Sei whales are the third-largest rorqual whales after the Blue and Fin whales. They are found in the deep offshore waters of most of the worlds oceans. Their annual migrations take them from their summer feeding grounds in cool subpolar waters to their winter breeding grounds in temperate, subtropical waters.
Like the Fin Whales Sei whales are big and fast reaching almost twenty metres in length. Their name comes from the Norwegian word for the Pollock fish which appear off the Norwegian coast at the same time of year as the whales do.
Common Name: Sei Whale
Other Names: Coalfish Whale, Pollack Whale, Japan Finner, Rudophi’s Rorqual
Scientific Name: Balaenoptera borealis
Conservation Status: Vulnerable/Endangered
Length: New-born calf 4.5 to 5 metres, Adults females up to 21 metres, Males up to 17.7 metres.
Weight: Birth weight is under 1 ton. Adults 45+ tonnes.
Gestation: 11 months (possibly as long as 13 months)
Weaning Age: Approximately 6 to 9 months / 8 to 9 metres in length
Calving Interval: 2-3 years
Sexual Maturity Age: 8 to 12 years / 8 to 9 metres in length
Physical Maturity Age:
Lifespan: 65 years
Mating Season: Spring and Summer
Calving Season: Summer and Autumn
Cruising Speed: Up to 50 kilometres per hour for short bursts
Protected Since:
Field Identification
– Dark grey in colour with white undersides.
– Low blow of about 3 meters height.
– Tall sickle shaped fin on their backs.
– Longitudinal ridge on head.
– Blowholes and fin visible at the same time.
– Rarely shows its peck fins.
– Does not arch its tail nor lift its tail out of the water when diving.
– Both sides of its head evenly dark grey.
– Often swims close to the surface.
Taxonomy
Sei whales are the third-largest rorqual whales after the Blue and Fin whales. Norwegians called the rorquals whales the “furrow whales” (røyrkval) because there longitudinal pleats or throat grooves.
The Sei Whale was first described by naturalist Karl Rudolphi in 1819 when he examined a whale that stranded near Grömitz, in Schleswig-Holstein (Germany) who thought it was a Minke Whale. Georges Cuvier described and called Rudolphi’s specimen “rorqual du Nord” in 1823. Later in 1828 Rene Lesson took Cuvier’s description of Rudolphi’s specimen as well as descriptions of another stranding in France (which later turned out to be a Fin Whale) and gave it the name Balaenoptera borealis.
There are now considered to be two subspecies of Sei whales, the northern hemisphere Sei Whale (Balaenoptera borealis borealis) and the southern hemisphere Sei Whale (Balaenoptera borealis schlegelli). Their ranges do not overlap.
Description
Sei whales are one of the largest whale species after the Blue and Fin Whales. They have sleek, streamlined bodies with a thin (laterally compressed) tail. Females are larger than the males and Southern Hemisphere Sei Whales are larger than the ones found in the Northern Hemisphere with males reaching lengths of nearly 18 metres and females up to 21 metres.
Sei whales are a bluish-grey to dark grey in colour and white or cream-coloured on the underside. They are often covered in scars that are thought to be healed ‘Cookie-cutter’ Shark or Lamprey bites that give them a subtle “mottling,” of discoloured spots and blotches. They have between 30 to 65 relatively short throat grooves, that extend from below the mouth to the naval area. About two-thirds of the way down their backs they have a distinct, tall and hooked dorsal fin.
In their mouths Sei Whales have 219 to 410 baleen plates that are dark in colour with grey/white fine inner fringes.
Sei whales are known to make a series of “growly” type calls.
Distribution and Movement Patterns
Sei whales are found in all the worlds oceans and are considered to be a cosmopolitan species. They migrate between polar, temperate and tropical waters following well defined north south migration routes but are rarely in polar or tropical waters.
Sei Whales are similar in appearance to Bryde’s Whales and even occasionally confused with Fin Whales so there is some confusion about their exact range, distribution and population size.
It has also been noted that Sei Whales distribution is unpredictable as they have been sighted in a location for a period of time then not seen there again for many years. This is behaviour is unusual for large whales species, who generally have predictable distribution patterns.
Australia – At this time there is insufficient data on the Sei Whale migration habits and patterns. It is believed that they follow the same general pattern of other baleen whales of spending the summer months in the Southern Ocean feeding grounds and then moving north during the winter months to breed. Timing may be a little later than other species as they do not travel to the high latitudes to feed.
The Australian Sei Whales winter breeding grounds are unknown. Whaling data suggests that there is a pronounced segregation of the sexes during migration with the pregnant females arriving and leaving the feeding grounds earlier than the males.
Summer feeding grounds are in the cool subantarctic waters of the Southern Ocean along the Antarctic convergences where there are concentrations of Krill. They do not travel as deeply south into the polar waters as Blue and Fin Whales. Opportunistic feeding has been reported off Albany in Western Australia as well as along the continental shelf in the Bonney Upwelling off Victoria and South Australia.
Another significant problem with any study of the Sei Whales is that they are often mistaken for Bryde’s Whales as they are very similar in appearance. Many whales identified by whalers as Sei Whales turned out to be Bryde’s. So correctly identifying species, their distribution and numbers, particularly in warmer waters (>20 °C) where Bryde’s whales are more common has been difficult.
Global – In the rest of the Southern Hemisphere Sei Whales are usually found between 40°S and 50°S latitude in the South Atlantic and southern Indian Oceans and 45°S and 60°S in the South Pacific during the summer months. They have been sighted along the Chilean coast with a possible feeding ground in the Aysen region.
It has also been found that many Sei Whales caught by whalers were actually misidentified Bryde’s Whales, so their winter breeding grounds and distribution is poorly understood.
