Balaenoptera borealis

Lesson, 1828 - Sei whale

Distinctive Characteristics

Sei whales can be confused with fin and Bryde's whales, both of which also have a prominent falcate dorsal fin. All 3 have typical rorqual body shapes. In both sei and Bryde's whales, the dorsal fin rises at a steep angle from the back. However, sei whales have only a single prominent ridge on the rostrum (Bryde's whales tend to have 3), and a slightly arched rostrum with a downturned tip. Unless the head can be seen at close quarters, however, Bryde's and sei whales can be especially difficult to distinguish.

Coloration is mostly dark grey, except for a whitish area on the belly. The back is often mottled with scars (possibly from lamprey bites), and the skin surface often resembles galvanized metal.
The 32 to 60 ventral pleats are short for rorquals, ending far ahead of the navel. The 219 to 402 baleen plates on each side are black with very fine fringes of light smoky grey to white. Sei whales produce a blow up to 3 m tall.

Can be confused with

Sei whales are most likely to be confused with Bryde's whales and, less likely, with fin whales. Attention to dorsal-fin shape and position, head shape, and colour patterns will help to distinguish among the 3. The 3 head ridges of Bryde's whales, and larger size and asymmetrical head coloration of fin whales will help make them distinguishable.


Adults are up to 18 m in length. Large adults may weigh 30 t. At birth, sei whales are 4.5 to 4.8 m long.

Geographical Distribution

Sei whales are open ocean whales, not often seen near the coast. They occur from the tropics to polar zones in both hemispheres, but are more restricted to mid-latitude temperate zones than are other rorquals.

Biology and Behaviour

Groups of 2 to 5 individuals are most commonly seen. Sei whales are fast swimmers, possibly the fastest of all cetaceans. When slow moving sei whales surface, their blowholes and dorsal fin are often visible above the water at the same time. Feeding sei whales tend to dive and surface in very predictable series, often remaining visible just below the surface between breaths.

Calving occurs in mid winter, in low latitude portions of the species' range.

Sei whales skim copepods and other small prey types, rather than lunging and gulping, like other rorquals. This may largely explain the relative fineness of the baleen fringes and the shortness of the throat pleats in this species.


As the larger rorquals became scarce in recent decades, hunting pressure on sei, Bryde's, and minke whales increased, largely in the Antarctic. Although heavily depleted, sei whales have recovered somewhat more successfully from hunting than other large baleen whales.

IUCN Status