Sei Whale

The sei (SAY) whale, Balaenoptera borealis, is the third largest rorqual, a group of large baleen whales that feed by straining food from the water.  They live 50 to 70 years and grow up to 60 feet in length, weighing 100,000 pounds.  They have bluish-gray backs, creamy bellies, a tall, hooked dorsal fin, and a large mouth with 200-400 baleen plates made of keratin.  Their skin is often marked with a series of curvy healed scars made by “cookie-cutter” sharks which attach themselves to large species with suction-cup lips hiding sharp teeth that cut out a section of flesh about two inches in diameter to eat.

There are two types of whales, toothed whales and baleen whales which are considerably larger, but have no teeth.  Toothed whales and other cetaceans including dolphins and porpoises, have one blowhole leading to the left nasal passage.  Baleen whales have both nasal passages open as two blowholes to accommodate their huge size, allowing a much greater air exchange when surfacing.  In all cetaceans, muscles are contracted to open a nasal plug in the blowhole allowing them to breathe.  When underwater, the muscles are relaxed, blocking the blowhole and preventing the animal from breathing in water.  Air sacs immediately below the blowholes allow for creation of long, loud, low-frequency calls within the hearing range of humans.  Most calls last less than one second and sound levels are equivalent to standing next to a jack hammer, carrying long distances underwater.

Sei whales are found in temperate oceans around the world, but not in polar or tropical waters.  The southern sub-species population is slightly larger than the northern one, and there is no evidence of interbreeding.  Current population estimates place a total of 80,000 individuals world-wide, but sei whales prefer deep ocean waters and rarely come near land making placement of tracking devices and accurate estimates difficult.  Tagging has been moderately successful, with many lost signals.  It is known that they migrate annually, following the available prey and using prevailing currents to move quickly, but exact migration routes and breeding areas are not yet known.

Sei whales generally travel alone or in small groups of up to six individuals.  They are one of the fastest swimmers of all ocean species, attaining speeds up to 35mph for short distances.  When feeding, they swim on their sides near the surface with one side of their huge mouths open taking in large amounts of water and straining out prey.  Their favorite food is copepods, a tiny planktonic crustacean, but they will also consume krill, squid, and small schooling fish such as anchovies, sardines, and mackerel.  They may make shallow dives to pursue schools of fish, quietly sinking below the surface; they can stay underwater for five to twenty minutes.  Each whale consumes about 1,000 pounds of food daily.

Sei whales reach sexual maturity between six and twelve years old, and a young adult is about 45 feet in length.  Females are slightly larger than males.  Gestation is 11 to 13 months, and each female mates once every two to three years.  One calf is born in winter.  A newborn is about fifteen feet in length and weighs 1500 pounds at birth.  Calves remain with their mother for six to nine months before being weaned off its mother’s milk.  Males are polygynous where one male has mating rights with several females.

Sei whales were overhunted in the mid-1960s for meat and oil after blue whale and fin whale populations were decimated in earlier years.  They were added to the IUCN Red List as endangered in 1970.  Their only natural predator is a pack of orcas.  Massive die-offs have been recorded in the last fifty years, and evidence suggests the cause is red tides, a harmful algal bloom that severely depletes oxygen levels in large areas and may last for several weeks or months.

Other threats include entanglement in fishing gear, leading to whales dragging the gear for long distances, ultimately tiring them to a point where feeding and breeding are impossible.  Climate change affecting ocean water temperatures and currents is altering prey distribution and availability resulting in less successful foraging.  Opening more shipping lanes in polar regions along the edges of sei whale territory, another result of climate change, contributes to more vessel strikes.  Shipping also increases ocean noise, making long-range communications for all species more difficult; however, the impacts from this on the sei whale population is not well understood.

The sei whale remains a species with more mystery than knowledge.  You can learn more about cetaceans in general from your local natural history museum or come to the Jurica-Suchy Nature Museum at Benedictine University to see a full skeleton of a sei whale on display.