei whale feeding frenzy

Sei whale research in the Falkland Islands

Behaviour

Sei whales are usually found alone or in small groups. During a summer survey along the Mid-Atlantic Ridge in 2004, most groups of sei whales encountered contained 2-5 animals, although up to 10 animals were observed at a time (Waring et al., 2008). In 2015, sei whales observed in Icelandic and Faroese waters were usually solitary or in pairs, but rarely also in groups as large as 8 individuals (Pike et al., 2019a).

Diving/swimming

Sei whales are believed to be the fastest of the large whales, reaching speeds up to 50 km/h (Shefferly, 1999). They do not dive very deep or long, and unlike other large whales, they usually just sink below the surface instead of arching their backs and showing their tail fluke (NOAA, n.d.).

Vocalisation

© Gísli Víkingsson / Marine and Freshwater Research Institute

Passive acoustic monitoring is used as an additional tool for management purposes, as recordings of vocalizations in an area is a less expensive way to pinpoint a species to a certain location than the more traditional use of human-manned surveys. It is also possible to deploy these monitoring devices over a longer period, only having to change batteries and collect the recordings once or a few times a year. Knowledge about the desired species’ vocalizations is therefore necessary.

Sei whales have been found to make more numerous calls during the day when their prey, the copepod C. finmarchicus, is at depth, than at night when the copepod is in surface waters. This suggests that the availability of this copepod at the surface waters influences sei whale vocalization behaviour. These vocalizations were assumed to be of social context, and not to locate prey (Baumgartner & Fratantoni, 2008).

Sei whale vocalizations recorded off the Azores averaged between 177 and 194 dB, at 20-120 Hz, with low-frequency downsweeps with maximum frequencies averaging 100 Hz down to 37 Hz (Romagosa et al., 2015). Downsweeps recorded in the south-eastern Pacific Ocean had a similar range, with an average maximum frequency between 93.3 and 105.3 Hz and an average minimum of between 35.6 and 42.2 Hz, although the maximum frequency was higher (129.4 Hz) and the duration was longer (2.27 s). However, calls recorded in Antarctic waters do not show similarities with the calls recorded in the south-eastern Pacific, with frequencies between 100 and 600 Hz and up to 3 s durations (e.g. Español-Jiménez et al., 2018).

Listen to sei whale sounds

Strandings

In 2014, a sei whale ended up in a field in Yorkshire, UK, after presumably foraging in shallow waters during high tide, the third stranding of a sei whale in 20 years in the UK (“How did a 33ft whale end up in the middle of a field in East Yorkshire?”, 2014).

At least 343 baleen whales, primarily sei whales, were found stranded in remote waters off Patagonia, Chile, in 2015. This is the biggest single baleen whale mass mortality event known to science, and investigations showed that the they died at sea, close to where they beached. There are indications that the cause was a harmful toxic algae bloom caused by a strong El Niño event, which could further indicate that marine mammals are among the first oceanic megafauna victims of climate change (Häussermann et al., 2017).

Predators

Like other large baleen whales, the sei whale does not have many natural predators due to its size. However, killer whales are known to also attack large whales.

Distribution and habitat

© Allison Henry/NOAA Fisheries

Worldwide distribution

Sei whales are found throughout the world’s oceans and are present in the North Atlantic, Pacific and Southern Oceans, but there are no records from the Northern Indian Ocean (Cooke, 2018). These whales prefer deep water throughout their range. They avoid polar and tropical waters, opting for a temperature range of 8-18ºC (Cooke, 2018).

As the main source of distribution of this species is whaling records, the current knowledge of sei whales does not reflect its entire distribution and might not even be accurate anymore for some areas (Prieto et al., 2012). Their close resemblance to the Bryde’s and fin whales makes distribution mapping somewhat challenging. The winter distribution, which seems to be widely dispersed, is in particular incomplete.

Distribution in the North Atlantic

Timing and extent of sei whale movements through western European waters have always fluctuated between seasons and years and whalers knew of “sei whale years” (Prieto et al., 2012). Typically, they occur north to the Davis Strait and the northern end of the Denmark Strait, and up to 70º N in the Norwegian Sea, in an arc running from the south of Nova Scotia, via the north-western British Isles and to Trondheim in Norway (Cooke, 2018).

Central & Eastern North Atlantic

Most sei whale sightings during summer are concentrated in the deep waters between Southeast Greenland and West Iceland, and some in the Faroe-Shetland Channel (Borchers & Burt, 1997; Cattanach et al., 1993; Sigurjónsson, 1997; Prieto et al., 2012).

