Sei whale (Balaenoptera borealis) COSEWIC assessment and status report: chapter 2

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Executive Summary

Species information

The sei (pronounced “say”) whale (Balaenoptera borealis) is the third largest member of the Balaenopteridae family, after the blue (B. musculus) and fin (B. physalus) whales. The name is an anglicization of “sejhval”, given by Norwegian whalers because its arrival in Scandinavian waters coincided with the “seje”, or pollock (Pollachius virens)(Andrews 1916).

The species is grey in colour, with a variable white region on the ventral and lateral sides from the chin to the umbilicus. These areas may appear mottled, with gray or white circular scars caused by various predators or parasites (Andrews 1916, Ivashin and Golubovsky 1978), lampreys (Pike 1951, Rice 1977), or cookie cutter sharks (Schevchenko 1977). The dorsal fin is tall and slender and further forward on the body when compared to blue or fin whales (Andrews 1916). The baleen is finer than that of the other rorquals, making it a reliable feature for species identification (Mead 1977).

In tropical waters, sei whales are difficult to distinguish from Bryde’s whales (B. brydei), while in temperate waters they are difficult to distinguish from fin whales. This potential for confusion with fin whales is noteworthy, since it could significantly impact our understanding of the species distribution and population size off Canada’s coasts.

Distribution

Sei whales are found in all the oceans of the world and make seasonal migrations from low-latitude wintering areas to high-latitude summer feeding grounds. While the locations of the wintering grounds are relatively unknown (Perry et al. 1999), summer distributions on the feeding grounds exhibit dramatic year-to-year differences (Andrews 1916, Jonsgård and Darling 1977, Waring et al. 2001). The species generally frequents more temperate waters than the other rorquals (Mizroch 1984, Horwood 1987).

There is evidence that at least three stocks of sei whales (western, central and eastern) occur in the Pacific (Masaki 1977). In Atlantic Canada, the Nova Scotia stock is presumed to be separate from the eastern North Atlantic stock (IWC 1977). Evidence for a second stock in Atlantic Canada – the Labrador stock – is equivocal.

There has been only a single confirmed sighting in the eastern North Pacific in recent years (Perry et al. 1999). However, the species is regularly sighted on the Atlantic seaboard. NMFS surveys have found concentrations of sei whales along the southern edge of Georges Bank in the Northeast Channel area (Waring et al. 2001). In Canadian waters, they are regularly seen during surveys of the Nova Scotian shelf. Difficulty distinguishing the species from fin whales may explain the lack of sightings in the eastern North Pacific.

Habitat

Sei whales use primarily pelagic habitats, and are most often found in deeper waters. They appear to be associated with the continental shelf edge in the northwest Atlantic (Hain et al. 1985). The main characteristic of sei whale feeding habitat is likely a high pelagic concentration of zooplankton, especially copepods. Characteristics of preferred breeding grounds are unknown.

Biology

Sei whales reach sexual maturity at between 5 and 15 years of age. The average size for adults is 15 m and 19 tonnes (Horwood 1987). They may live to 60 years of age (Lockyer 1974), and are the fastest of the rorquals, capable of short bursts in excess of 30 knots (Andrews 1916). The gestation period is estimated at 10 – 12 months (Masaki 1976) with conception and calving occurring at lower latitudes. The calving interval is 2-3 years, and calves are weaned on the feeding grounds prior to the fall migration. This suggests a lactation period of about 6 months. Migrations in both hemispheres are segregated based on age, sex and reproductive condition (Gambell 1968, IWC 1977, Gregr et al. 2000). Sei whales exhibit a diversity of feeding strategies. This may allow them to develop a more generalist diet than other balaenopterids, and likely explains the differences in diet composition reported in the different oceans of the world.

