I. Identification and Nomenclature
The diversity of thought about our great whales is characterized by quotes from the biologist R. C. Haldane on the sei whale. He described the species as the “most graceful of all the whales, as its proportions are so perfect, and wanting die clumsy strength of the two larger Balaenoptera, spenns and Megaptera.” He added, “it is also far the best to eat, the flesh tasting of something between pork and veal and quite tender.”
The sei whale (Balaenoptera borealis, Lesson, 1828) is a typical, sleek rorqual, illustrated in Fig. 1. It is the third largest whale, reaching a maximum length of almost 20 m. More typically it is 15 ni, weighing 20 metric tons.
Identification of the sei whale at sea can be difficult. By size alone, adult blue and fin whales are obviously larger and minke whales smaller. The dorsal fin is a useful cue, being relatively taller than that of blue and fin whales. It is also strongly concave on its dorsal edge, similar to a minke whale. For a long time it was not distinguished from its close relative, the warm-water Bryde’s whale (B. edeni). The Bryde’s whale has three distinct ridges, running the length of the head, whereas the sei whale has only one (Fig. 1).
The color helps in identification. It is dark gray dorsally and on the ventral surfaces of the flukes and flippers, and there is no whitening of the lower lip as found in fin whales. However, in a few individuals some white baleen plates occur. Often the body can be heavily scarred with healed lamprey bites. Sei whales dive more by sinking than an arched dive, but the other rorquals can also dive in this quiet manner.
A more detailed external inspection allows a more definite identification. In sei and minke whales the ventral grooves end well before the umbilicus. In other Balaenoptera spp., including Bryde’s whale, they end at, or posterior to, the umbilicus. The number of ventral grooves varies considerably from whale to whale. In sei whales they vary between 40 and 65 with a mean number of about 50. This is less than in blue (B. muscu-lus), fin (B. physalus), and minke whales (B. acutorostrata and B. bonaerensis) but about the same as in Bryde’s whales.
The baleen of sei whales is a dark gray, but often with a yel-lowing-brown hue, and often with some anterior white plates. The plates number about 350 on each side of the jaw, and the largest is less than 80 cm long. The width of the plate is relatively narrow compared to blue, fin, and Bryde’s whales. In the sei whale the length-to-breadth ratio is typically over 2.2, whereas in the Bryde’s whale it is always less than 2.2. The bristles of the sei whales’ baleen are particularly fine. At their base they are about 0.1 mm in diameter compared with 0.3 mm for the other rorquals.
The sei whale is closely related to the Bryde’s whale. Wada and Numachi (1991) showed that genetic differentiation of the rorquals took place over 4 million years ago, but the sei and Bryde’s whales separated less than 300,000 years ago.
The sei whale is derived from the Norwegian “sejhval,” as it would arrive off Norway at the same time as the “seje” or saithe (Pollachius virens). There are a variety of other common names, but English forms have disappeared from usage in favor of the sei whale.
II. Distribution
The sei whale can be found in all ocean basins. It undertakes extensive, seasonal, latitudinal migrations, spending the summer months feeding in the subpolar higher latitudes and returning to the lower latitudes to calve in winter. Figure 2 shows the global distribution of sei whales in summer and winter. In the Southern Hemisphere, they rarely penetrate as far south as blue, fin, and minke whales, with summer concentrations mainly between the sub-Tropical and Antarctic convergences. Sei whales are seen, and have stranded, in the northern Indian Ocean, but their distribution and migrations are undetermined.
Figure 1 Smaller than blue or fin whales and larger than minke whales, sei whales (Balaenoptera borealis) can be distinguished by a relatively larger dorsal fin and by only one distinct ridge extending the length of the head. Sei whales occur in all oceans.
Figure 2 Global distribution of sei whales. Filled areas are the summer feeding distributions, and hatched areas represent breeding areas.
III. Populations and Biology
Genetic studies show the existence of different populations in the Northern and Southern Hemispheres but have failed to identify separate populations within ocean basins. However, biologists have separated populations, for management purposes, on the basis of different migrations and biological characteristics.
In the Southern Hemisphere, six populations are assumed, as for the other rorquals. In the North Pacific, two or three populations have been proposed. In the North Atlantic, as many as eight populations have been suggested, but only three are considered for management purposes.
The migratory behavior, spatial distribution, and fine baleen structure of the sei whales are associated with the nature of their preferred copepod prey. They feed on patches of cope-pods. near the surface, usually by skimming, and predominantly at dawn. The other rorquals would probably find such food too scarce, and the sei whale’s feeding habits are nearer to those of the right whales than the other rorquals. Sei whales also feed on shoals of fish and squid if they are encountered. Sei whales have a specific feeding niche different from, but sometimes overlapping, that of the other baleen whales.
Maturity is at about 10 years for males and females. Females are larger than males. Sei whales are bigger in the Southern Hemisphere, and here males mature at about 13-14 m and females at 14 m. In most seas, the age of maturity declined by 2 to 3 years, after the populations were depleted by whaling. In the Southern Hemisphere, young are conceived in June and births are in December in northern waters. The young are carried for almost a year and are born at a size of 4.5 m. Most calves are weaned in 7 months, after they have migrated to colder waters with their mothers.
As for most mammals, sei whales can be expected to have increased rates of mortality when very young or old, but actual rates are poorly known. From observations of age compositions, the rate of natural mortality is typically about 5-10% per year. They die naturally from predators, such as killer whales, and weakening from disease and parasites.
IV. Status of Populations
A fast rorqual, the sei whale was not exploited until the era of modern whaling at the end of the 1800s. Off north Norway. 4000 sei whales were killed between 1885 and 1900. Since then, sei whales were caught in the North Atlantic from land stations in Canada, Faeroes, Iceland, Ireland, Iberia, Norway, and Scotland. In the North Pacific, they were caught from California, Canada, Japan, Kamchatka, and Kuril and by pelagic fleets. In the Southern Hemisphere, they were caught from Brazil, Chile, Peru, South Africa, and South Georgia. The largest catches were made by the Antarctic pelagic fleets, after the numbers of blue and fin whales had been reduced, and between 1960 and 1970 over 110,000 sei whales were killed. Whaling is regulated by the IWC. and whaling for sei whales ceased in the Southern Hemisphere in 1979 and in the North Pacific in 1975. In the North Atlantic, whaling was prohibited from 1986, but limited catches continued for a few years, at Iceland, under a scientific special permit and through subsistence whaling from Greenland.
The size of populations is poorly determined, but whaling significantly depleted populations in all areas. In the Southern Hemisphere the original population was about 100.000, and in 1980 was thought to be 24,000. In the North Pacific, a population of over 60,000 was reduced to about 15,000. By now there may be 70,000 in both areas. The status of the North Atlantic sei whale is more uncertain, but recent sightings surveys indicate about 10,000 sei whales in the central and northeastern North Atlantic.
