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What is a Sei Whale?

K. K. Lowen
K. K. Lowen

The sei whale is an aquatic mammal present in nearly all of the world’s oceans, preferring to live and travel in non-polar and non-tropical waters. Balaenoptera borealis, the formal name of the sei whale, is most often deep gray in color with a lighter gray area covering a portion of its underside and bottom jaw. The sei derives its name from the Norwegian term for pollock, a type of fish that congregates alongside the sei whale in coastal Norway. It is one of the fastest marine animals in the Cetacea order, which consists of whales and dolphins, yet the speedy and majestic whales appear on the endangered species list.

Rather than using teeth to catch prey, sei whales have an oral structure known as a baleen to aid in screening the digestible contents from a mass of seawater. People sometimes refer to baleen as "whalebone," though it is actually made of fibrous proteins. The sei whale feeds by consuming a large amount of water and utilizing a series of baleen plates to filter out edible contents, such as plankton and small types of fish. Although sei whales typically travel alone or in small groups, they occasionally assemble in large groups to feed in areas with plentiful resources.


The sei whale is the third largest rorqual whale in existence. Rorqual is another name for the group of whales that belong to the Balaenoptera genus, the largest group of baleen whales. One common trait of all rorquals is vertical throat grooves that expand, allowing the animals to swallow enormous amounts of water. The two largest rorquals are blue and fin whales, respectively.

The sei whale’s mating season takes place during the winter months in warm waters. After an estimated ten to twelve month gestation period, females generally produce a single offspring, referred to as a calf. They may give birth to more than one calf at a time, but it is a rare occurrence.

The calves stay close to the mothers for no more than nine months before ceasing their maternal feeding. The young whale is usually sexually mature by the time it is 10 years old, but it usually does not achieve its full size and weight until it is approximately 25 years old. The animals may live for up to 65 years.

The sei whale appears on the endangered species list because, beginning in the 19th century, it was subject to hunting by the whaling industry. Heavy whaling led to the creature’s drastic population decrease. In the 1970s, sei whales began acquiring various protection statuses and legal hunting of the creatures ceased, but illegal hunting may continue, despite international conservation efforts.

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