Bowhead whale

Distribution

The bowhead is the only full-time arctic resident among the baleen whales. It is a species that lives in close association with sea-ice on a year-round basis and it has a patchy circumpolar distribution that varies seasonally as the ice extent contracts and expands. Bowhead whales usually remain close to the southern boundary of winter ice; as the ice cover recedes the whales in most populations move northwards and disperse in pack ice areas during spring, summer and autumn.

There are five defined stocks of bowhead whales in the Arctic. One is the Spitsbergen stock, which is confined to the Northeast Atlantic where they have been observed from the Greenland Sea between Greenland and Svalbard, eastward past the Barents and Kara Seas, well into the Russian Arctic.

General ecology

Bowheads number over 10,000 individuals in the Alaskan stock, and in the Eastern Canadian Arctic and West Greenland there is another stock that numbers approximately 5,000 individuals, while other bowhead stocks are much smaller, consisting of hundreds or tens of animals. In Svalbard sightings of this species are rare. No estimate is available, but it is known that the population is very small, probably less than 100 animals.

The bowhead’s adaptations to living in the Arctic are extensive. It has the greatest development of the blubber layer seen among whales (up to 50 cm thick), and it has complex blood circulatory mechanisms for conserving heat. Their raised nostrils and lack of dorsal fin are thought to be adaptations to living in ice. The bowhead is a very slow swimmer compared to the more stream-lined baleen whales.

Bowheads also have extreme development of baleen (reaching lengths of four to five metres), which is a series of filtering plates that hang in the mouth from the top jaw. These feeding plates are adapted to filter small prey out of large volumes of water. The muscular tongue helps clear the prey off of the hairy inside surface of the plates and help to direct food into the throat. Bowheads are assumed to obtain most of the food necessary for annual growth and maintenance, during the summer months, although some feeding almost certainly takes place outside the summering areas. They feed throughout the water column, skimming the surface, but also feeding near the sea floor.

Bowhead whales have a remarkable vocal repertoire, which they use frequently, suggesting that sound serves an important function in their lives. It is thought that bowheads communicate acoustically with one another over very long distances.

Bowhead whales, similar to many other cetaceans, show some sex and age segregation. Mothers and calves migrate independently of male groups and often feed in different areas as well during the summer months.

This species appears to be ”loosely social”, travelling in very small groups most of the time and without long-term associations outside the mother-calf pair between individual animals. It has been suggested that there is co-ordination among activities of individuals separated by considerable distances (up to 100s of kilometres), so long-distance communication of some sort is almost certainly taking place. Bowheads feed mainly on planktonic crustaceans, including copepods and euphausiids (ranging in size from three to 30 mm), although they will also take small schooling fishes such as polar cod in addition.

Killer whales are their only known natural predator.

Life history and reproduction

Bowhead whales are the only baleen whale species that gives birth to their calves in arctic waters. In the Canadian Arctic (and likely elsewhere too) they mate and calve during their northward spring migration and spend the summer feeding and raising their young. Mating groups often consist of a female and several males, similar to right whales. A lot of boisterous activities take place during mating, such as breaching and fluke slapping, but it is not clear whether most of this is competitive or whether some form of cooperation takes place among males during mating. Gestation lasts 13–14 months, after which the single calf is born.

Calves remain with their mothers during the summer and subsequent autumn migration and probably through the winter as well. This species has a long inter-calving interval of about four years. Bowheads have a delayed age at first reproduction; they do not commence breeding before they are approximately 25 years old.

They grow slowly, reaching final body size when they are between 40 and 50 years old and they are thought to live to ages in excess of 200 years.

Management status and monitoring

Peoples of the North have hunted bowheads along their migratory routes for centuries. They were also the first arctic whale species to be harvested intensively by commercial whalers. Because they are slow swimmers, and because they are so fat that they float when killed, early whalers that were operating out of small man-driven boats working off sailing ships targeted them. The Spitsbergenstock was the first to receive the attention of these early whalers and this species was the first of the Great Whales to be severely depleted. During intensive hunting that took place primarily from the 17^th century up into the 19^th century, bowheads were driven to near extinction throughout their range.

Today, native people of Alaska, Canada and Greenland hunt bowheads based on quota systems; the species is showing positive signs of recovery throughout much of its range. Bowheads are protected in Svalbard. A bowhead whale from the Spitsbergen stock was satellite tagged in the spring of 2010 in the Fram Strait at approximately 80 ^oN. This animal travelled about 1,000 km south and west through the summer and moved back north to the tagging latitude again by midwinter. This seasonal pattern is ‘up-side-down’ compared to other population of this species that travel north in summer and south in winter. But, the pattern seen for this one animal is exactly what was described by whalers hundreds of years ago for the animals in this region.

Passive acoustic listening devices placed in the Fram Strait in recent years have confirmed that this is a routine wintering site for this population, and the frequency of calling and the number of differ calls provide glimmers of hope that this population might be increasing. This little network of acoustic monitoring devices is being expanded in hopes of learning more about the abundance, distribution and behaviour of bowheads in the Svalbard area.