Overview
The Sei Whale (Balaenoptera borealis) is the third-largest rorqual on Earth, surpassed in length only by the blue whale and fin whale. Sleek, hydrodynamic, and exceptionally rapid, this majestic pelagic cetacean is capable of swimming bursts reaching up to 30 miles per hour (50 km/h). Favoring deep offshore oceanic trenches over shallow coastal shelf waters, the sei whale undertakes long annual migrations between warm winter breeding grounds and productive temperate feeding zones.
Biology and Physical Characteristics
Adult sei whales possess a slender, torpedo-shaped body adorned in dark steel-gray or charcoal tones with mottled grayish-white belly markings. A prominent, tall, sickle-shaped dorsal fin sits approximately two-thirds of the way down the back. Unlike other rorquals with coarse baleen, the sei whale features exceptionally fine, silky inner baleen bristles numbering between 300 and 400 plates per side, specifically adapted for skimming microscopic zooplankton such as calanoid copepods and euphasiids.
Behavior and Ecology
Sei whales exhibit flexible foraging techniques, employing both gulp-feeding and surface skimming (skimming forward with mouths partially open along dense plankton slicks). They travel either singly or in small pods of two to five individuals. Diving profiles are typically shallow and synchronized, rarely exceeding 15 minutes in duration, with the dorsal fin and blowhole breaking the ocean surface simultaneously in an elegant rolling motion.
Conservation and Human Impact
Subjected to intensive commercial whaling throughout the mid-20th century after populations of blue and fin whales had collapsed, over 300,000 sei whales were slaughtered between 1950 and 1980. International commercial moratoriums established by the IWC in 1986 have permitted gradual recovery, yet the species remains listed as Endangered. Current anthropogenic hazards encompass vessel collisions, entanglement in heavy commercial fishing gear, ocean noise pollution, and climate-induced shifts in copepod blooms.
Interesting Factoid
The name ‘Sei’ originates from the Norwegian word for pollock (‘seje’), because sei whales traditionally arrived along the coasts of Norway at the exact same time as massive annual migrations of pollock fish.
Last updated: October 2026