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Recovery Strategy for Blue, Fin, and Sei Whales in Pacific Canadian Waters [Proposed]

4. The Sei Whale

4.1. Current status

Common name: Sei whale

Scientific name: Balaenoptera borealis

Legal listing (SARA): January 2005 (Endangered)

Assessment summary: May 2003

COSEWIC status:Endangered

Reason for designation:  This was one of the most abundant species sought by whalers off the British Columbia coast (with over 4000 individuals killed) and was also commonly taken in other areas of the eastern North Pacific. Sei whales have not been reported in British Columbia since whaling ended and may now be gone. There are a few, if any, mature individuals remaining in British Columbia waters, and there is clear evidence of a dramatic decline caused by whaling and no sign of recovery. (www.cosewic.gc.ca)

Occurrence in Canada:North Atlantic and North Pacific

Status history:Designated Endangered in May 2003.

4.2.  Species description

The sei whale is the third largest member of the Balaenopteridae, after the blue and fin whales. Sei whales are cosmopolitan in their distribution, though they appear somewhat restricted to temperate waters, occurring within a more restricted range of latitudes than all other rorquals except Bryde’s whales (COSEWIC 2003). There is evidence for three stocks of sei whales (western, central and eastern) in the Pacific (Masaki 1977).

An average adult sei whale is 15 m long and weighs 19 tonnes (Horwood 1987). Females are larger than males. Animals in the northern hemisphere appear to be smaller than those in the southern hemisphere (Tomilin 1967). The maximum reported lengths for a female were 18.6 m in the northern hemisphere and 20 m in the south (Gambell 1985a).

Sei whales are dark to bluish grey dorsally and white to cream coloured ventrally. The ventral grooves commonly have a white or light-coloured area extending from the chin to the umbilicus, although colouration is extremely variable. Oval-shaped scars from cookie-cutter shark bites and lampreys, and infestations of ectoparasitic copepods often occur on the lateral and ventral sides. The curved, slender dorsal fin is prominent measuring 0.25-0.75 m, and is set further forward on the body compared to blue and fin whales. The pectoral flippers are relatively short measuring only 9-10 per cent of the body length, dark grey ventrally, and pointed at the tips. The dark grey flukes are rarely raised to “fluke-up” before dives (COSEWIC 2003).

In the eastern North Pacific, fin and sei whales overlap morphologically in body size, colouration, and dorsal fin shape making the two easily confused. However, sei whales lack the asymmetric white colouration of the right jaw and ventral side that is diagnostic for fin whales. Confusion with fin whales, and to a lesser degree with Bryde’s and minke (B.acutorostrata) whales, implies that sei whale population size and range could easily be underestimated (COSEWIC 2003).

Sei whales migrate from low-latitude wintering areas to high-latitude, summer feeding grounds. Catch records indicate that migrations are segregated according to length (i.e., age), sex, and reproductive status, with pregnant females leading the migration to the feeding grounds. The youngest animals arrive last and leave first, and travel to lower latitudes than adults. The wintering grounds of sei whales are largely unknown, though they are thought to occur far offshore (COSEWIC 2003).

Males and females reach sexual maturity between 5 and 15 years of age and live to approximately 60 years of age. In both hemispheres, the age of sexual maturity declined from 10-11 years to 8 years between the 1930s and the 1960s, likely in response to exploitation. Mating, followed by a gestation period of 10.5 to 12 months, and calving occur in winter. Calves nurse for about 6 months and are weaned on the feeding grounds. The calving interval is 2-3 years (COSEWIC 2003) .