In Norwegian and British waters, occurrence is scarce (e.g. Hammond et al., 2013, 2017; Leonard & Øien, 2019a,b,c,d; Pike et al., 2019a, Rogan et al., 2018), and only a few sightings have been made in Norwegian waters in recent years. Most recently, a single individual was sighted each of the years 2006, 2014 and 2018 (Øien et al., 2009; Prieto et al., 2012; Leonard et al., 2019). Additionally, sei whales have been sighted infrequently in the Svalbard archipelago, including at the northern tip of Spitzbergen. This is the northernmost record of this species, and suggests a possible range expansion (Storrie et al., 2018).

Sei whales have also been detected during acoustic surveys in Irish waters. These detections were rare, but considered a minimum, and only occurred in autumn and especially southwest of the Porcupine Shelf (Berrow et al., 2018). Additionally, 2 sei whales were sighted on the Porcupine Shelf during a winter survey in 2016 Rogan et al., 2018).

The sei whale was the most commonly encountered species during a survey run along the Mid-Atlantic Ridge between Iceland and the Azores in summer 2004, and all the 53 sightings, totalling 85 animals, were north of the Charlie Gibbs Fracture Zone (Waring et al., 2008).

Further south

Sei whales in low numbers are observed in late summer and early winter in the Bay of Biscay, and they have been detected in small numbers off north-western Spain. There are discrepancies in historical whaling data from the Iberian Peninsula, indicating that sei whales were never common off central Portugal. Whaling data from the 1970s gives the best indication of a well-defined sei whale wintering area off the Iberian Peninsula (e.g Prieto et al., 2012).

Sei whales have also been shown to be regular visitors to the Azores Archipelago during the spring migration (e.g. Silva et al., 2003; Prieto et al., 2012), with the first record in 1989 (Gordon et al., 1990). Their peak abundance in this area has been shown to be about 16 weeks after the onset of the spring bloom, coinciding with the peak in secondary producers (Visser et al., 2011). This study also recorded sub-adults frequently, indicating an extensive use as a foraging area for females with calves or juveniles.

There is very limited information on sei whales off the western African coast. There have been sporadic sightings and strandings in this area (e.g. Prieto et al., 2012), and one individual was recorded off the Moroccan coast during a sighting survey in April 2005 (Boisseau et al., 2010).

Western North Atlantic

Sei whales were considered rare in the Western North Atlantic during the whaling years, and currently, they seem to be present in low densities in most of the areas covered by dedicated sighting surveys. In Greenland, sightings are concentrated off Lille Hellefiske in the southwest and off the southwestern tip. In Canada, sei whales have been confirmed in the Davis Strait, the continental shelf areas along the coasts of Labrador and Newfoundland. Off the north-eastern US coast they concentrate in deeper waters, taking occasional trips into shallower waters (e.g. Prieto et al., 2012).

© Peter Duley/NEFSC/NOAA Fisheries

Migrations and movements

Like other large whales, sei whales are thought to undertake seasonal migrations from low to high latitudes, using the latter as a summer feeding ground and the former as a wintering area (e.g. Prieto et al., 2012). Migration routes are poorly known, and sei whales seem to exhibit an often irregular migration pattern, but there are indications that they breed in warmer waters close to the Equator (Ellis, 1991).

In the period June-September 1979-1985, sei whales showed up in August west and southwest of Iceland, at the time the blue, fin and humpback whales were leaving the area (Sigurjónsson, 1997).

In 2005, a large amount of sei whales were sighted off southeast and southwest Greenland, these were likely roaming animals from a larger North Atlantic population that some years reach these waters (Boertmann, 2007).

A presumed “foraging hotspot” was found around the Charlie Gibbs Fracture Zone along the Mid-Atlantic Ridge, coinciding with areas of high calanoid abundance (Waring et al., 2008).

Azores – Labrador

Sei whales in the Azores have been equipped with satellite tags, and all whales tagged during spring and early summer moved to the Labrador Sea, indicating a clear migratory corridor between these two areas (Prieto et al., 2014). This backed up the results of the tagging of one whale by Olsen et al. (2009) that showed the same migration route. However, the former results indicated no foraging behaviour while migrating until they reached the Labrador Sea, whilst the latter observed feeding behaviour along the Mid-Atlantic Ridge. The whale tagged by Olsen et al. (2009) also seemed to be closely associated with oceanographic features, “hitching a ride” on the ocean current conveyor belts on its way to the Labrador Sea.

