Sei Whale – Balaenoptera borealis
Habitat: Mainly offshore in deep waters, 500 – 3000 m.
Description: Slender dark grey whale with greyish white on middle part of throat grooves. Relatively small pointed flippers. 30 – 60 throat grooves (50 on average) end well before navel. Dorsal fin is fairly erect, usually 25 – 60 cm tall, strongly recurved, and located a little more than 1/3 along back from tail. Relatively small tail flukes are broad and triangular, with median notch. Baleen plates relatively short and narrow, 300 – 410 on each side of upper jaw, uniformly grey-black but with fine, almost silky white fringes.
Size: 14 – 14.5 m; females larger. Maximum size 19.5 m. Individuals in the northern hemisphere tend to be smaller on average than those in the southern hemisphere.
Weight: 20,000 – 30,000 kg.
Lifespan: around 65 years.
Distribution: Worldwide. Seasonal migrations from polar and cold temperate regions (around Iceland, Greenland Sea, west Barents Sea) in summer to warm temperate and subtropical waters (Spain, Portugal, north west Africa) in winter. Rarely seen in coastal waters of British Isles, although probably under-recorded due to difficulties in identification. Most records from from waters >200 m deep between N Isles and Faroes and in Rockall Trough; occasionally occurs in coastal waters off Shetland, Hebrides, and between south Ireland and south west England. Sightings around British Isles generally July – October; off south Ireland, July – November.
Diet & Feeding: Feeds mainly on copepods and euphausiids. Small schooling fishes and squid important in some areas. Prey is taken near surface, either by skimming with half-open mouth or taking large mouthfuls of water.
Breeding: Mainly gives birth in winter, particularly Novermber – December, possibly offshore from north-west Africa or west of Iberia. Gestation lasts for 10.5 – 13 months; lactation for 6 – 9 months; calving interval is 2 – 3 years. Sexual maturity is reached at 5.6 – 11.7 years for females and 7 – 11.7 in males, although in areas following exploitation the average is around 6 – 8 years.
Conservation Status & Population: Listed by IUCN as endangered. No current estimate for the North Atlantic population; recent surveys indicate > 13,500 individuals. Noise and disturbance from vessels and industrial activities potentially damaging; seismic soundings in oil and gas exploration along Atlantic Frontier also a potential problem. No information exists on fisheries interactions or pollutant levels.
Historically, heavily exploited wherever it was common, particularly between 1955 – 1975. Pre-exploitation population in northern hemisphere estimated at 66,000 individuals.
Dark steely-grey back, often with grey or white round scars. Blow moderately tall (around 3 m), cone shaped. Blow and fin often visible for long period before making shallow dive. Dorsal fin more than 1/3 along back from tail – slightly further forward than in fin whale, much further forward than in blue whale.
Right lower lip and mouth cavity uniformally grey, unlike fin whale, and no white on underside of flippers or flukes.
Head is slender, slightly arched forehead but not as rounded as blue whale.
Single prominent median ridge on head, unlike Bryde’s whale which has 3. Baleen plates relatively narrow; length:breadth ratio >2:2; always less in Bryde’s whale).
Vocalisations: Little studied. Includes 3 kHz pulsed clicks < 1 s long. Recorded sounds consisted of 2 phrases of 0.5 – 0.6 s duration, spaced 0.4 – 1 s apart; each phrase consisted of 210 – 220 FM sweeps in 1.5 – 3.5 kHz range.