Bowhead Whale (marine mammals)


Bowhead whales (Balaena mysticetus), sometimes called – Arctic right whales, Greenland right whales, great polar whales, or ahvik, are the only members of the family Balaenidae (suborder Mysticeti, order Cetacea) that live most of the year associated with sea ice in northern latitudes. Bowheads have never been seen in the Southern Hemisphere.

I. Description

Bowheads are readily identifiable by their large size, rotund shape, lack of a dorsal fin, dark color, white chins, triangular head (in profile), and neck (an indentation between the head and back). They are predominantly black, but most have characteristic white patterns on their chins, undersides, around their tail stocks, and/or on their flukes (Fig. 1). These patterns distinguish them from the similar-appearing right whales (Eubalaena spp.) and are unique to each individual. The white patterns around the tail and on the flukes increase with age. In addition, most bowheads accumulate distinctive, permanent marks on their backs, perhaps resulting from contact with sea ice. The bowed appearance of the mouth gives them their name.

These huge marine mammals are among the largest animals on earth, weighing as much as 75-100 tons. Males grow to 14-17 m in length and females 16-18 m, perhaps as much as 20 m. Their flukes are 2-6 m across. The heads of these whales constitute over a third of the bulk of their bodies, and their baleen may be as long as 4 m (no other whale has baleen longer than 2.8 m) with 230 to 360 plates on each side of the mouth, making their capacious mouths quite possibly the largest of any animal ever. To insulate them from the icy water, bowheads are wrapped in blubber 5.5 to 28 cm thick covered by an epidermis up to 2.5 cm thick. This combination of blubber and skin is the thickest of any whale species.

Bowhead whales are large, black cetaceans with various amounts of white on their chins (the arched, bright white area on the left), tail stocks (the paired white spots on the far right), and ventral surfaces (out of sight in aerial photographs). Note that the whale's left eye is visible deep underwater directly below the blow (exhalation vapor).

Figure 1 Bowhead whales are large, black cetaceans with various amounts of white on their chins (the arched, bright white area on the left), tail stocks (the paired white spots on the far right), and ventral surfaces (out of sight in aerial photographs). Note that the whale’s left eye is visible deep underwater directly below the blow (exhalation vapor). .

Bowheads are well adapted to the risky occupation of being air-breathing mammals in seas often covered with thick ice: they can withstand breaking through ice as much as 60 cm thick, and their diving abilities are exceptional—possibly exceeding an hour—which is critical to finding breathing holes when swimming under sea ice. The very low and very loud calls that bowheads produce may help them find mates or assist in following each other while navigating through sea ice. The only other whales commonly found as far north as bowheads are belugas (Delphinapterus leucas) and narwhals (Monodon mo-noceros), which are toothed whales with some of the same characteristics seen in bowheads: smooth backs and relatively thick blubber.

II. Breeding

Bowheads probably mate in later winter or early spring, but sexual activity may occur in any season. Mating groups consist of a male-female pair or several males and a female. Acoustics probably play a vital role in reproduction as bowheads are vocally active during the mating season and can hear each other 5-10 km away. Breaching (leaping completely out of the water) and fluke slapping (where the tails smash down on the water surface) may also play a role in attracting a mate or asserting dominance. Dominance is sometimes expressed through physical contact or sperm competition. However, there is a possibility that cooperation occurs among males during mating, making it more likely that at least one of them is able to inseminate a female. Over a year after mating (13-14 months), calves are born, usually during the spring migration between April and June. Calves are about 4 m long at birth. Females have calves 3 to 4 years apart. The following spring, the young whales, now 6 to 8 m long, are weaned from their mothers. After this, growth is slow compared to other baleen whales. At roughly 15 years of age, when 12 to 14 m long, females become sexually mature, and males become sexually active when 12 to 13 m long. Bowhead whales may live longer than other mammals: ancient harpoon points collected in whales recently indicate that the whales may have lived for more than a century.

III. Feeding

Bowheads feed throughout the water column, sometimes on the surface (called “skimming”) and sometimes at or near the seafloor (as evidenced by mud smeared across their heads and backs). A bowheads huge mouth can engulf large volumes of water, including prey, and, as the tongue rises, the water is pushed out, trapping prey on the inside fringed surfaces of the baleen, which serves as a filter all the way around the mouth. The massive tongue (as much as 5 m long and 3 m wide) then sweeps the food off the baleen into a very narrow digestive tract. As many as 60 species of animals have been found in bowhead stomachs, but their preferred prey are copepods (11 species) and euphausiids (2 species), as well as mysids and gammarid amphipods. Sometimes as many as a dozen bowheads will feed together in an echelon formation, similar to a line of migrating geese. Perhaps this coordinated effort helps the whales entrap their prey.

The only predators of bowhead whales, other than humans, are killer whales (Orcinus orca). Killer whale scars were found on approximately 4 to 8% of the whales taken by Alaskan Eskimos. In part, the bowheads’ close association with sea ice may be a way of seeking refuge from killer whales.

IV. Distribution and Abundance

Bowhead whales may have once been a single panmictic (randomly interbreeding) population that emerged in the Northern Hemisphere during the Pliocene (roughly 8 million years ago), according to fossil records. Genetic mixing between stocks in different areas was possible during the relatively warm interglacial periods (such as in a.d. 1000-1200) when reduction in sea ice meant whales could move between the Atlantic and Pacific Basins. Bowheads could have moved freely between the Beaufort Sea and Hudson Bay until the “Little Ice Age” between 1400 and 1850. Today’s temperatures are cool enough to keep ice across most of the east-west passages of the Arctic, isolating these whale stocks.

While ice may have contributed to this isolation, commercial whaling had a more profound effect. The bowheads large size, long baleen, thick blubber, slow speed, and gentle disposition have made them such a valuable commodity that whalers went to great lengths to harvest them. Commercial whalers from the 17th to 19th centuries were so efficient that they eliminated stock after stock of these whales. In fact, even a century after commercial whaling ceased, all bowhead stocks are still considered endangered.

Currently there are five stocks of bowheads defined as geographically distinct segments of the species’ total population: the Bering Sea (or Bering-Chukchi-Beaufort stock around Alaska), Okhotsk Sea (eastern Russia), Davis Strait (northeastern Canada), Hudson Bay (perhaps a part of the Davis Strait stock), and Spitsbergen (North Atlantic). The largest remnant stock, the Bering Sea stock, consists of approximately 8000 whales that migrate from the Bering Sea in the winter through the Chukchi Sea to the Beaufort Sea in the summer. This is the only stock that appears to be recovering from commercial whaling, growing at an annual rate of 3%. Originally there may have been 10,000 to 23.000 whales in this stock. Currently. Native Alaskans harvest approximately 40 whales per year through quotas set by the International Whaling Commission (IWC). The Chukotka Natives of Siberia have been allotted 5 bowheads per year from the Alaska quota. Independent of the IWC quota, the Canadian government has allowed a limited hunt of bowheads in the Bering Sea stock as well as from the stocks in Hudson Bay and Davis Strait.

There is very little known about other stocks of bowheads. Available evidence indicates that most stocks are very small: only about 300-400 currently live in the Okhotsk Sea (originally more than 3000): approximately 350 (originally 11.700) are in the Davis Strait stock: roughly 270 (originally about 580) live in Hudson Bay; and the Spitsbergen stock numbers “only in the tens” where there may have been as many as 24,000.

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