Sei Whale


Class: Mammalia
Order: Cetacea
Suborder: Mysticeti
Family: Balaenopteridae
Genus: Balaenoptera
Species: borealis
Common Names: Sei whale; Pollack whale; Coalfish whale; Sardine Whale
Irish Name: Droimeiteach na Saíán

Key Identification Features

Maximum body length: 20m
Average body length: 12-16m
Blow: Narrow cloud, 3m high, but bushier than that of fin whale
Head: Slender head, single longitudinal ridge extends from blowhole to snout, head size can be between 1/5 and ¼ of it's body size
Beak: No beak
Dorsal fin: The dorsal fin is well-defined, slender, erect, and slightly hooked and is located about two-thirds back along the body.
Colouration: It is a mottled blue to grey dorsal side, with paler grey to white underside, often scared by parasites
Markings: Distinctive throat grooves, 32-62 usually end just behind flippers, two distinctive blowholes, narrow pointed snout.

Field Identification

This is a slender cetacean, although more robust than the fin whale. The dorsal fin is well defined and slightly hooked and is located about two-thirds back along the body. The head and jaws are rather narrow and slightly arched, unlike the other rorquals. Rarely seen breaching but when breaching, their body comes out of the water at a shallow angle and belly flops onto water. Normally blows once every 40-60sec for 1-4 minutes, then dives for 5-20min. During short dives rarely descends deeper than a few meters, so it's progress can be followed by fluke prints or swirls left by the beat of it's tail just below the surface. The whale's blow and dorsal fin can appear simultaneously, which is in marked contrast to the fin whale, whose dorsal fin appears long after the blow.

Species Similar in Appearance

Most likely to be confused with minke (20ft smaller) and fin whale (20ft bigger)


Sei Whales tend to swim in pods of 3-5 animals, though sometimes larger groups may gather if food is abundant and during migrations. Although little is known about how this species communicates, it has been found that low-frequency pulses are common. Sei whales are among the fastest cetaceans, swimming at speeds of up to 50 kilometres per hour.

Status and Distribution

Worldwide distribution, but mostly in deep temperate waters. Not normally found in extreme polar waters although sub-arctic and sub-Antarctic are favoured summer feeding grounds. Little is known about their migration but they appear to move to lower latitudes in winter. This species is essentially a dweller of the open ocean, not generally found inshore or in coastal waters. The Sei Whale tends to follow shelf contours and plankton gatherings. Current estimation of population is 40-60,000.

Where and When Best Seen in Ireland

Occasionally observed from headlands around the same winter period as the fin whales off the Cork coast.

Food and Feeding

The Sei whale obtains food by skimming through the water and catching prey in its baleen plates. These whales feed near the surface of the ocean, swimming on their sides through swarms of prey. An average Sei whale eats about 900 kilograms of small fish (up to 30cm), squid and plankton per day.

Reproduction and Life Cycle

Young reach sexual maturity at 10 years of age, mating occurs during the winter months. Sei whales in the northern hemisphere mate between November and February, whereas mating in the southern hemisphere occurs between May and July. Gestation lasts from 10 to12 months. Females typically give birth to a single calf measuring approx. 450 cm in length and weights of approx.750kg. The calf nurses for six or seven months, but do not reach adult size until they are about 25 years old. Females typically give birth every other year, but a recent increase in pregnancies has been noted. Sei whales may live as long as 75 years.

Conservation Issues

Sei whales have been heavily exploited by the whaling industry. The take of these animals peaked in the 1964-65 season, when 25,454 of these whales were taken. The reported global catch of sei whales in the 1978-79 season was a mere 150, reflecting the dramatic reduction in their numbers. They are now listed as endangered.

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