The Sei Whale is identified by a dorsal fin, 38 to 56 ventral grooves (throat grooves that allow their throat to expand during the hug intake of water during filter feeding). Sei whales are also baleen whales, meaning that instead of the common mammal tooth form, baleen whales have plates for filtering foods in and water out, specifically sei whales have two rows of 300 to 380 baleen plates. This whale is known to be 14 to 20 m long and weighs about 20 tons – females are generally 1-2 m longer than males. The sei whale is dark gray or bluish grey on the back and sides with a greyish white area on the ventral grooves of the lower jaw and underbelly.
The sei whale is one of the fastest cetaceans, reaching speeds of up to 50 km/hour. Sei whales have a more regular dive sequence than most others and is known to stay near the surface more consistently. The sei whale normally blows once every 40 to 60 seconds, for about 1 to 4 minutes, and then can dive for 5 to 20 minutes. During the shorter dives, the sei whale rarely descends deeper than a few feet, so its progress can be followed by “fluke prints” or swirls left by the beat of the tail just below the surface. The sei whale seldom breaches, however the dorsal fin and back remain visible for longer periods of time than with other large whales.
Sei whale groups mostly consist of 2-5 individuals, however thousands may aggregate where plenty of food is available. Like the other baleen whales, sei whales feed by skimming and swallowing surface plankton, mainly copepods (tiny marine crustaceans) but also euphausiids (krill, shrimp like crustaceans).
Mating season ranges from November to February in the Northern Hemisphere and from May to July in the Southern Hemisphere. Females generally give birth to a single calf every other year in winter, after a gestation period of 10.5 to 12 months. Although little is known about their breeding habits, some data indicate that sei whale migration is loosely organized around sex, age, or reproductive function. This presumably relates to mating strategies, but at this time nothing is known of their mating habits or calving grounds.
The sei whale species was intensively exploited worldwide after blue and fin whale stocks have been reduced. In 1980, it was estimated that the Southern Hemisphere population had been reduced to around 24,000 from an initial level of 100,000 or so. In the North Pacific there was a decline from 42,000 in 1963 to just 8,600 in 1974, while figures in the North Atlantic are the most uncertain, although some surveys have suggested a figure around 10,000. Since 1985, the International Whaling Commission has officially halted all commercial whaling of this species. However, today 50 sei whales are killed annually by the Japanese whalers in the North Pacific in Japan’s “scientific whaling” program.
Sei whales inhabit all ocean and adjoining seas except in polar regions, feeding in cold water during the summer and migrating to warm tropical and subtropical waters during the winter. In the western North Pacific, sei whales are most common in the south-west Bering Sea to the Gulf of Alaska, and offshore in a broad arc between about 40 degrees North and 55 degrees North across the Pacific.
In the North Atlantic, sei whales can be found from the coast of Labrador, and along the coasts of Greenland and Iceland. In the East Atlantic, sei whales migrate north to northern waters off Norway, Shetland, Orkney and the Faeroe Islands an occasionally, Svalbard. Sei whales are also present in the Denmark strait
Sei Whale Feeding Frenzy video: