Sei whale with a calf

Species Profile: The Sei Whale

The Sei Whale is the fourth largest baleen whale and an endangered species.

Sei whale with a calf
Sei Whale Mother With Her Calf (Photo: NOAA)

The Sei Whale is another baleen whale and it’s the fourth largest of them after the Blue Whale, the Fin Whale, and the Humpback Whale.

Although it prefers deeper offshore waters, it’s often sighted in most oceans and adjoining seas.

Typically, its body is colored dark steel gray with irregular light grayish to white markings towards the front of the lower body. The Sei whale’s skin is commonly marked with distinct crater-shaped scars caused by wounds inflicted by Cookie-cutter sharks (Isistius Brasiliensis). Also, these whales have a tall, sickle-shaped dorsal fin.

This whale’s common name comes from the Norwegian dialect and refers to the word “pollock.” The pollock is a type of fish that appears off the Norwegian coast at the same time every year as the Sei whale does.

Because of relentless hunting during the whaling era of the late 19th and 20th centuries, this whale is now protected internationally. However, the Japanese still hunt a limited number of them under their “research” program.

The IUCN lists the Sei Whale as an Endangered Species.

1) Scientific Name

Balaenoptera Borealis

2) Scientific Classification:

  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Mammalia
  • Order: Cetacea
  • Family: Balaenopteridae
  • Genus: Balaenoptera
  • Species: Balaenoptera Borealis

3) Life Expectancy

The Sei whale is able to live for up to 65 years. There are reports of exceptional individuals up to 70 years old.

4) Average/Maximum Length

Sei whales are sexually dimorphic mammals with adult females being generally larger than adult males. Their average sizes are also region specific.

The species grows to an average length of about 14 to 20 meters (46 to 67 feet).

5) Average/ Maximum Weight

Adults can attain weights of up to 20 tons. However certain female individuals weighed up to about 30 tons.

6) Maximum Swimming Speed

The Sei are among the fastest cetaceans.

They can reach swimming speeds of 31 mph (50 km/h). Though that kind of speed is only sustainable over short distances.

7) Interaction With/Danger To Humans

The species poses no immediate danger to human beings.

Human beings, on the other hand, initially posed no danger to the species until the post-1880s depletion of blue and fin whales. From then on till around the 1980’s, whalers hunted Sei whales in great number to satisfy the demands for the whale’s meat and oil.

Technological advancements in the whaling industry at the start of the 20th century aided in maximum whaling practices. Sei whale meat was (and still is) also more preferred by exotic restaurateurs making it a more profitable venture for whalers.

Massive whaling of the species across all territorial waters led to rapid depletion of the whale’s numbers. Estimates show that between 1910 and 1980 hunters took at least 200,000 whales in all its territorial habitats.

Presently, there are approximately just 80,000 whales remaining. They are still periodically hunted, more for so-called research reasons than anything else.

 Whalers took over 200,000 Sei Whales during the commercial whaling era.

8) Reproduction Details

Both sexes within the species attain sexual maturity between the ages of 8 and 10 years.

Mating occurs in subtropical waters during the cold season of autumn and winter. The gestation period in the species ranges from between 11 and 12 months.

Females are only able to give birth to one calf per periodic birth. They also give birth only once every 2 to 3 years.

Calves measure about 4.5 meters at birth. Their mothers nurse them until they are ready for weaning usually at 9 months, measuring about 8 meters long. Interestingly, weaning is timed to takes place between summer and autumn when shoals of fish are in abundance.

9) Diet/Hunting Pattern Of The Sei Whale

Like other baleen whales, the species feed by filtering their food. They scoop large amounts of water containing food then skim the water out using their baleen plates hence trapping food behind in their mouths.

These are heavy eaters and an individual can eat an average of about 900 kg (1200 lbs.) of food per day. Their preferred prey include krill, Japanese flying squid, sardines, mackerels, various zooplankton, among other organisms.

10) Alternative Names

  • Pollack Whale
  • Japan Finner
  • Coalfish Whale
  • Rudolphi’s Rorqual
  • Sardine Whale

11) Population And Conservation Status

Due to high whaling rates in the pre-1980’s period, the species witnessed a swift decrease in its population. The most recent estimates have the number of individuals at between 80,000 and 90,000 individuals.

That’s just a third of its population prior to the 1910s.

The Southern hemisphere region witnessed the highest whaling rates with more than 120,000 members of the species being hunted. The North Atlantic region had the lowest number of hunts with 12,000 individuals recorded between 1920 and 1985.

This high whaling coupled with the low reproduction rate has led to the species being considered under threat. Therefore, the IUCN designates the Sei Whale as Endangered.

As from 1970, this whale fell under the protection of the International Whaling Commission (IWC).

Ban on commercial whaling of the species commenced in 1976 within the North Pacific waters. A total ban on commercial whaling in all waters remains in place since 1986.

 The latest population figures for the Sei Whale showed just a third of their pre-whaling population exists today.

12) Ancestry And History

The Sei whale is a reclusive whale whose bio-data is still not sufficient enough. In terms of body size, it is the fourth largest whale after the blue, fin, and humpback whales.

It’s also closely related to the Bryde whale, a species it is sometimes confused with.

The species is composed of two subspecies, the northern Sei whale (Balaenopteraborealis Borealis) and the southern Sei whale (Balaenoptera Borealis Schlegelii).

Though similar in almost all aspects, the two species are mainly defined by their habitat range.

13) Distribution and Habitat

Sei whales inhabit almost all territorial oceanic bodies except Polar regions and tropical regions. Their migratory patterns are controlled by thermal seasons. They are found in cold waters during warm seasons due to the abundance of food sources in such waters. During cold seasons they move to warmer tropical waters.

In the North Pacific region the whale has been sighted in the Gulf of California, the Sea of Japan and the larger Korean peninsula.

They are also common within the Gulf of Alaska.

In the North Atlantic regions, Sei whales are commonly sighted on the coasts of Iceland and Greenland. They are also found off the northern coast of Norway as well as the Denmark Strait. Though not common, sightings have been recorded in the Gulf of Mexico, the Mediterranean Sea and the Gulf of Antilles.

The southern hemisphere is not as predominantly inhabited as it once was.

Southern coastal regions of the Indian Ocean feature occasional sightings of the Sei whale. Southern Atlantic region such as Chilean coast accounts for more sightings than any other southern regions.

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