Sei whales are very similar in external appearance to fin and Bryde’s whales, both of which also have a prominent falcate dorsal fin. All three have typical rorqual body shapes. In both sei and Bryde’s whales, the dorsal fin rises at a steep angle from the back. Sei whales have only a single prominent longitudinal ridge on the rostrum (Bryde’s whales tend to have three), 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 at sea.
The 32-60 ventral pleats are short for rorquals, ending far ahead of the navel. The 219-402 baleen plates on each side are black with very fine fringes of light smoky-gray to white. Sei whales produce a blow up to 3 m tall. Coloration is mostly dark gray, except for a whitish area on the belly. The back is often mottled with scars (probably from cookie-cutter shark bites), and the skin surface often resembles galvanized metal.
Adults can be up to 18 m in length, although 15 m is a more typical length for adults. Large adults may weigh 30 tons. At birth, sei whales are 4.5-4.8 m long.
Can be Confused With
Sei whales are most likely to be confused with Bryde’s whales, less likely with fin whales . Attention to dorsal fin shape and position, head shape, and color patterns will help to distinguish among the three. The three head ridges of Bryde’s whales, and larger size and asymmetrical head coloration of fin whales will help make them distinguishable.
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. They do undergo seasonal migrations, although they apparently are not as extensive as those of some other large whales.
Ecology and Behavior
Groups of two to five 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 midwinter, in low latitude portions of the species’ range.
Feeding and Prey
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.
Threats and Status
After blue and fin whales, the sei whale was next in the line of fire of the modern whaler’s harpoon. The heaviest period of exploitation was between the 1950s and 1970s. Whaling took place in the North Pacific and North Atlantic oceans, but most hunting was in the Southern Hemisphere. Although fully protected by the International Whaling Commission since 1985, a few were taken in the North Atlantic by Iceland in the last few decades of the 20th century. Sources of mortality other than direct exploitation include probable vessel strikes. Current global abundance of the sei whale is considered to be about 80,000.
Currently, sei whales are ‘Endangered’ (IUCN) and ‘Endangered’ (ESA).