The Sei Whale, Balaenoptera borealis SALLY A. MIZROCH, DALE W. RICE, and JEFFREY M. BREIWICK Introduction balaenopterids have fringed baleen from high-latitude summer feeding plates instead of teeth, and feed on grounds to lower latitude winter areas The sei whale, Balaenoptera swarms of small zooplankton which (Fig. 1). Populations north and south borealis Lesson, 1828, can range in they capture in their baleen as water is of the Equator are presumed to be length up to 18.5 m (60 feet), which filtered through. Sei whales, like other separate, as their migration schedules makes it the third largest whale in the baleen whales, do not have a well­ are 6 months out of phase. family Balaenopteridae, following the defined school or social structure, and Unlike most other balaenopterids, blue, B. musculus, and fin, B. are generally found in small groups or sei whales tend to be restricted to physalus, whales. Sei whales are gray as solitary individuals (Tomilin, more temperate waters, and conse­ with a variable white area extending 1957). quently are generally found within a from the chin to the umbilicus. All the smaller range of latitudes. Although Distribution and Migration The authors are with the National Marine there have been sporadic reports of sei Mammal Laboratory, Northwest and Alaska Like most balaenopterids, sei whales close to the ice edge, this is Fisheries Center, National Marine Fisheries thought to be uncommon (Jonsgllrd, Service, NOAA, 7600 Sand Point Way N.E., whales are found in all oceans and Bin CI5700, Seattle, WA 98115. migrate long distances north-south 1966). Figure 1. -Geographical distribution of the sei whale. Simple hatching indicates the summer feeding grounds. Stippling indicates distribution during autumn, winter, and spring; records are scarce during these seasons, and the distribution is to a large extent speculative. 46(4),1984 25 A sei whale surfacing in Antarctic waters, showing the tall, sickle-shaped dorsal fin characteristic of this species. Photo by G. Joyce. In the North Pacific, sei whales Although sei whale feeding areas Whaling Commission (lWC) con­ winter in waters from lat. 20 0 N to lat. are fairly well defined in all oceans, sidered the whales in the North 23°N, and summer from lat. 35°N to the location of wintering areas re­ Pacific as two stock units, but since 40 0 N (with a few individuals found at mains to a large degree a mystery, the capture of sei whales was pro­ lat. 50 0 N) (Masaki, 1976). In the Ant­ since sei whales migrate in the open hibited, the IWC considers only one arctic, the summer distribution (in­ ocean and are difficult to observe stock unit for management purposes. ferred from catch statistics) is mainly from shore. Research indicates, however, that from lat. 40 0 S to 50 o S, and the winter there may be at least three stocks Stock Identity distribution is as yet unknown. In the found within this large area. Masaki North Atlantic, the northern (sum­ Since most hunting (and hence (1976; 1977) suggests boundaries west mer) limit is thought to be lat. nON, research, which is usually conducted of 175°W between 175°W and although little is known of southern in association with whaling activities) 155°W, and east of 155°W, based on (winter) distribution (Jonsglird, 1966). occurs on feeding rather than mark-recapture studies, catch There is evidence of differential breeding grounds, the groupings distribution, and differences in baleen migration by reproductive class, with observed are generally feeding rather plate morphology. pregnant females leading waves of than breeding aggregations. The rela­ North Atlantic migration both into and out of the tionship of breeding groups to feeding feeding grounds (Matthews, 1938; groups is poorly understood. The IWC recognizes the following Gambell, 1%8). Pregnant females stocks in the north Atlantic: Nova North Pacific also tend to be found in higher Scotia, Iceland-Denmark Strait, and latitudes (Masaki, 1976). At one time the International northeast Atlantic. There is also an in- 26 Marine Fisheries Review dication, inferred from catches, mark however, and if copepods are absent, causes them to shed their baleen recoveries, and migration patterns, of they feed on euphausiids, or krill, that plates; this greatly impairs their a separation between the Nova Scotia congregate in dense shoals near the feeding ability. Predation on sei stock and the one off northeast New­ surface-notably Euphausia superba whales by killer whales, Orcinus orca, foundland and Labrador (Mitchell and E. vallentini in the Antarctic; E. appears to be rare. Natural mortality and Chapman, 1977). Schmidly pacifica, Thysanoessa inermis, T. rates are difficult to estimate, but ap­ (1981) speculates that a Gulf of Mex­ longipes, and T. spinifera in the pear to be about 7.5 percent per year ico/Caribbean stock exists, but