Sei Whale

About the Species

Sei whales occur in subtropical, temperate, and subpolar waters around the world. Often found with pollack in Norway, the name "sei" comes from the Norwegian word for pollack, "seje."

The sei whale population has been greatly decreased by commercial whaling. During the 19th and 20th centuries, sei whales were targeted and greatly depleted by commercial hunting and whaling, with an estimated 300,000 animals killed for their meat and oil.

Commercial whaling ended for this species in 1980. Although whaling is no longer a major threat to this species, some scientific whaling continues today in Iceland and Japan. Vessel strikes and entanglement pose the biggest threat to sei whales today. The sei whale is listed as endangered under the Endangered Species Act and depleted under the Marine Mammal Protection Act.

NOAA Fisheries and our partners are dedicated to conserving and rebuilding the sei whale population. We use a variety of innovative techniques to study, protect, and rescue these endangered whales. We engage our partners as we develop regulations and management plans that foster healthy fisheries and reduce the risk of entanglements, create whale-safe shipping practices, and reduce ocean noise.

Status

Today, there are around 8,600 sei whales in the North Pacific. This is only little more than 20 percent of the original population estimate of 42,000 for this area.

The total population of sei whales in all U.S. waters is unknown.

The most recent population assessments can be found on the sei whale stock assessment reports.

Appearance

Sei whales have a long, sleek body that is dark bluish-gray to black in color and white or cream-colored on the underside. The body is often covered in oval-shaped scars (probably caused from cookie-cutter shark and lamprey bites) and sometimes has subtle "mottling," or discolored spots or blotches.

Sei whales have a tall, hooked dorsal fin located about two-thirds down their back. Sei whales have 219 to 410 baleen plates (long, finger-nail like plates instead of teeth) that are dark in color with gray/white fine inner fringes in their enormous mouths. They also have 30-65 relatively short accordion-like creases, or throat grooves, that extend from below the mouth to the naval area. The number of throat grooves and baleen plates may differ depending on geographic population.

At the water's surface, sei whales can be sighted by a columnar or bushy blow that is about 10 to 13 feet in height. The dorsal fin usually appears at the same time as the blowhole when the animal surfaces to breathe.

Behavior and Diet

Sei whales are usually observed alone or in small groups of two to five animals. They are fast swimmers that can reach speeds of over 34 miles per hour.

Sei whales dive differently than most whales. They do not arch their backs or show their flukes before diving; they simply sink below the surface. They often leave “fluke prints”—smooth circles on the surface created by the movement of the fluke underwater.

An average sei whale eats about 2,000 pounds of food per day. They can dive 5 to 20 minutes to feed on plankton (including copepods and krill), small schooling fish, and cephalopods (including squid) by both gulping and skimming. They prefer to feed at dawn and may exhibit unpredictable behavior while foraging and feeding on prey.

Where They Live

Sei whales have a wide distribution and live in subtropical, temperate, and subpolar waters around the world. They prefer temperate waters in the mid-latitudes, and can be found in the Atlantic, Indian, and Pacific Oceans. During the summer, they are commonly found in the Gulf of Maine, and on Georges Bank and Stellwagen Bank off the U.S. coast in the western North Atlantic. The movement patterns of sei whales are not well known, but they are typically observed in deeper waters far from the coastline. Sei whales have an unpredictable distribution. Many whales may be found in one area for a period and then not return for years or decades. This behavior is unusual for large whales, who generally have a predictable distribution. No one knows where sei whales breed.

Lifespan & Reproduction

Sei whales become sexually mature at 6 to 12 years of age when they reach about 45 feet in length. They generally mate and give birth during the winter in lower latitudes.

Females breed every 2 to 3 years, with a gestation period of 11 to 13 months. Calves are about 15 feet long and weigh about 1,500 pounds at birth. Mothers burse their calves for 6 to 9 months before weaning them when at their preferred feeding grounds.

Threats

Vessel Strikes

Inadvertent vessel strikes can injure or kill sei whales. The projected increase in ship traffic arising from the opening of trans-polar shipping routes (as arctic sea ice melts) will increase the risk of vessel strikes as well as ambient noise and pollution.

Entanglement

One of the main threats to sei whales is getting caught in fishing gear. They can become entangled gear including traps, pots, and gillnets. Once entangled, whales may swim for long distances with gear attached, resulting in fatigue, compromised feeding ability, or severe injury. These conditions can lead to reduced reproductive success and death.

Learn more about entanglement

Ocean Noise

Underwater noise threatens whale populations, interrupting their normal behavior and driving them away from areas important to their survival. Increasing evidence suggests that exposure to intense underwater sound in some settings may cause some whales to strand and ultimately die.

Scientific Classification

Kingdom	Animalia
Phylum	Chordata
Class	Mammalia
Order	Cetacea
Family	Balaenopteridae
Genus	Balaenoptera
Species	borealis