Sei Whales (Balaenoptera borealis) are streamlined and relatively slender whales known for their fast swimming speeds. Sei Whales are among the largest whales, measuring up to 20 meters (66 feet) in length and weighing as much as 30 tonnes (66,000 pounds), yet they move with remarkable agility.

Sei Whales are baleen whales. They feed by skimming the water for plankton, krill, and small fish, using their finely fringed baleen plates to filter prey. Sei Whales are often seen alone or in small groups. Unlike some baleen whales, they rarely breach but can make sudden, powerful lunges when feeding.