Sei Whale
Balaenoptera borealis

Other Names: Pollack Whale, Coalfish Whale, Sardine Whale, Japan Finner, Rudolphi's Rorqual

Habitat: Offshore

Status: Locally Common

Population: c.40,000-60,000

Threats: Unknown

Group Size: 2-5, larger groups of up to 30 at good feeding grounds

Fin Position: Far behind center

Newborns: 14 ½-15 ¾ ft (4.4-4.8 m), c.1,600 lb. (725 kg)

Adults: 39 ½-52 ½ ft (12-16 m), 20-30 tons

Diet: Fish, squid or octopus, and krill or other crustaceans

Baleen: 300-410 baleen plates on each side

Description

Dorsal fin:

- slender, erect dorsal fin
- slightly further forward on body than on other rorquals
- some fins more falcate in shape
- tip points backward
- smooth trailing edge

Flukes:

- pointed tips
- distinct notch in middle
- almost straight trailing edges
- roughly triangular shape
- flukes small in relation to body size

Flippers:

- dark upper sides and undersides of flippers
- pointed tips
- slender, relatively short flippers, one-tenth of body length

Head:

- low splashguard in front of blowholes
- gray-black baleen may have metallic sheen
- slightly arched head shape
- plates near front of mouth may be lighter in color
- throat grooves

Head (from above):

- 2 distinct blowholes
- single longitudinal ridge extends from blowholes to near tip of snout
- narrow, pointed snout, but not as acutely pointed as in Fin Whale
- similar coloring on either side of head

Other characteristics:

- both sides of mouth same color
- slightly arched head
- predominantly bluish gray, dark gray, or black body may appear brownish in certain lights
- thick tail stock
- dark body may have patches of lighter gray
- underside and sides may appear mottled, with gray or white circular scars caused by parasites, lampreys, or Cookie-cutter Sharks
- 32-62 throat grooves, usually end just behind flippers
- paler gray or grayish white area on throat grooves

Behavior

- more regular dive sequence than most other rorquals and stays near the surface more consistently
- normally blows once every 40-60 seconds, though may blow every 20 to 30 seconds for 1 to 4 minutes, and then dive for 5 to 20 minutes
- during shorter dives, rarely descends deeper than a few feet, so its progress can be followed by "fluke prints" or swirls left by the beat of the tail just below the surface
- seldom breaches
- dorsal fin and back remain visible for longer periods of time than with other large whales
- swimming behavior less erratic than Bryde's Whale, but capable of great speed

Distribution

- worldwide distribution, but primarily in deep, temperate waters
- less easy to predict occurrence at specific localities than most other rorquals, though there tend to be sporadic annual invasions known as "Sei Whale years," in particular areas
- not normally found in extreme polar waters, although subartic and subantartic are favored summer feeding grounds; believed to migrate into warmer, lower latitudes for the winter
- migrations are poorly known and probably quite irregular
- there appears to be little or no mixing between northern and southern hemisphere populations
- it is most common in the southern hemisphere
- may be seen around islands but rarely close to shore elsewhere


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This page was created by:Candice Orca Mottet