|Scientific Name:||Balaenoptera borealis|
|Species Authority:||Lesson, 1828|
|Taxonomic Notes:||The Sei Whale is a recognized species, but phylogenetically it falls within a clade including the Bryde's Whale and Omura's Whale (B. omurai) (Wada et al. 2003). Older catch and sightings records of Sei Whales, particularly prior to 1972, tended to include Bryde's Whales, and possibly Omura's Whales, in counts of "sei whales" in areas where one or both of the other two species also occur. Tomilin (1957) proposed two subspecies, B. b. borealis in the northern hemisphere and B. b. schlegeli Flower, 1884 in the southern hemisphere, but these are not widely recognized.|
|Red List Category & Criteria:||Endangered A1ad ver 3.1|
|Assessor/s:||Reilly, S.B., Bannister, J.L., Best, P.B., Brown, M., Brownell Jr., R.L., Butterworth, D.S., Clapham, P.J., Cooke, J., Donovan, G.P., Urbán, J. & Zerbini, A.N.|
|Reviewer/s:||Taylor, B.L. & Notarbartolo di Sciara, G. (Cetacean Red List Authority)|
The cause of the population reduction in this species (commercial whaling) is reversible, understood, and is not currently in operation. For this reason, the species is assessed under criterion A1, not under A2, A3 or A4. The global mature population is estimated to have declined by about 80% over the last three generations (Figure 1 in linked PDF document). This is supported by observed declines in abundance in several regions. Most of this decline is attributable to the southern hemisphere. While a higher intrinsic rate of increase than that given in Table 1 (in linked PDF document) may be possible and could result in a less threatened category, a precautionary approach is to use the value given in Table 1, which, based on the available data, is plausible for Sei Whales, particularly given the absence of direct evidence of any increase in the population.
|Range Description:||The Sei Whale is a cosmopolitan species, with a mainly offshore distribution, occurring in the North Atlantic, North Pacific and Southern Hemisphere, but probably not in the Northern Indian Ocean (Rice 1998); it is an occasional visitor to the Mediterranean (Reeves and Notarbartolo di Sciara 2006). Sei Whales migrate between tropical and subtropical latitudes in winter and temperate and subpolar latitudes in summer, staying mainly in water temperatures of 8-–18°C, and tend not to penetrate to such high latitudes as other rorquals. Their winter distribution seems to be widely dispersed and is not fully mapped (Horwood 1987, 2002).
In the North Pacific, Sei Whales in summer are distributed mainly north of 40°N, including the Gulf of Alaska and Aleutian Islands (US), and to some extent into the Bering Sea, but not into the Okhotsk Sea. The wintering grounds are poorly known, but Sei Whales were formerly caught in winter off the Bonin Islands (Japan) (IWC 2006); animals tagged there have been recaptured throughout the summer range (Masaki 1977).
In the North Atlantic the summer distribution seems to be quite variable from year to year; however, Sei Whales typically occur north of an arc running from south of Nova Scotia in the west, to the northwestern British Isles and Trondheim (Norway) in the east. They occur as far north as the Davis Strait and the northern end of the Denmark Strait. Occasional incursions into other areas have been noted (e.g. the Gulf of Maine, Schilling et al. 1993). Peaks of catch rates in early and late summer suggested a northward and southward migration through the former Nova Scotia whaling ground (Mitchell 1975). Sei Whales have been scarce in Norwegian waters in recent years despite significant catches there in the past. Sei Whales were caught in limited numbers, mainly in late summer, off northwestern Spain (Aguilar and Sanpera 1982), and have been recorded in winter as far south as the Caribbean Sea and Cap Blanc, Mauritania (Rice 1998).
The summer (Jan–Feb) distribution in the southern hemisphere is mainly in the zone 40–50°S in the South Atlantic and southern Indian oceans, and 45–60°S in the South Pacific (Miyashita et al. 1995). Known wintering grounds include a number of former low–latitude whaling grounds, including northeastern Brazil at 7°S (da Rocha 1983), Peru at 6°S (Valdivia et al. 1982), and in earlier years off Angola and the Congo (IWC 2006). Catches off western South Africa (Donkergat) and eastern South Africa (Durban) showed peaks in spring and autumn, suggestive of populations on migration routes (Horwood 1987).
