SEI WHALE

(BALAENOPTERA BOREALIS)

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Sei whales occur from the tropics to the poles in all oceans, but their distribution appears more limited to mid-latitude temperate zones than most other baleen whales. They migrate seasonally between warm sub-tropical wintering areas used for mating and calving, and cooler higher-latitude feeding grounds in the summer. The Falkland Islands appear to be a globally-important region for sei whales, and are one of very few localities worldwide where the species is observed regularly and within shallow, coastal waters. Elsewhere, the species is usually found offshore and has an unpredictable occurrence, making it challenging to study.

Species identification

Sei whales have average adult body lengths of 15m (maximum 19.5m).

The blow is tall and vertical, although in windy conditions it may appear angled or bushier. The dorsal fin is located two-thirds of the way along the back and is tall and erect, rising steeply from the back.

Overall colouration appears a dark grey-brown in good light, with a pale-grey chevron marking extending from the flank and over the back behind the head. A pale, forward-sweeping “brush mark” pattern is also present on the mid-flank. The jawline is slightly arched and downturned at the tip.

Falklands distribution

Sei whale sightings in the Falklands have increased since the 1990s, although it is unclear whether that is due to growing population size or a shift in distribution.

They primarily occur during a six month period spanning the summer and autumn (December to May), although a few sightings occur in other months.

Their distribution likely includes the entire Falklands shelf, although they appear to be less numerous where the shelf is narrow. They can approach within a few hundred metres of the shoreline (in depths as shallow as 15m), and enter semi-enclosed waters in large bays and inlets.

Ecology

In the Falklands, sei whales usually occur singly, or in groups of 2 to 7 animals.

They appear to use Falkland waters as a feeding ground, diving for up to 13 min duration during subsurface foraging. However, individuals are occasionally observed skimming or lunge-feeding at the surface.

Faecal samples collected in East and West Falkland suggest that lobster krill (Munida gregaria) are a major prey item. Sei whales are an important component of the Falklands marine ecosystem, consuming a significant amount of prey biomass, recycling nutrients via defecation, and themselves being prey for top predators, namely killer whales.

Photo-identification

During photo-identification the dorsal fins and flanks of sei whales are photographed, to detect natural markings by which individual whales can be uniquely-identified and catalogued. Over 100 sei whales have been photo-identified in East and West Falkland respectively, providing information on their movements.

Two sei whales have been matched between opposite coasts of the Falkland Islands and in different years, showing that individuals repeatedly return to Falkland waters and range widely around the Islands. Some animals are photographed repeatedly in small areas of a few kilometers, while others travel greater distances. For example, one whale swam 27 km overnight.

Falklands Conservation has carried out sei whale field research since 2017, aiming to document their distribution, ecology and population structure in order to implement evidence-based conservation and management.

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