Population sizes and trends

The best estimates of population size, still cited today, are 7,260 – 12,620 for the entire North Pacific (Tillman 1977), and 1,393 – 2,248 in the western Atlantic (Mitchell and Chapman 1977). These estimates are based primarily on catch per unit effort data collected during commercial whaling. Because of the lack of contemporary data, there are no recent abundance estimates or population trends available for this species in either the eastern North Pacific or the Northwest Atlantic. During commercial whaling, as many as 1,000 sei whales may have been killed by Newfoundland and Nova Scotia whaling stations, and at least 4,002 were taken off British Columbia.

Limiting factors and threats

Factors that would limit the recovery of this species include inter-specific competition, disease, and human-induced stresses. The species is known to carry both endo- and ectoparasites, and is susceptible to an infection causing the shedding of baleen plates (Andrews 1916, Rice 1974, 1977). The degree to which these infections are present today is unknown.

The level of competition between sei whales and other whale species is not known, but the most likely competitors would be fin and right whales, given their sympatric distribution and similarities in diet. However, the global depletion of all whale stocks and the sei whale’s polyphagous feeding habits make it unlikely that competition with other whale species is currently a limiting factor. Rather, competition with planktivorous fishes may be a limiting factor for all balaenopterids (Payne et al. 1990, Trites et al. 1999).

Threats from human activities include ship strikes and entanglement, as well as acoustic and chemical pollution (Clapham et al. 1999). While very few ship strikes have been reported, they may go undetected because of the pelagic nature of the species. Similarly, while there are no reports of fisheries-related mortality, entanglement or injury on either coast (Perry et al.1999, Carretta et al.2001), offshore drift net fisheries pose a potential threat (Barlow et al. 1997).

The effects of acute and chronic acoustic and chemical pollution on this species are not well studied. Responses to vessel noise appear to depend on the behaviour of the animals and the approaching vessel (Perry et al. 1999). Habitat degradation resulting from chronic exposure to acoustic and chemical stresses may pose the greatest threats to this species.

Special significance of the species

Since the sei whale is rarely found near shore, it is not the primary target of whale watching operations. Similarly, this offshore distribution means the species never played a role in the lives of coastal Aboriginal groups, except perhaps intermittently in the eastern North Pacific. However, the species likely has a significant ecological role.

Existing protection or other status designations

The sei whale is listed as “Endangered” by the IUCN on the basis of large and rapid population declines and a reduction in range caused by 20th century exploitation (1960 – 1986). CITES lists the species under Appendix 1, the category that includes species threatened with extinction. Both the Nova Scotian and the eastern North Pacific stocks are listed as endangered under the United States Endangered Species Act. The IWC has classified both the North Pacific and the Nova Scotia stocks as “Protected”. This designation prohibits commercial whaling, but not whaling for the collection of scientific data.

In Canada, the existing federal Fisheries Act and Marine Mammal Regulations prohibits disturbance of marine mammals except for purposes of hunting, for which a permit is required. None of the species’ range is currently protected in Canadian waters.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) determines the national status of wild species, subspecies, varieties, and nationally significant populations that are considered to be at risk in Canada. Designations are made on all native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fish, lepidopterans, molluscs, vascular plants, lichens, and mosses.

COSEWIC Membership

COSEWIC comprises representatives from each provincial and territorial government wildlife agency, four federal agencies (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biosystematic Partnership), three nonjurisdictional members and the co-chairs of the species specialist groups. The committee meets to consider status reports on candidate species.

Definitions

Species

Any indigenous species, subspecies, variety, or geographically defined population of wild fauna and flora.

Extinct (X)

A species that no longer exists.

Extirpated (XT)

A species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)

A species facing imminent extirpation or extinction.

Threatened (T)

A species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)*

A species of special concern because of characteristics that make it particularly sensitive to human activities or natural events.

Not at Risk (NAR)**

A species that has been evaluated and found to be not at risk.

Data Deficient (DD)***

A species for which there is insufficient scientific information to support status designation.

* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.
** Formerly described as “Not In Any Category”, or “No Designation Required.”
***  Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994.

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list.

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.