Sei whale tagging off Portugal

North Atlantic stocks

Little is known on the stock identity of sei whales within the North Atlantic. Three stocks are recognized by the IWC:

  • Nova Scotia (including the waters off eastern USA);
  • Iceland-Denmark Strait; and
  • Eastern (including the waters of Spain, Portugal, British Isles, the Faroe Islands and Norway).

These divisions were, however, chosen on largely arbitrary grounds, mainly based upon historical catch and sighting data with a desire to match ICES boundaries (Donovan, 1991). Sei whales have also been observed in the West Greenland-Labrador Sea area, an area not assigned to any of these stocks (Cooke, 2018).

Genetic comparisons of sei whales from different North Atlantic locations found no evidence for population structure consistent with the extensive range of movement observed in satellite tagged sei whales. However, due to the high uncertainty of the genetic divergence estimates it was not ruled out (Huijser et al., 2018).

Sei whales from the North Atlantic are genetically distinct from those of the North Pacific and the Southern Hemisphere (Husijer et al., 2018).

© Christin Khan/NEFSC/NOAA Fisheries

Current abundance and trends

Recent surveys suggest in excess of 10,000 sei whales in the North Atlantic, but this number should be considered a minimum (Pike et al., 2019a). Pre-whaling levels have been estimated at around 20,000 individuals (Sigurjónsson, 1997).

Globally

There are indications that the global mature population has recovered to around 30% of the level in 1948, making the sei whale status at the border between Endangered and Vulnerable. Current number of mature individuals is estimated at 50,000 globally (Cooke, 2018).

Eastern North Atlantic

There have been several surveys in the Central Eastern North Atlantic since the 1980s. However, most of these surveys have not been optimised for the sei whale, running earlier and further north than the main sei whale summer distribution. Thus far, only one survey has run later in the season and farther south; the North Atlantic Sightings Survey in 1989. This survey resulted in an estimate of 10,300 sei whales (CV=0.27, 95% CI 6,150-17,260) for the Icelandic and Faroese area (Cattanach et al., 1993). The survey run in 1995 yielded an estimate of 9,249 sei whales (95% CI 3,700-23,116), which could suggest a northward shift in distribution (Borchers & Burt, 1997). However, this surprisingly high estimate could also be attributed to the fluctuations in sei whale abundance and distribution between seasons, the so-called “sei whale years”, especially taking into consideration previous and later estimates (e.g. Pike et al., 2011, 2019).

The latest survey, run in 2015, resulted in an estimate of 3,767 animals (CV=0.54, 95% CI 1,156-12,270) for the Central North Atlantic (Iceland and the Faroe Islands) (Pike et al., 2019a).

Most surveys run in Norwegian, UK, Irish and European waters have yielded none or too few sightings for an abundance estimate (e.g. Hammond et al., 2013, 2017; Leonard & Øien, 2019abcd, Pike et al., 2019b, Rogan et al., 2018). However, one survey run in the European Atlantic in 2007 gave an estimate of 366 sei whales (CV=0.33, 95% CI 176-762), with sightings limited to the survey block off Spain (Macleod et al., 2009).

All available abundance estimates

Area Year Estimate 95% Confidence interval CV Source Comment
Central NA 1987 1,293 434-3,853 0.60 Cattanach et al., 1993 Did not extend far south, resulting in a low estimate
1989 10,300 6,150-17,260 0.27 Cattanach et al., 1993 More sei whale specific survey (later in the season and farther south)
1989 1,590 N/A 0.60 Cattanach et al., 1993 The same survey as the previous, only without the southern blocks
1995 9,249 3,700-23,116 N/A Borchers & Burt, 1997 Surprisingly high estimate, perhaps a “sei whale year”
2001 1,494 843-2,245 0.24 Pike et al., 2011 High certainty estimate, uncorrected
2007 5,159 1,983-13,423 0.47 Pike et al., 2019b T-NASS core area, not corrected for bias
2007 9,737 4,189-19,665 N/A Pike et al., 2019b T-NASS core area + extension vessels, uncorrected. 97% of sightings in the extension areas were made in the area southwest of Iceland.
2015 3,767 1,156-12,270 0.54 Pike et al., 2019a Corrected estimate
European Atlantic 2007 366 176-762 0.33 Macleod et al., 2009 The first unbiased estimates (corrected for g(0) and responsive movement) for this area
Survey effort during NASS 1987 (Pike et al., 2011).
Survey effort during NASS 1989 (Pike et al., 2011).
Survey effort during NASS 1995 (Pike et al., 2011).
Survey effort during NASS 2001 (Pike et al., 2011).
Survey effort during NASS 2007 (Pike et al., 2011).
Survey effort during T-NASS 2015 (Pike et al., 2019a).

Western North Atlantic

Estimates based on sightings made along the eastern US coast and southern Nova Scotia in 2010-2014 were as follows (Palka et al., 2017):

Spring: 6,293 (CV=1.02, 95% CI 1,209-32,733)

Summer: 1,872 (CV=0.42, 95% CI 849-4,129)

Autumn: 2,489 (CV=0.49, 95% CI 1,006-6,158)

There are no other available abundance estimates for Canadian waters, as there have been too few sightings during surveys (e.g. Lawson & Gosselin, 2009, 2018).

Stock status

No assessment has yet been performed by NAMMCO for the sei whale in the North Atlantic, in part because of lack of abundance estimates for the whole area.

The Greenlandic Red List (2007) lists the species as Data deficient.

The Icelandic Red List (2018) lists the  species as Least Concern.

The Norwegian Red List (2015) list the species as Not assessed, too small population.

STATUS ACCORDING TO OTHER INTERNATIONAL ORGANISATIONS

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) listed the sei whale as Endangered in May 2019, when it was reassessed from its status as “Data Deficient” from 2003. This species is also listed as endangered by the Canadian Species at Risk Act (SARA).

The Convention on the Conservation of Migratory Species of Wild Animals (CMS) lists the sei whale on its Appendix I – Endangered migratory species and Appendix II – Migratory species conserved through Agreements.

The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) has the species in its Appendix I, which lists species that are the most endangered among CITES-listed animals and plants. This includes species threatened with extinction for which CITES prohibits international trade in specimens, with the exception of when the purpose of the import is not commercial.

The International Whaling Commission (IWC) has not carried out an assessment due to “Insufficient data for assessment”. They note that “Surveys reveal little sign of recovery in the Northeastern Atlantic” (IWC, n.d.).

The IUCN global Red List (2018) classifies the species as Endangered, but increasing, bordering on Vulnerable (Cooke, 2018).

The IUCN Red List for Europe (2007) classifies the sei whale as Endangered with unknown population trend (IUCN, 2007).

Management

Sei whales fall under the managements of both the International Whaling Commission, and, since its inception in 1992, NAMMCO.

A temporary, complete ban on whaling around Iceland for whales larger than common minke whales was declared in 1916, but whaling was resumed again in 1948 (NAMMCO, 2017).

The sei whale has been protected from commercial whaling by the IWC since 1975 in the North Pacific and 1979 in the Southern Hemisphere, as well as the IWC’s moratorium on whaling since 1986 (Cooke, 2018). However, the moratorium does not include catches taken under scientific permit.

Sei whales are now protected in all NAMMCO member countries.

Hunting and utilisation

Historically

When the blue and fin whale stocks were depleted, hunters started focusing their efforts onto the sei whale (Ellis, 1991; Prieto et al., 2012). It was exploited up until the 1970s, to a much less severe extent in the Central than in the Northeast Atlantic (IWC, n.d.; Cooke, 2018).

Since the 1860s and up until the moratorium on commercial whaling, about 16,000 sei whales were landed in the North Atlantic. The peaks were around the turn of the century (more than 1,200/year) and a few hundred catches during the years after that (Sigurjónsson, 1995). These numbers do not include the high struck and lost rates, which numbered approximately 13,000 unspecified large whales (Cooke, 2018). The sei whale seems to have been depleted in the Northeast Atlantic, as the majority of landings (nearly 10,000 in Norwegian and British waters, again not counting struck and lost) were taken here (Cooke, 2018).

Western North Atlantic

Sei whales were not commonly taken in Western Greenland and the Davis Strait, with only 8 reported catches in the periods 1924-39 and 1946-50. However, it was heavily exploited off Canada especially from the 1960s and up until 1972 (IWC, n.d.; Prieto et al., 2012).

