Sei whales are the least known of the rorqual family, and at sea they may be confused with other rorqual whales, including the fin whale and the Bryde's whale. It has also been known to mate with the fin whale and produce at least first generation young. The names ‘pollack whale', ‘coalfish whale', and ‘sardine whale' arose from the fact that the sei whale generally showed up at the same time as those species. In fact the name "sei" comes from the Norwegian word for pollack, "seje".
Unlike the Bryde's whale with three ridges on the top of its head, the sei whale has a single ridge that runs from the tip of the snout to its two blowholes. It also lacks the asymmetrical white lower jaw found in fin whales. It skims the water for food while swimming and has short ventral pleats and fine baleen with densely packed bristles which increase the efficiency of feeding. The sei has a sickle-shaped dorsal fin. The tail fluke is rarely seen above the surface, though during shorter, shallower dives, the 'fluke prints' it leaves in the water enable it to be followed. Colouration of the sei whale ranges from greyish blue to dark grey to brownish in some individuals.
The sei whale does not gulp-feed like the other whales in its family and while swimming, the dorsal fin and back remain visible for longer periods of time than with the other large whales. The sei will breach occasionally, and at a low angle, after which it flops onto its belly. Sei whales are very fast. When the animal is swimming the blowholes and dorsal fin are visible at the same time. Sei whales are found in nearly all waters of the world, including the sub-Arctic and sub-Antarctic where they go for summer feeding, but not in the Mediterranean or Baltic seas. They are usually seen in small groups, although when food is plentiful many more may be seen. They are thought to migrate into warmer, lower latitudes for the winter. They are most common in the southern hemisphere. Northern hemisphere and southern hemisphere populations do not mix. Sei whales may be seen close to island shores, but are rarely seen inshore elsewhere.
Sei whale populations were severely depleted by the commercial whaling industry. They are classified as Endangered (IUCN 2008). As with other large whales, they are threatened by noise and chemical pollution, vessel strikes, global warming, sonar and entanglement in fishing gear.