The sei whale (pronounced “say”) is the third-largest animal on Earth, reaching lengths of 40 to 60 feet and weighing up to 100,000 pounds. It belongs to the rorqual family of baleen whales, the same group that includes blue whales and humpbacks, and it holds the title of fastest-swimming large whale, with recorded top speeds of 34.5 mph. Despite its size and speed, the sei whale is one of the least familiar of the great whales, partly because it prefers deep, open ocean waters far from coastlines.
Size and Appearance
Sei whales have a long, sleek body that is dark bluish-gray to black on top and white or cream-colored underneath. Their skin is often marked with oval-shaped scars, likely left by cookie-cutter sharks and lampreys, along with subtle mottled patches of discolored skin. A tall, hooked dorsal fin sits about two-thirds of the way down the back, which is one of the most reliable ways to spot one at sea.
Instead of teeth, sei whales filter food through 219 to 410 baleen plates, stiff structures made of keratin (the same protein in your fingernails). These plates are dark with fine gray-white fringes on the inner edge. Running along the underside of the throat are 30 to 65 accordion-like grooves that expand when the whale takes in water during feeding. When a sei whale surfaces to breathe, its blow shoots up in a tall, bushy column roughly 10 to 13 feet high.
How to Tell It Apart From Similar Whales
Sei whales are commonly confused with two close relatives: fin whales and Bryde’s whales. Fin whales are noticeably larger, often exceeding 70 feet, and have an asymmetrical jaw coloring (white on the right side, dark on the left) that sei whales lack. Bryde’s whales are closer in size but have three distinct ridges running along the top of their head, while the sei whale has just one central ridge. The sei whale’s dorsal fin is also proportionally taller and more upright than a Bryde’s whale’s, and it tends to surface more smoothly, rarely arching its tail fluke above the water before a dive.
A Unique Feeding Strategy
Most rorqual whales feed by lunging, opening their enormous mouths and engulfing huge volumes of water and prey in a single gulp. Sei whales can lunge-feed too, but they’re unusual because they also skim-feed, a technique more commonly associated with right whales. During skim feeding, the whale swims forward at a steady pace with its mouth partially open, generating continuous suction that draws water and tiny organisms across the baleen plates. This filters out the food without the explosive energy cost of a full lunge.
This dual strategy gives sei whales dietary flexibility. They skim-feed when targeting smaller prey like copepods (tiny crustaceans barely visible to the naked eye) and switch to lunge feeding when they encounter denser patches of krill or small schooling fish. Research into skull shape has shown that the sei whale’s moderately wide, straight snout is a physical adaptation for this multi-prey approach, distinct from the narrow, curved snout of dedicated skim feeders like right whales and the broad, flat snout of pure lunge feeders like blue whales.
During summer, sei whales feed on copepods, krill, and small pelagic fish across their range. This flexible diet means they can exploit whatever food source is most abundant in a given area, which may partly explain their wide geographic distribution.
Where Sei Whales Live and Migrate
Sei whales are found in every major ocean basin, from subarctic waters to the subtropics, though they generally avoid both polar ice and the warmest tropical zones. They spend summer months at higher latitudes, feeding in productive, nutrient-rich waters, then migrate toward warmer, lower-latitude waters in winter to breed.
Satellite tracking in the western-central North Pacific has mapped this cycle in detail. Whales leave their feeding grounds between November and December, traveling southward from waters off Japan to a region west of Hawaii. Their paths converge in the waters around the Marshall Islands and north of Micronesia, between roughly 7°N and 20°N latitude, which appear to serve as breeding grounds. After a relatively brief stay, they head north again from January to February, reaching about 30°N by March and arriving back at feeding grounds north of 35°N by May or early June. Notably, none of the tracked whales entered the Bering Sea or the Sea of Okhotsk, suggesting these enclosed northern seas are not part of their range.
In the North Atlantic, sei whales follow a different corridor, feeding in the Labrador Sea during summer and wintering off northwestern Africa, passing through the waters around the Azores. Southern Hemisphere populations have been documented moving between feeding areas off Brazil and breeding grounds near the Falkland Islands.
Speed and Movement
Sei whales are considered the fastest of all large cetaceans. Their top recorded speed of 34.5 mph (about 55.5 km/h) outpaces even fin whales, which were once nicknamed “the greyhounds of the sea.” This speed historically made sei whales difficult for early whalers to catch, though that changed once modern steam-powered and diesel vessels entered the industry. In everyday travel, sei whales cruise at far slower speeds, but their bursts of acceleration help them chase schooling fish and evade threats.
Conservation Status
Sei whales are classified as endangered by both the IUCN Red List and the U.S. Endangered Species Act. The current global population is estimated at roughly 80,000, a fraction of pre-whaling numbers. Commercial whaling in the 20th century hit sei whale populations especially hard. As whalers depleted larger, more profitable species like blue and fin whales, they turned increasingly to sei whales, killing tens of thousands in the 1960s and 1970s alone.
International protections have been in place since the 1980s moratorium on commercial whaling, and some populations show signs of slow recovery. Today, the main threats include ship strikes, entanglement in fishing gear, ocean noise pollution, and climate-driven shifts in prey availability. Because sei whales depend on dense concentrations of small organisms, even subtle changes in ocean temperature and plankton distribution can disrupt their feeding success across an entire season.