Sei Whale

Balaenoptera borealis

KEY FACTS
Gaelic name: Muc-mhara-sei
Length: 14-15 metres (females about 0.5 m longer than males)
Weight: about 20 tonnes
Range: worldwide
Threats: environmental (noise & chemical pollution)

Physical Description
Sei whales are generally 14-15 m long, weighing about 20 t, although they can reach lengths of 18.6 m (with males) and 21.0 m (with females) in the Antarctic, and weigh up to 30 t.
The body is slim and streamlined and is more bulky than that of the fin whale, but still laterally compressed in the caudal region. The head is less pointed than that of the fin whale and less rounded than that of the blue whale. Viewed from the side it appears slightly arched, with the tip turned slightly downward. The head of the sei whale can be one fifth to one quarter of the animals’ body length.
Sei whales are dark grey or bluish grey in colour on their back and sides, and on the belly there is a region of greyish white that is almost always confined to the area of the ventral grooves.

Life span
The life span of the sei whale, determined from annual growth layers in the earplug, can range up to 65 years. Killer whales may prey upon sei whales and interactions between sei whales and swordfishes have been reported.


Distribution
The sei whale is a cosmopolitan species with a patchy oceanic distribution. It favours temperate, deep offshore habitat more than other species of large whales. In summer, sei whales do not move as far towards the polar waters as other rorqual whales, and do not usually enter icy water.

Like most other baleen whales, sei whales spend the winters in warm, sub-tropical waters, migrating to temperate or polar seas to feed in summer, although they may depart the winter grounds later, and generally do not penetrate into the polar seas as far as some other balaenopterids. Sei whale abundance in the Antarctic peaks later than blue and fin whale abundance.


Behaviour
Sei whales are normally found alone or in groups of 2-5 individuals, though larger aggregations are found on feeding grounds.
Sei whales are regarded as the fastest swimmers among the great whales, reaching bursts of speed of over 20 knots.
Sei whales often swim horizontally near the surface when feeding, in contrast to blue, fin and humpback whales that often turn on their side to feed.
Breeding

Male sei whales attain sexual maturity at 12.8 to 13.6 m in length and females at 13.3 to 14.0 m in length, and at 6 to 8 years of age. Calves are usually born in winter, after a gestation period of 10.5 to 13 months. A female gives birth to a single calf which is nursed for 6-9 months. The calving interval is 2-3 years.


Feeding
Sei whales feed primarily on surface plankton, mainly copepods but also euphausiids. The food type of the sei whale varies according to the temperature and latitude.
Sei whales appear to feed opportunistically on whichever food species is most prevalent in the area, including small schooling fishes and squid in some places. Regardless of species of plankton, small fish or squid, the sei whale seems to select prey which occur in dense aggregations and in surface waters.


Current Situation
The sei whale has been heavily exploited in all areas where it is known to have been abundant. Original population numbers may have been approximately 190,000 in the Southern Hemisphere and 66,000 in the Northern Hemisphere. The status of stocks is poorly known except that some are now well below pre-exploitation level. The biology and ecology of sei whales has been poorly studied, compared to many other species of whale, and information of sei whale behaviour, migrations and population is scarce.

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