In the Northern Hemisphere Sei Whales are found mostly in the eastern North Pacific off the US states of California, Washington and British Columbia. Some have been also been seen within the Gulf of California. Rare sightings have also been made around Japan and Korea.
Sei Whales have been recorded in the northern Indian Ocean near Sri Lanka and the Indian coast.
In the North Atlantic Sei Whales are found from northwestern Africa to Norway in the east and from the southern United States to Greenland in the west. As the Sei Whales prefer deeper waters off the continental shelf they do not frequent semienclosed bodies of water, such as the Gulf of Mexico, the North Sea and Mediterranean Sea.
Population
Given the Sei Whales deep ocean habitat, wide range and that there have been no dedicated surveys scientists population numbers are based on rare observations and old whaling data. So it is thought that there might be around 80,000 Sei Whales left, which would be about a third of the prewhaling population.
Commercial whaling absolutely decimated their numbers with a reported number of 141,553 Sei Whales killed in the Southern Hemisphere between 1947–1980. This number did not include the gross under-reporting by Soviet whalers who killed an additional 23,615 Sei Whales. So the true estimated minimum number of Sei Whales killed in the Southern Hemisphere for this period is 165,168.
Sei Whale only breed every 2–3 years so population recovery is very slow.
Habitat
Sei Whales live in the deep ocean far from land. They prefer the more temperate waters of the mid-latitudes and are not usually found in the tropics or deep polar regions. They also don’t like semienclosed bodies of water so will not be found in such areas as the Gulf of Mexico, the North Sea and Mediterranean Sea.
Significant feeding grounds are found in the Southern Ocean along the Antarctic convergences where there are concentrations of Krill. Opportunistic feeing has been observed in the Bonney Upwelling off the Victorian and South Australian coasts during the upwelling season from November to May.
Breeding is thought to occurs in tropical and subtropical waters, but where exactly is still unknown.
Behaviours
At this time little is known about Sei Whales social structure. In general Sei Whales are found traveling alone or in small pods of three to five whales. Larger pods may gather in the feeding grounds. Long distance migrations appear to be driven by the search for food and the whales appear to be often segregated by age, sex and their reproductive status. Pods in temperate waters are mainly made up of lactating females and juveniles. Adult whales, most males are found in the higher high latitudes.
Like Fin Whales Sei Whales are fast swimmers and have been known to reach speeds of up to 50 km/h over short distances. Sei Whales are not what you’d call active at the surface as they tend not to rise high out of the water when surfacing, although both the blowholes and dorsal fin are often exposed above the water surface almost simultaneously. They tend to lay in the water breathing every minute or so and then sink back below the surface seldomly lifting their tails clear of the surface. Dives are shallow and last for five to fifteen minutes. They also rarely breach.
Sei Whales feed by skimming pelagic crustaceans from near the surface by swimming horizontally near the surface. They may also use a side lunging technique as other whales do. It is thought that they will feed on smaller concentrations of food that are inadequate for other whales.
Vocalisations – Sei Whales are known to make long, loud, low-frequency sounds, as well as other calls sometimes described as “growls” or “whooshes”. Most calls lasted about a half second and consist of multiple parts at different frequencies in the 240–625 hertz range, which is well within the range of human hearing. These multiple parts at different frequencies distinguishes Sei Whale calls from those of other whales species. The meaning of particular calls is unknown.
Feeding
In the summer months Sei Whales migrate south into their feeding grounds between the Antarctic and subtropical convergences where they feast upon planktonic crustacea, particularly copepods and amphipods as well as Krill. The further south they go the higher the percentage of Krill in their diets. This said the overall percentage of Krill consumed is much lower than in other species of whales suggesting that by having a varied diet not heavily dependant on Krill Sei Whales avoid competing directly with them for food.
As a baleen whale Sei’s eat by taking giant mouthfuls of water containing their prey which are then strain through their baleen plates. Sei Whales feed close to the surface ’skimming’ or sometimes lung feeding on concentrations of food. Again they may target lower concentrations of food that maybe inadequate for other whales. The Sei Whale main competitors for food are fish such as Herring, Basking Sharks, and Right Whales. They consume around 900kg of food per day.
Life Cycle
As Sei Whales live in the deep ocean far from shore as well as having no known breeding grounds scientist have been unable to observe their full life cycle. Sadly much of the information we do know comes from the data recorded by the whalers who killed them. So assumptions are based on this information as well as any other chance observations.
It is thought that Sei females breed every 2 to 3 years and that the pregnancies lasting around 11 months (possibly as long as 13 months) with only one calf being born. Calves are around 4.5 to 5 meters and 1 ton in weight at birth. Mothers will nurse their calves for 6 to 9 months by which time they will have reached 8 to 9 metres in length. This means that weaning occurs in the summer feeding grounds. Sei whales become sexually mature at 8 to 12 years of age, when males have reached 13 metres and females 14 metres in length. Sei Whales lifespan is thought to be up to 65 years.
Threats
Like all marine mammals Sei Whales have a number of natural and man made threats.
These include whaling, pollution, climate change, acoustic disturbance, ship strikes, entanglement and bycatch, over-harvesting of food sources such as Krill.
Mass Deaths – There have also been mass death events such as the one in June 2015 in southern Chile where 337 dead Sei Whales. Scientist believe this might have been caused by a ‘red tide’ algal blooms which was toxic to the whales.
References/Sources
Australian Department of the Environment, Canberra
NSW Department of the Environment
NOAA Fisheries
Whales, Dolphins and Porpoises by Mark Carwardine
Whales, Dolphins & Seals: A Field Guide to the Marine Mammals of the World by Hadoram Shirihai and Brett Jarrett
Copyright 2020 David Jenkins – Whale Spotter