Sei whales (Scientific name: Balaenoptera borealis) are the third-largest rorqual after the blue whale and the fin whale and the sixth longest overall after sperm whales, right whales and bowhead whales. Sei whales and humpback are about the same length (13 meters, 52 feet) but humpack whales are heavier. Sei whales are also very fast. Their name is derived from the Norwegian word for pollack (seje) because they are found in the same waters with these fish. Their lifespan is thought to 50 to 70 years.
Sei whales (pronounced SAY) are pelagic (open ocean) whales usually found far from shore but are different seen in coastal areas. They are are found in all oceans, including the the Indian Ocean, Atlantic Ocean and Pacific Ocean, and adjoining seas, except polar and tropical regions. They live temperate and subpolar regions in the summer, but migrate to sub-tropical waters during the winter. [Source: Nancy Shefferly, Animal Diversity Web (ADW) /=]
Sei whales prefer temperate waters in the mid-latitudes.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.
There are an estimated 65,000 sei whales today, including 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. 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. [Source: NOAA]
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures
Sei Whale Physical Characteristics
Sei whale are 12.2 to 18.2 (40 to 60 feet) in length and weigh up to 50 US tons (45,360 kilograms, 45.7 tonnes, 100,000 pounds) Few exceed 15.2 meters (50 feet) in length. Of this length, the head and body make up about 13 meters. Males are slightly smaller than females. [Source: Nancy Shefferly, Animal Diversity Web (ADW) /=\, NOAA]
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 compressed tail stock that abruptly joins the flukes. The snout is pointed, and the pectoral fins are short. The dorsal fin is sickle shaped and ranges in height from 25 to 61 centimeters (10 inches to two feet).
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. These expand when the whales are feeding. 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 three to four meters (10 to 13 feet) in height. The dorsal fin usually appears at the same time as the blowhole when the animal surfaces to breathe.
Sei Whale Behavior, Swimming and Diving
Sei whales are motile (move around as opposed to being stationary), migratory (make seasonal movements between regions, such as between breeding and wintering grounds), and social (associates with others of its species; forms social groups). They are usually observed alone or in small groups of two to five animals. They sense using touch and chemicals usually detected with smell. [Source: NOAA; Nancy Shefferly, Animal Diversity Web (ADW) /=]
Sei whales are fast swimmers that can reach speeds of over 55 kilometers per hour (34 miles per hour). They are among the fastest cetaceans, Although their speed is remarkable they are not great divers. They dive only to shallow depths, and they remain submerged only five to ten minutes at a time. 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.
Little is known about the actual social system of sei whales. Groups of two to five individuals are typically observed, but sometimes thousands may gather if food is abundant. However, these large aggregations may not be dependent on food supply alone, as they often occur during times of migration. Norwegian workers call the times of great Sei whale abundance "invasion years."
Sei Whale Food and Eating Behavior
An average sei whale eats about 900 kilograms (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. [Source: NOAA]
The Sei whale mainly obtain food by skimming through the water and catching prey in its baleen plates. They whales feed near the surface swimming on their sides through swarms of prey, using their speed to engulf prey before the prey can move away.[Source: Nancy Shefferly, Animal Diversity Web (ADW) /=]
In the North Atlantic, they feeds primarily on calanoid copepods, specifically Calanus finmarchicus, with a secondary preference for krill (euphausiids). In the North Pacific, they feed on similar zooplankton, including the copepod species Neocalanus cristatus and N. plumchrus, and Calanus pacificus, and euphausiid species Euphausia pacifica and E. similis. In addition, they eats larger organisms, such as the Japanese flying squid, and small fish, including anchovies, sardines, Pacific saury, mackerel, jack mackerel and juvenile rockfish. Some of these fish are commercially important. Off central California, they mainly feed on anchovies between June and August, and on krill during September and October. In the Southern Hemisphere, prey species include the copepods such as Neocalanus tonsus and Calanus simillimus and krill such as Euphausia superba and Euphausia vallentini and the pelagic amphipod Themisto gaudichaudii. [Source: Wikipedia]
Sei Whale Sounds
The sei whale makes long, loud, low-frequency sounds. Relatively little is known about specific calls or what they mean. In 2003, observers noted sei whale sounds described as "growls" and "whooshes" off the coast of the Antarctic Peninsula. Many of these sound consisted of multiple parts at different frequencies, which distinguishes them from those of other whales. Most calls lasted about a half second, and occurred in the 240–625 hertz range, well within the range of human hearing. The maximum volume of the vocal sequences is reported as 156 decibels — the equivalent of jackhammer operating two meters away. [Source: Wikipedia]
In November 2002, scientists recorded calls in the presence of sei whales off Maui. All the calls were downswept tonal calls, all but two ranging from a mean high frequency of 39.1 Hz down to 21 Hz of 1.3 second duration. These calls closely resembled and coincided with calls previously been attributed to fin whales. Between 2005 and 2007, low frequency downswept vocalizations were recorded in the Great South Channel, east of Cape Cod, Massachusetts, which were only significantly associated with the presence of sei whales. These calls averaged 82.3 Hz down to 34 Hz over about 1.4 seconds in duration.
Roddy Morrison, a former whaler active in South Georgia, told the BBC: "When we killed the sei whales, they used to make a noise, like a crying noise. They seemed so friendly, and they'd come round and they'd make a noise, and when you hit them, they cried really. I didn't think it was really nice to do that. Everybody talked about it at the time I suppose, but it was money. At the end of the day that's what counted at the time. That's what we were there for."
Endangered Sei Whales
Sei whales are listed as “Endangered” on International Union for Conservation of Nature (IUCN) Red List and the Endangered Species Act (ESA, U.S. Fish and Wildlife). The Convention on International Trade in Endangered Species (CITES) lists them in Appendix I, which lists species that are the most endangered among CITES-listed animals and plants, from the equator to Antarctica. All other populations are listed as Convention on International Trade in Endangered Species (CITES) Appendix II, which lists species not necessarily threatened with extinction now but may become so unless trade is closely controlled.. The Marine Mammal Protection Act (MMPA) describes sei whales as depleted throughout their range. One distinct population segment is listed under Specially Protected Areas and Wildlife (SPAW) Annex II
The sei whale population was 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. [Source: NOAA]
The global population of these whales is estimated at only 57,000. Hunting of these whales by humans has been high since the 1950s. The take of these animals peaked in the 1964-65 season, when 25,454 of these whales were taken. The reported global catch of Sei whales in the 1978-79 season was only 150, showing the dramatic drop in whale populations. Some researchers have concluded that Sei whale populations are rising as a result of decreases in blue and fin whale populations. However, data to back this up is scarce. [Source: Nancy Shefferly, Animal Diversity Web (ADW) ]
Threats to Sei Whales
Threats to sei whales include vessel strikes, entanglement in fishing gear and ocean noise.
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. [Source: NOAA]
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.
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.
Image Sources: 1) NOAA 2) Wikimedia Commons; 3) Institute of Cetacean Research
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated May 2023