Chilean whaling records (32–38°S) show catches and sightings of “Sei” Whales throughout the year including summer until the early 1980s, but some of these may have been Bryde’s Whales. However, some Sei Whales have been observed feeding in summer at around 42°S off Chile in recent years (Galletti et al. 2005).
Native:Angola (Angola); Argentina; Australia; Bahamas; Bermuda; Brazil; Canada; Cape Verde; Chile; Congo; Congo, The Democratic Republic of the; Cuba; Dominican Republic; Falkland Islands (Malvinas); Faroe Islands; French Southern Territories; Gibraltar; Greenland; Haiti; Iceland; Ireland; Japan; Kenya; Korea, Democratic People's Republic of; Korea, Republic of; Madagascar; Mauritania; Mexico; Morocco; Namibia; New Zealand; Northern Mariana Islands; Norway; Peru; Portugal; Réunion; Russian Federation; Saint Helena, Ascension and Tristan da Cunha (Tristan da Cunha); Saint Pierre and Miquelon; South Africa; South Georgia and the South Sandwich Islands; Spain; Turks and Caicos Islands; United Kingdom; United States; Uruguay; Western Sahara
|FAO Marine Fishing Areas:||
Atlantic – western central; Atlantic – northeast; Atlantic – northwest; Atlantic – eastern central; Atlantic – Antarctic; Atlantic – southwest; Atlantic – southeast; Indian Ocean – Antarctic; Indian Ocean – eastern; Indian Ocean – western; Pacific – southwest; Pacific – southeast; Pacific – northeast; Pacific – Antarctic; Pacific – northwest; Pacific – eastern central; Pacific – western central
|Range Map:||Click here to open the map viewer and explore range.|
North Atlantic. The International Whaling Commission (IWC) recognizes three stock divisions for sei whales in the North Atlantic: Nova Scotia; Iceland-Denmark Strait; and Eastern (including the waters of Spain, Portugal, British Isles, Faeroes and Norway), but the divisions were chosen on largely arbitrary grounds (Donovan 1991).
About 14,000 Sei Whales are recorded caught by modern whaling in the North Atlantic. In addition, an unknown proportion of the approximately 30,000 unspecified large whales caught in the North Atlantic in the late 19th and early 20th centuries were Sei Whales.
Sei Whales appear to have been depleted in the eastern North Atlantic, with over 7,500 recorded taken in Norwegian and British waters prior to World War II (not counting the unspecified whales), but fewer than 200 since then. The species seems to be virtually absent there now: Norwegian surveys of northeast Atlantic waters in 1987, 1989, and annually from 1995–2005 have yielded only one Sei Whale sighting in the Eastern stock area. Small numbers of sei whales were caught during 1957–80 off northwestern Spain, mainly in late summer/autumn (Aguilar and Sanpera 1982); the stock identity of these animals is unclear.
By contrast, Sei Whales are still abundant in the central North Atlantic (Iceland-Denmark Strait IWC stock area), especially southwest of Iceland south to 50°N in summer. The only survey with reasonably complete coverage, designed specifically for the purpose of estimating sei whale abundance, was in 1989, and yielded a population estimate of 10,300 whales (CV 0.27) (Cattanach et al. 1993). Areas of Sei Whale abundance seem to shift markedly between years relative to the northern extent of the distribution, more so than for other baleen whale species (Gunnlaugsson et al. 2004).
No recent abundance estimates are available for the western North Atlantic. The population size during 1966–69 was estimated at 2,078 (Mitchell and Chapman 1977) from sightings surveys, using strip transect methodology. About 1,200 Sei Whales were taken off Nova Scotia during 1962–72.
