A Sei whale is a slender cetacean, although it is more robust than a fin whale. These rorquals are long and slender, and so are much more streamlined than other large whales. They have a compressed tail stock which abruptly joins the flukes. Their snout is pointed, and their pectoral fins are short. Their dorsal fin is shaped like a sickle. They typically have a dark steel gray body with irregular white markings on the underside. These plates have whitish fine inner bristles.
Sei whales live in all oceans and seas, except tropical and polar regions. They occupy sub-polar and temperate regions during the summer, migrating to sub-tropical waters in the winter. These whales occur in the open ocean and generally avoid coastal waters.
Sei whales gather in groups that number two to five individuals, although larger groups can form where food is very plentiful. They prefer to eat at dawn and may demonstrate unpredictable behavior during foraging and eating. They are fast swimmers, and are believed to migrate to warmer waters in lower latitudes during winter. Sei whales are one of the fastest cetaceans. However, they are not remarkable divers, descending only to shallow depths, remaining submerged for just five to ten minutes. Sei whales typically do not rise high above the water when they dive, usually only sinking below the surface. Their dorsal fin and blowholes are often visible above the surface of the water almost simultaneously. A Sei whale almost never lifts its flukes up above the surface, it is generally less active at the surface of the water and it rarely breaches.
Sei whales are polygynous, one male having exclusive mating rights with many females. Mating takes place during the winter. Females mate every 2-3 years and gestation lasts 11-13 months. Females typically give birth to one calf. Mothers nurse their calves and look after them until they are weaned at the age of 6 - 9 months. Sei whales are sexually mature when they are about 10 years old.
Sei whales are under threat by climate change and also by pollution, entanglement in fishing gear and ship strike.
According to the U.S. National Marine Fisheries Service, the total size of the Sei whale population is around 80,000 individuals. This species is classified as Endangered (EN) on the IUCN Red List.