Sei Whale: where and when to see them

The sei whale is a streamlined baleen whale of temperate and subpolar seas, heavily depleted by twentieth-century commercial whaling. It remains a marine animal of predominantly offshore waters, not a regular coastal attraction. Long-distance movements are influenced by feeding conditions, and its distribution can change markedly between years. Identification is often difficult because brief views may resemble fin or Bryde's whales, particularly when distance conceals the head and body proportions.

Size

12–18 m total length

Weight

15–30 tonnes

Lifespan

50–70 years

Status

Endangered

Sei whales often travel alone or in small, loose groups, with larger aggregations forming around concentrated prey. They are fast swimmers and commonly surface with a relatively smooth rolling motion. Feeding can involve skimming or engulfing prey near the surface. Tail flukes are seldom raised clearly before a deeper descent. Seasonal movements are widespread, but routes and timing remain less completely understood than those of several better-studied baleen whales. A characteristic surfacing pattern involves a series of shallow dives followed by a deeper dive with little or no fluking. They are known for their rapid swimming capabilities, sometimes reaching speeds of up to 50 km/h over short distances.

• Its common name derives from a Norwegian word associated with pollock.
• The baleen has unusually fine fringes suited to filtering small crustaceans.
• Sei whales generally show their tail flukes less often than humpback whales.
• Commercial hunting intensified after larger whale species had already been severely depleted.

Around the Azores, spring and early summer can offer a plausible regional passage window, though shore detection is difficult. Southern Ocean travel mainly occurs during austral summer, but this does not establish a dependable sei whale season along Antarctic visitor routes. In the North Atlantic, particularly around oceanic islands, late spring and early summer offer the highest chance of encounters as whales migrate through productive areas. However, predictability remains low compared to other large whale species.

They primarily use deep oceanic waters and productive shelf-edge regions, usually in temperate or subpolar conditions. Around Antarctica they are more associated with northern waters and the Subantarctic than dense pack ice, so standard Antarctic Peninsula routes offer limited prospects. Though generally avoiding ice, sei whales may occasionally venture into waters with scattered ice floes at the edges of their foraging grounds. Their distribution is closely linked to oceanographic features that concentrate their small prey, such as currents and upwellings.

Copepods and other small crustaceans are major foods, with krill and schooling fish important in some regions. Fine baleen fringes help retain small prey. Diet varies geographically, reflecting the dense, seasonally shifting concentrations needed to sustain such a large animal. When feeding on schooling fish, sei whales are capable of lunging through schools at high speeds, gulping large volumes of water and prey. They are one of the most rorqual-like whales in their feeding adaptability, shifting prey as availability changes seasonally or geographically.

Common name

Sei Whale

Scientific name

Balaenoptera borealis

Animal group

Mammals

Size

12–18 m total length

Weight

15–30 tonnes

Lifespan

50–70 years

Status

Endangered

Identification

A single head ridge, relatively upright dorsal fin and absence of the fin whale's conspicuous white right lower jaw help distinguish sei whales, although distant views are often inconclusive. The sei whale's typically darker, sleek body and proportionally smaller pectoral fins can be helpful clues when compared to other rorquals. The dorsal fin is also set further forward on the back compared to minke whales, but further back than blue whales.