Sei Whale

Balaenoptera borealis

SEI_WHALEClass: Mammalia
Order: Cetacea
SubOrder: Mysticeti
Family: Balaenopteridae
Genus: Balaenoptera
Species: Borealis

Physical Description
Sei whales are very similar in external appearance to fin and Bryde’s whales, both of which also have a prominent falcate (curved and tapering to a point; sickle-shaped)dorsal fin. All three have typical rorqual body shapes.

In both sei and Bryde’s whales, the dorsal fin rises at a steep angle from the back. Sei whales have only a single prominent longitudinal ridge on the rostrum (Bryde’s whales tend to have three), and a slightly arched rostrum with a downturned tip. Unless the head can be seen at close quarters, however, Bryde’s and sei whales can be especially difficult to distinguish at sea.

The 32-60 ventral pleats are short for rorquals, ending far ahead of the navel. The 219-402 baleen plates on each side are black with very fine fringes of light smoky-gray to white. Sei whales produce a blow up to 3 m tall. Coloration is mostly dark gray, except for a whitish area on the belly. The back is often mottled with scars (probably from cookie-cutter shark bites), and the skin surface often resembles galvanized metal.

Length & Weight
Adults can be up to 18 m in length, although 15 m is a more typical length for adults. Large adults may weigh 30 tons. At birth, sei whales are 4.5-4.8 m long.

Mating and Breeding
Sei whales mate in tropical latitudes during the autumn and winter, although sporadic mating is thought to occur throughout the year. Individuals reach sexually maturity between 6 to 12 years of age. Cows give birth to a single calf after an 11 to 12 month gestation period and calves nurse for the next 6 to 7 months before they are weaned. Cows have a 2 to 3 year calving interval.

Feeding
Sei whales skim copepods and other small prey types, rather than lunging and gulping, like other rorquals. This may largely explain the relative fineness of the baleen fringes and the shortness of the throat pleats in this species.

Distribution and Migration
Sei whales are open ocean whales, not often seen near the coast. They occur from the tropics to polar zones in both hemispheres, but are more restricted to mid- latitude temperate zones than are other rorquals. They do undergo seasonal migrations, although they apparently are not as extensive as those of some other large whales.

Natural History
Sei whales, swimming at speeds of up to 30 knots, are amongst the fastest baleen whales. When diving, they do not arch their back nor show their flukes, but simply sink below the surface. They often leave a series of tracks or “fluke prints”, a smooth circle of water created by the movement of the fluke just below the surface of the water.

Status
After blue and fin whales, the sei whale was next in the line of fire of the modern whaler’s harpoon. The heaviest period of exploitation was between the 1950s and 1970s. Whaling took place in the North Pacific and North Atlantic oceans, but most hunting was in the Southern Hemisphere. Although fully protected by the IWC since 1985, a few were taken in the North Atlantic by Iceland in the last few decades of the 20th century. Sources of mortality other than direct exploitation include probable vessel strikes. Current global abundance of the sei whale is considered to be about 80,000.

Acknowledgements
Australian Museum; American Cetacean Society; National Geographic Society; Department of the Environment, Water, Heritage and the Arts