this is North Pacific; and Meganyctiphanes in adults, perhaps somewhat greater questionable due to possible confu­ norvegica and T. inermis in the North in immature animals (Allen, 1980). sion with Bryde's whales, Atlantic. In some parts of the Nor­ Balaenoptera edeni (Leatherwood thern Hemisphere, they also feed ex­ Exploitation and Population Size and Reeves, 1983). tensively on small schooling fishes History of Exploitation such as anchovies, Engraulis mordax; Southern Hemisphere sauries, Cololabis saira; and jack The earliest exploitation of sei By convention, sei whales are con­ mackerel, Trachurus symmetricus. whales likely occurred in waters off sidered to belong to six separate In autumn, sei whales migrate northern Japan, starting around the stocks in the Antarctic, based on the several thousand miles toward middle of the 17th century. The six IWC statistical areas developed us­ equatorial waters. During the winter Japanese method of capturing the ing blue, fin, and humpback whale, they eat very little or fast for several nonbuoyant, fast-swimming rorquals Megaptera novaeangliae, data months, living off their fat reserves (including sei, fin, and humpback (Brown, 1962; Mackintosh, 1966). (Mackintosh, 1965). whales) involved netting the animal Mark-recovery data indicate links be­ before killing it, and then towing it to Reproduction tween sei whales in various areas, e.g., shore for processing for human con­ the Brazilian coast and the western The reproductive strategy of sei sumption. Until the introduction of half of Area II; the Natal coast of whales is similar to that of most other modern whaling in Norway in 1864, South Africa with the eastern half of balaenopterid whales. The mating the Japanese were the only whaling Area III and the western half of Area season covers about 5 months during nation that could effectively capture IV; and the western and southeastern the winter. Most females ovulate only rorquals (Tjijnnessen and Johnsen, Australian coasts and Area IV (lWC, once, but if they do not conceive, they 1982). 1977). Other than this information, may ovulate two or three times during we know nothing of potential one season. The single calf is born North Pacific breeding areas in the Southern after a gestation period of about 1 Since modern whaling was intro­ Hemisphere, and therefore cannot year, when it is about 4.4 m (14.5 feet) duced in Japan at the beginning of the delineate breeding stocks. Conse­ long. The calves are weaned on the 20th century, the sei whale has ac­ quently, sei whales are managed summer feeding grounds when they counted for a large proportion of the separately by IWC area. are 6-9 months old, and have attained total whale catch in Japanese waters. a length of about 9.0 m (30 feet). Both Catches ranged from about 300 to 600 Life History and Ecology sexes attain sexual maturity between 5 per year from 1911 through 1955, and Feeding and 15 years of age. Adult females rose to 1,340 in 1959, remaining at bear a calf every 2 or 3 years. high levels until a large drop in 1971. During the summer, sei whales in­ (Gambell, 1968; Masaki, 1976; Rice, Sei whales off Japan were protected habit much the same range as fin 1977; Lockyer and Martin, 1983). after 1975. Catches of sei whales by whales in the higher latitudes and the Natural Mortality Japanese and Soviet fleets in the cold currents on the eastern sides of North Pacific and Bering Sea jumped the oceans, where food production is Important natural mortality factors from 260 in 1962 to over 4,500 in 1968 high. In general they range even far­ are unknown. The sei whale is usually and 1969, after which catches declined ther offshore than fin whales, and relatively free of ectoparasites, but is rapidly. Whaling ceased after 1975 tend to be nomadic. Sei whales very often heavily infected with en­ when this stock of sei whales was pro­ specialize on copepods when available doparasitic helminths; presumably tected. (mainly Calanus tonsus, C. this is because its diet is more catholic Except for harvests off British Co­ simillimus, and Drepanopus pec­ than that of the fin or blue whale lumbia in the late 1950's through the tinatus in the Antarctic; C. cristatus, (Rice, 1977). Some of these en­ mid-1960's, sei whale catches off the C. plumchrus, and C. pacificus in the doparasitic worms are frequently coasts of North America were fairly North Pacific; C. finmarchicus in the pathogenic, affecting especially the insignificant compared to fin and North Atlantic) (Nemoto, 1959; liver and kidneys. A disease of humpback whale catches. Catches in Kawamura, 1973, 1974). They are unknown origin affects 7 percent of this region rose from 39 in 1958 to more euryphagous than fin whales, the sei whales off California, and over 600 in 1964 and 1965, after 46(4),1984 27 .0000 30000 / .................·/l_fin blue- .