Taxonomy/Description

Sei whales belong to the Mammalian Order Cetacea, in the suborder Mysticeti.  All baleen whales belong to the suborder Mysticeti, which is Latin for "mustached whales".  The sei whale belongs to the rorqual family, Balaenopteridae.  The word "rorqual" means "tube whale or furrowed whale", referring to the pleats on the lower jaws of rorquals.  All rorquals have baleen, a dorsal fin and throat grooves.  Other members of this family include the humpback whale, fin whale, and minke whale.  The sei whale's scientific name is Balaenoptera borealis.  Its generic name, Balaenoptera, means "winged whale", which refers to the sei whale's dorsal fin.  The sei whale's specific name, borealis, means "northern".  The common name "sei", pronounced "say".  It is derived from seje, the Norwegian word for pollock, a relative of the codfish that composes part of the whale's diet.  Historically, sei whales returned to the Norwegian coast each year to feed on pollock swarms feeding on plankton.  Sei whales and Bryde's whales are very similar in appearance.  Upon closer inspection, sei whales have one ridge on their head, while Bryde's whales have three ridges.  Sei whales may exceed 60ft in length and weigh 14-17 tons.  Female sei whales are usually larger than males of the same age.

Habitat/Diet

Sei whales are found in all oceans, but do not venture into polar regions.  Their major food sources, krill and copepods, live in all regions.  Sei whales living in northern waters occasionally feed on small fish.  As the whale opens its large mouth to gulp up the krill, the pleats of its lower jaw expand to contain the large volume of water.  The whale then presses its tongue up against the roof of its mouth, straining the water through its 640-760 baleen plates.  These 19 inch plates are composed of keratin, the same substance of fingernails and hair.  The ends of baleen are brush-like, preventing the prey from escaping.  The copepods remain inside, and are consequently swallowed.  While feeding on plankton, the sei whale will strain water at the ocean surface.

Behavior/Reproduction

The sei whale leads a solitary life.  However, small groups of 2-3 animals are also commonly observed.  Large congregations of one hundred or more whales have been witnessed.  Such large groupings usually surround a common resource.  The migration of sei whales to a location rich with prey is typical.  They are very fast swimmers, reaching speeds of more than 30 knots.  Sei whales do not arch their backs or raise their flukes to dive.  Instead, they gracefully sink beneath the ocean surface.

The breeding location of this species is unknown.  Gestation lasts about 12 months.  Calves are 14-15ft in length, weighing 2,000lbs at birth.  They nurse for six months.  Little is known about sei whale breeding and calving.  Sexual maturity is reached at ten years by males and females. 

Conservation

Sei whales are slimmer than blue whales and fin whales because they have less blubber.  Consequently, sei whales were not hunted until blue whale and fin whale populations were depleted.  Sei whale hunting began during the 1950's.  Within two decades, the Antarctic population of sei whales crashed.  Since that crash, sei whales have been protected.  One-fifth of the original population exists today, numbering around 54,000.  A moratorium on all whaling has been established, and the greatest current threat to sei whales is ocean pollution.