This fast cetacean inhabits all oceans and adjoining seas except in tropical and polar regions. The sei whale became a major target for commercial whaling after the preferred stocks of blue and fin whales had been depleted.
Today, although commercial whaling has been officially halted, the species is subject to "scientific whaling" by Japan. It also remains vulnerable to pollution, shipping strikes and entanglement in fishing gear.
Recently a number of South Pacific countries, including five island nations, have declared their EEZs (Exclusive Economic Zone) to be whale sanctuaries in which commercial whaling is prohibited and where additional research on large whales is encouraged.
This species is identified by a dorsal fin, 38 to 56 ventral grooves, and two rows of 300-380 baleen plates. The sei whale is one of the fastest cetaceans, reaching speeds of up to 50 km per hour. A sei whale marked in the Antarctic and killed by whalers 10 days later had traveled more than 4,000 km (2,200 miles) during that period.
It is not usually a deep diver and periods of submergence generally last 5-10 minutes. To feed, the sei whale swims through swarms of prey, twisting on its side and also uses the skimming method of filtering food with its baleens. Although the species does not use echolocation to search for prey, it has been heard to emit a sonic burst of 7-10 pulses.
Like other great whales, the sei whale prefers to spend the summer feeding in the cooler northern waters before migrating south to warmer waters to breed and calve.
This whale is 14 to 20 m long and weighs about 20 tons. Females are about 1-2 m longer than males.
The species is dark grey or bluish-grey on the back and sides, with a greyish white area on the ventral grooves of the lower jaw and underbelly. Its similarity to the Bryde's whale caused much confusion among whalers in the 19th Century, who often reported sei kills as Bryde's, or either as fin whales.