A sei whale by the whale station in Jarfjord, Norway, 1897. Photo: Ellisif Rannveig Wessel. License: Attribution-NonCommercial-NoDerivs (CC BY-NC-ND) (https://creativecommons.org/licenses/by-nc-nd/4.0/deed.en)
A fin whale and a sei whale at Skorøy, an old whaling community outside Tromsø, Northern Norway. Vintage photographic postcard, c.1903 (the picture is from c. 1890). © Casas-Rodríguez Collection, 2009. Some rights reserved. https://creativecommons.org/licenses/by-nc-nd/3.0/

Today/after the moratorium

Except for one sei whale caught in Greenland in 2006, there have been no reported catches in NAMMCO member countries since its inception. Iceland caught a total of 70 sei whales for scientific research in the 1980s (the IWC list of total catches).

Year Area Type Catch
1986 West Iceland Special permit (scientific whaling) 40
1987 West Iceland Special permit (scientific whaling) 20
1988 West Iceland Special permit (scientific whaling) 10
1989 West Greenland Aboriginal subsistence hunting 2
2006 West Greenland Aboriginal subsistence hunting 1

Other human impacts

© Christin Khan/NEFSC/NOAA Fisheries

Entanglements & ship strikes

Although the reporting is not systematic, there does not seem to be any reported ship strikes or entanglements involving sei whales in NAMMCO areas (NAMMCO’s National Progress Reports, the IWC’s Progress Reports). There are, however, records of both in the Western North Atlantic in US and Canadian waters (e.g. Henry et al., 2014; Laist et al., 2001; the IWC’s Progress Reports).

Given the sei whale’s offshore distribution, entanglements and ship strikes could be less of a problem for this species than the species that frequent areas with high fishing activity and/or shipping lanes. However, ship strikes could go unnoticed due to both the sei whale’s negative buoyancy, making them sink when dead, and the lack of visibility immediately in front of ships. There have been reports of dead sei whales being detected only when the ship arrived at port with the whale on its bow (Laist et al., 2001).

Noise

As other whales do, sei whales use vocalisations for communication and, perhaps, underwater navigation and prey location. Increased noise from ship traffic, seismic surveys and military sonars could all have an impact on sei whales. It could, among other things, lead to behaviour alterations (e.g. fleeing, diving, cessation of feeding) or to damages in the structure of the ears, causing temporary or permanent loss of hearing.

Persistent organic pollutants

Persistent organic pollution levels in sei whales are not known. As with other marine mammal species, it may have an impact on reproduction and limit the recovery of certain populations.

Climate change

El Niño is a naturally occurring weather pattern mainly impacting the equatorial Pacific, but that can also affect climate elsewhere. A warming climate seems to affect the intensity of the events, thus also enhancing the impact (Fasullo et al., 2018). The 2014-2016 El Niño was the worst in 15 years, contributing to record-breaking seasons in tropical cyclone basins.

There are indications that the cause of the biggest single baleen whale mass mortality event was a harmful toxic algae bloom caused by this El Niño event. At least 343 baleen whales, primarily sei whales, were found stranded in remote waters in Chile. Investigations showed that the they died at sea, close to where they beached., which could further indicate that marine mammals are among the first oceanic megafauna victims of climate change (Häussermann et al., 2017).

Warming oceans could also affect prey distribution, in turn altering sei whale distribution and/or cause lack of prey and a shift in diet.

Research in NAMMCO member countries

There is no active research on the sei whale in NAMMCO member countries. However, sei whale sightings during the North Atlantic Sightings Surveys (NASS) are recorded. This has provided the basis for several recent abundance estimates in the Central North Atlantic (e.g. Pike et al., 2011, 2019a,b). As these latest surveys only have covered parts of the sei whale distribution area, and not the core range, these estimates are only partial. See also 6. Current Abundance and Trends.

The sei whale’s status is also regularly discussed in the various Working Groups of NAMMCO.

Baumgartner, M.F. & Fratantoni, D.M. (2008). Diel periodicity in both sei whale vocalization rates and the vertical migration of their copepod prey observed from ocean gliders. Limnol. Oceanogr. 53(5, part 2):2197-2209


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Boisseau, O., Lacey, C., Lewis, T., Moscrop, A., Danbolt, M. & McLanaghan, R. (2010). Encounter rates of cetaceans in the Mediterranean Sea and contiguous Atlantic area. Journal of the Marine Biological Association of the United Kingdom 90(8):1589-1599. doi:10.1017/S0025315410000342


Borchers, D., & Burt, M. (1997). Sei and fin whale abundance in the North Atlantic, estimated from NASS-95 shipboard survey data. SC/5/AE/1 for the NAMMCO Scientific Committee.


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