North Pacific. The last assessment of North Pacific Sei Whales by the IWC Scientific Committee was in 1974 (IWC 1977). The exploitable population (animals of legally catchable size) was estimated to have declined from 42,000 in 1963 to 8,600 in 1974 (Tillman 1977). Over 40,000 Sei Whales were caught during this period (IWC 2006). A 75% reduction in Sei Whale catch rates of Californian whaling stations during the 1960s is consistent with an ocean-wide decline (Rice 1977). Exploitation ceased in 1975. The extent to which the population has recovered since then is unclear.
Hakamada et al. (2004) gave an estimate of 4,100 animals from one area of the western North Pacific, but attempts to extrapolate this to produce an estimate for the entire western North Pacific have not been accepted. It is likely that there has been some increase in the population since the end of exploitation, but given the tendency for sei whale distribution to change from year to year, it is hard to interpret the limited survey data that currently exist.
Sei Whales appear not to have formerly been uncommon in the eastern North Pacific. However, a preliminary estimate for the US west coast based on surveys in 1996 and 2001 is only 56 whales (Barlow 2003). In the Eastern Tropical Pacific, all potential sei/Bryde’s whales that could be specifically identified were found to be Bryde’s, suggesting that Sei Whales are now rare (Wade and Gerrodette 1993).
Southern Hemisphere. Over 200,000 Sei Whales are recorded taken by modern whaling in the southern hemisphere during 1905–1979. The IWC arbitrarily divides southern hemisphere Sei Whales, along with other southern hemisphere baleen whales, into six longitudinally defined management Areas. Sei Whales in all of the six Areas have been classified as Protection Stocks by the IWC since 1978. The greatest catches were during 1960–72, when they exceeded 5,000 in every year, topping at nearly 20,000 in 1964 alone. Most catches were taken in summer by pelagic fleets operating south of 40°S, but winter catches were also taken from land stations in Brazil, Peru and South Africa (and smaller numbers in Chile, where there is ambiguity with Bryde’s Whales).
The last assessment of southern hemisphere Sei Whales by the IWC Scientific Committee was conducted in 1979 (IWC 1980). Some extra information on these assessments is given by Horwood (1987). Based on catches/sightings per unit effort from Japanese catch and scouting vessels respectively, the “exploitable” stock size (Sei Whales of legal size, approximately 2/3 of total population), excluding Area II (South Atlantic sector), was estimated to have declined from about 64,000 in 1960 to about 11,000 by 1979. The population model used failed to produce an estimate for Area II, because nearly all the catch for this area was taken in just the two seasons 1964/65 and 1965/66, with nearly 30,000 Sei Whales being taken in these two years, yet the abundance indices suggested a more continuous decline during the 1960s and 1970s (IWC 1980). This suggests that the management Areas do not correspond to biological populations.
Other available evidence also suggests a severe and rapid decline in Sei Whale stocks during the 1960s and 1970s. The catch and sighting rates from winter whaling operations off Brazil and South Africa show even sharper declines. Abundance indices from the Brazilian whaling ground for Sei Whales during 1966–81 (da Rocha 1983) declined by ca. 90% during 1966–72.
No recovery of sei whales in Brazilian waters has been detected since that time (Zerbini et al. 1997, Andriolo et al. 2001). Catch and sightings indices off Durban, South Africa for 1965–74 (Best 1976) declined by over 95%. Tag-recapture data from pelagic whaling suggested an approximately 6-fold decline between 1962–76 (IWC 1980, Fig. 1).
IWC (1996, Table 2) gives a total population estimate for Sei Whales south of 30°S of about 10,000 based on a combination of IDCR and Japanese Scout Vessel (JSV) sightings data. It is difficult to know how much reliance to place on this estimate, because no variance is given, but it is consistent with a severe decline. No more recent summer data are available, except for the area south of 60°S, which is outside the main summer range of Sei Whales. In the absence of dedicated surveys in Sei Whale habitat, and resulting abundance estimates, it is not possible to assess whether there has been any increase in southern hemisphere sei whales since the cessation of whaling.
Generation time and maximum rate of increase. The generation time is estimated to be 23.3 years (Taylor et al. 2007). The 3-generation time window for applying the decline criterion (A) is 1937–2007.
