The sei whale's streamlined shape makes it a very fast swimmer, reaching speeds of 50 km/h (26 mph).
Possibly two (Northern and Southern).
Up to 74 years.
Northern subspecies of sei whales can reach lengths of up to 18m and Southern subspecies can reach up to 21m in length. They weigh 20-30 tonnes.
Sei whales have a relatively slender body with a sickle-shaped dorsal fin. The body is dark grey with irregular white markings on the underside and they have a longitudinal ridge on the head.
They are found in all oceans and adjoining seas, except polar and subtropical regions. They occupy temperate and sub-polar regions in the summer.
Sei whales feed on krill, small fish and squid.
Mating occurs during the winter months and females typically give birth every year.
Sei whales are classified as Endangered by the 2000 Red List. They were heavily exploited, especially in the 1960s and 70s.