Share | facebook twitter instagram flickr flickr

Did you know?

Ospreys are an indicator species. The health of their population has implications for the health our coastal ecosystems.

Image of Instagram logo


New Jersey Endangered and Threatened Species Field Guide

Sei whale

Balaenoptera borealis

Species Group: Mammal

Conservation Status

State: Endangered

Federal: Endangered



The sei whale is a large baleen whale. Reaching lengths of up to 62 feet, the sei whale's dark steel gray body frequently looks galvanized. As with other baleen whales, the female is larger than the male. The body is frequently covered with small pit-like circular scars, originating in the bites of cookie-cutter sharks. Underneath, the belly is grayish white around the ventral grooves, which stretch only midway between the base of the flippers and the navel. The right lower lip is gray and baleen plates are primarily grayish black. The whale's snout is barely arched, while its slightly pointed rostrum (the forward extension of the upper jaw) sports a single median dorsal ridge. Two-thirds of the body length behind the head, a tall dorsal fin is very falcate, or curved like a sickle. The leading edges of the flukes are occasionally white.

Instead of teeth, it has great plates of horny baleen which extend from the upper jaw. These are used to strain food from large mouthfuls of water. It has two blowholes and the blow is either bushy or columnar and is usually quite tall (up to 10 feet high). Unlike the blue and fin whales, when the sei whale surfaces, the blowholes often appear at the same time as the dorsal fin, rather than before. They also do not usually arch their back or raise their flukes when diving.

Image of Range of the Sei whale off of New Jersey's coast.Zoom+ Range of the Sei whale off of New Jersey's coast.

Distribution and Habitat

Sei whales live within the Atlantic, Indian, and Pacific Oceans, primarily in subtropical, temperate and subpolar waters. Their distribution and migratory patterns are not well known. They are usually observed in deeper waters, far from most coastlines. They are not usually encountered within New Jersey’s coastal waters. According to the New Jersey Endangered and Nongame Species Program, there are no sightings currently documented within New Jersey waters for this species.


Sei whales feed on tiny crustaceans known as krill as well as small schooling fish such as herring, capelin, and sand lance. They also feed on squid. They feed by filter feeding with their mouthful of baleen. They do so by taking a large mouthful of both prey and water, closing their mouth, and then pushing the water out of their mouth using their enormous tongue. The prey items are then left within the mouth, trapped by the strips of baleen, and ready to be swallowed. The sei whale eats an average of 2,000 pounds of food per day. They may often be seen feeding within groups of 2-5 individuals. In the North Atlantic Ocean, they will also be seen feeding near other species of whale, such as the humpback, and dolphins.

Life Cycle

Little is known about the sei whale life cycle and behavior. Like other baleen whales, it is likely that long-term bonds are rare. Females will give birth once every 2-3 years beginning when they are 10 years old. Most calves are born in the winter and grow quickly feeding on the milk of their mother. They will nurse on their mother’s milk for 6-9 months. Males play no role in raising their young.

Sei whales are one of the fastest swimming species of whale. They can reach speeds of 34.5 miles/hour. Their only predator, aside from humans, is the killer whale (Orchinus orca). Sei whales may live to at least the age of 50 years if not longer.

Current Threats, Status, and Conservation

All of the large whale species have been at risk of extinction due to a long history of whaling. The principal attraction of whaling was the whale's blubber, which yielded oil ideal for lamp oil and, much later, in the production of margarine. Baleen was also of value. Whalebones were also used in the manufacture of glue, gelatin and manure. Besides being eaten by humans, the meat has also been used in dog food and, when dried and crushed, cattle feed.

Commercial whaling of sei whales ended in 1987 due to the decline of the species. It was listed by the federal government as endangered in 1970 and, as a result of that federal status, was automatically added to the New Jersey endangered species list following enactment of the New Jersey Endangered and Nongame Species Conservation Act in 1973. Sei whales are provided with additional protection with the Marine Mammal Protection Act of 1972. It is unknown whether sei whales are recovering throughout much of their range. Ship strikes and entanglements may be slowing their recovery. The global population is currently thought to be approximately 80,000.