For assessment purposes, the IWC Scientific Committee has used a natural mortality rate of 0.06, an age at first reproduction declining (with stock depletion) from 12-13 years to 10-11 years, and an annual pregnancy rate increasing (with stock depletion) from 0.27 to 0.37-0.39 (IWC 1980; Horwood 1980). Assuming a maximum annual pregnancy rate of 0.40, a minimum age of first reproduction of 10 years, and no juvenile mortality, the maximum rate of increase is 2.7% p.a. Horwood and Millward (1987) also conclude that the maximum rate of increase for sei whale populations is less than 3% per year.
Population assessment. Because the available published assessments for this species are not up to date, an updated population assessment is conducted here to enable assessment of the population reduction over the period 1937-2007 relative to the A criterion. While the available data do not permit a scientifically rigorous estimation of the extent of population reduction, it is reasonable to use conventional population assessment methods to provide a crude indication of the extent of possible reduction relative to the criteria. A conventional deterministic age-structured model with an age at first capture (“recruitment”) (ar) and an age at first reproduction (am), and linear density-dependence was applied to the North Pacific, North Atlantic and Southern Hemisphere regions separately. The results are provided in the linked PDF document, and constitutes an integral part of this assessment.
|Habitat and Ecology:||Sei Whales exhibit a greater variety in diet than, say, Blue Whales, but tend to feed on only one type of prey at a time. Of 21,713 stomachs examined in the North Pacific, 82.7% contained copepods only, and 12.6% contained euphausiids only; of 31,494 stomachs examined in the southern hemisphere, 54.3% contained euphausiids only, 30.5% contained copepods only, and 14.4% contained amphipods only (Nemoto and Kawamura 1977). Sei whale stomachs examined in the Northeast Atlantic contained copepods and euphausiids (Jonsgård and Darling 1977). Sei Whale stomachs analysed in the North Pacific off California contained euphausiids and anchovies (Rice 1977). Sei Whales are noted for their erratic appearance in specific feeding grounds, being plentiful in some years and absent (sometimes for years or even decades) in others (Horwood 1987).|
Sei Whale exploitation by modern whalers was particularly intensive in the southern hemisphere and the North Pacific from the late 1950s to the mid-1970s, following the depletion of Blue, Fin and Humpback Whales. Sei Whale stocks were seriously depleted during this period. Exploitation in the North Atlantic occurred over a longer period and was less intensive, except in the eastern North Atlantic, where the population appears not to have recovered from early modern whaling. Commercial exploitation ceased in the North Pacific in 1975, in the southern hemisphere in 1979 and in the North Atlantic in 1989. Catches by Japan under scientific permit resumed in the North Pacific in 2002: since 2004, the annual take has been 100 animals.
Reports of other human-caused deaths of Sei Whales are rare. Two fatal ship strikes (Sei Whales found dead on the bows of ships) were reported on the US East Coast during 2000–2004 (Cole et al. 2006). It is hard to extrapolate from known cases to an estimated total, but Sei Whales appear to be at relatively low risk of human impacts, probably because of their largely offshore distribution.
Rice (1961, 1974) reported a pathological condition in several North Pacific sei whales which resulted in deterioration or loss of baleen; however, the current frequency of this condition, and its impact (if any) on the population, are unknown.
|Conservation Actions:||Sei Whales have been specifically protected from commercial whaling by the International Whaling Commission since 1975 in the North Pacific and 1979 in the southern hemisphere. They have also been protected by the general moratorium on commercial whaling since 1986, although this does not cover catches taken under scientific permit. This species is included in CITES Appendix I although Iceland has held a reservation against this listing since 2000. The species is listed in Appendix II of CMS.|
|Citation:||Reilly, S.B., Bannister, J.L., Best, P.B., Brown, M., Brownell Jr., R.L., Butterworth, D.S., Clapham, P.J., Cooke, J., Donovan, G.P., Urbán, J. & Zerbini, A.N. 2008. Balaenoptera borealis. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 10 March 2014.|
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