Length: 12 – 16 m (40-53’)
Weight: 20 – 30 tons
Population size in Canada: Atlantic – unknown     Pacific – unknown
Population size in World: ~ 54,000
Conservation status in Canada: Atlantic – Data deficient (COSEWIC), Pacific – Endangered (COSEWIC/SARA)
Conservation status in World: Endangered (IUCN)

Classification

Latin Name: Balaenoptera borealis
Other Names: Rudolphi’s Rorqual, Pollack whale, Coalfish whale, Sardine whale, Japan finner
Suborder: Mysticete (Baleen Whale)
Family: Balaenopteridae

Field Identification
  • up to 53 feet in length
  • large, like mid-sized fin whale
  • streamlined body
  • v-shaped head, long ridge on head
  • two blowholes
  • body and bottom of tail is dark
  • dark colouration, both sides of head and jaw are dark
  • ventral grooves end before navel around tip of pectoral flippers
  • tall dorsal fin, higher and farther back than a fin whale’s; sometimes very pointed
  • body often has oval-shaped scars
  • greyish-black baleen with fine white bristles on inside, ~75-80 cm in length
  • tall, columnar blow
  • blowhole & dorsal fin appears simultaneously
  • very elongated “footprint” on surface after dive
  • rarely shows fluke & doesn’t arch tail stock
  • group size 2-5 (less commonly 1-5), groups up to 30 at good feeding grounds
Description

The Sei whale is the third largest baleen whale (after the blue whale and fin whale). They are slim and streamlined, with relatively short pectoral fins (1/10 of their body length) that are pointed at the tips. The dorsal fin is tall and falcate (very curved) and is usually visible when the blowholes surface. There is a single, long rostral ridge on the v-shaped head, and 30-60 ventral grooves that end well before the navel.
Their large bodies are dark bluish-gray dorsally, and creamy-white on the underside, and are often characterized by the presence of many white/gray oval scars caused by bites from cookie-cutter sharks and lampreys. They have between 300-410 baleen plates on each side of the mouth. The baleen is dark gray or black, with a fine, white inner edging.
Adult male Sei whales can reach between 45-53 feet and weigh 14-17 tons, while adult females are smaller, reaching up to 50 feet and weighing around 13 tons.

Distribution

The Sei whale has an extremely wide distribution, and can be found from subtropical waters to high latitudes, inhabiting both shelf and oceanic waters. They are famous for their unpredictable movement patterns, and are characterized by sudden influxes into an area, followed by absence from it for many years. Most rorquals are migratory, and it is generally accepted that Sei whales show seasonal migrations between high latitudes (summer) and tropical waters (winter), although much of this remains unclear. It is currently unknown where breeding takes place.

Life History

Male and female Sei whales can live up to 70-80 years, and reach sexual maturity around the age of 10. In most seas, the age of maturity declined by 2 to 3 years after the populations was depleted by whaling. It is speculated that mating may occur year round, although little is known about their mating and breeding patterns. Females usually give birth every 2 to 3 years.  Calves are born in winter, presumably in tropical water, after a gestation period of 11 to 12 months. Most calves are weaned at 6 to 8 months at which time they separate from their mothers.

Diet

Sei whales feed on small fish, squid, krill, and small zooplankton, especially copepods.  They are the only mysticete whale that uses a combination of two feeding methods: they will feed (1) by gulping (taking one big mouthful of water and prey at a time), and (2) by skimming and constantly filtering food.

Behaviour

There is very little known about the social behaviour of Sei whales. Much like other baleen whales, they travel in small transient groups, and have only been observed aggregating in larger groups on feeding grounds. Sei whales are thought to be one of the fastest of the large whales (along with the Fin whale) and can swim at speeds up to 25 knots in short bursts. When beginning a deep dive, the Sei whale does not raise its fluke and very rarely arches its back.

Population size

Sei whales are thought to be fairly abundant in the North Atlantic and North Pacific. However, Southern Ocean populations are considered to be much depleted. There are currently an estimated 54,000 Sei whales globally. The best estimates of population size in the western North Atlantic are 1400 – 2300.

Threats

Although not initially a target for commercial whales, Sei whales became severely exploited in the decades following the collapse of the fin and blue whale stocks. At the same time, faster whaling boats made hunting rorquals easier, and Sei whales became the target of a worldwide hunt that killed over 200,000 individuals during the 20th century.
Currently, Sei whales face many threats from increasing ocean noise, including sonar, military operations and oil exploration. They are also at risk of boat strikes, entanglement in fishing gear and exposure to pollution. They are currently at one-fifth their estimated historical population size.

References

American Cetacean Society. 2006. Sei Whale and Bryde’s Whale Fact Sheet. Pgs. 1-4.

COSEWIC report

Fisheries and Oceans Canada. 2005. Sei Whale Backgrounder. BG-PR-05-015c-e. http://www-comm.pac.dfo-mpo.gc.ca/pages/release/bckgrnd/2005/bg015c_e.htm [accessed February 3, 2007]

National Audubon society’s Guide to marine mammals of the world.

Reeves, R. R., Stewart, B.S., Clapham, P.J., Powell, J.A. 2002. Pilot Whales in: Guide to Marine Mammals of the World. Alfred A. Knopf: New York. Pgs. 440-443.