Sei whales face a number of threats, all of which are caused by humans. These threats include entanglement in fishing gear, collisions with ships, and habitat impacts including noise pollution.

One of the greatest threats to sei whales is entanglement in fishing gear. Sei whales may become wrapped-up in nets and/or fishing line around their tail, mouth, or other body part. Discarded nets and lines may float at sea for decades or become snagged on rocks or debris at the ocean bottom. Once entangled in fishing gear, the whale may face an agonizing death, pulling the gear along while it swims for many days, months, or even years, as the gear slowly cuts through their body and causes swimming to become more difficult. Scars on whale bodies are often an indication of a previous entanglement from which they escaped.

Collisions with ships are an increasing threat to sei whales. The increasing number of large and fast ships results in whales and ships being in close proximity more often. Unfortunately, whales do not always know or have time to react to the approach of large ships and they get hit, usually resulting in their death.

Negative impacts to whale habitat may take the form of development, pollution, noise, overfishing, and climate change. Shipping channels, aquaculture, offshore energy development, and recreational use of marine areas may destroy whale habitat or displace whales which would normally use the area. Oil spills and other chemical pollutants are also a threat to whales and the prey which they feed on.

Another form of pollution is noise pollution. Whales’ primary means of communication, navigation, locating food, locating mates, and avoiding predators and other threats is through their sense of hearing, which is much more highly developed than that of humans. Noise pollution created by ship traffic or offshore construction may negatively impact whales by disrupting otherwise normal behaviors associated with migration, feeding, alluding predators, rest, breeding, etc. Any changes to these behaviors may decrease survival, simply by increasing efforts directed at avoidance of the noise and the perceived threat. Active sonar, such as that used by the Navy, also threatens marine mammals by disrupting navigation, foraging and communication abilities. There have been instances of whale stranding and death caused by acoustic trauma. This may be due to a fatal injury within the structure of the ear, or may result from the distressed animal surfacing too rapidly and developing nitrogen bubbles within their blood (decompression sickness). In addition to the direct threat posed by active sonar, it may indirectly harm marine species by causing changes in behavior.

Overfishing of the small fish and squid which sei whales feed upon is another potential cause for concern as well as the potential impacts of global climate change. This issue may be the greatest long-term threat to the marine habitat and its species. Climate change may significantly alter the chemical balance of the seas, off-shore currents, and plankton distribution and abundance, thereby affecting migration routes of marine species and impacting the entire food web.

Although sei whales are large animals, we don’t currently know a great deal about their habitat use off the coast of New Jersey. It is likely, in fact, that they may not occur within New Jersey’s waters which only extend 3 nautical miles off the coastline. It is likely, however, that they do occur within the ocean waters beyond 3-nautical miles. It is unknown whether they may be using those offshore waters as a migratory pathway between their summer feeding grounds in the north and their winter breeding grounds in the south. Surveys are currently being conducted off the New Jersey coastline in order to determine where whales are located, how many individuals are there, and during what time of year. Based on these findings, further knowledge regarding their habitat use in New Jersey waters may be gained and attention may then focus on protecting important habitats.

Text derived from the book, Endangered and Threatened Wildlife of New Jersey. 2003. Originally edited by Bruce E. Beans and Larry Niles. Edited and updated Michael J. Davenport in 2010.

Scientific Classification

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

Find Related Info: Endangered, Mammals

Report a sighting

Image of Red knot.

Report a sighting of a banded shorebird or rare species.


Become a Member

Join Conserve Wildlife Foundation today and help us protect rare and imperiled wildlife for the future.


Wildlife Photographers

Join our Endangered Wildlife of New Jersey group on

Image of Flickr logo


Download the complete list of New Jersey's Endangered, Threatened, & Special Concern species.