Zoology

Migratory Patterns of the Humpback Whale



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The migratory patterns of the humpback whale (Megaptera noviaeangliae) span some of the greatest distances of any species, covering on average some 25,000 kilometres every year from the tropics to polar waters and back again. One population of humpback whales winters off the coast of Costa Rica but travels all the way to Antarctica in the summer, making it the longest documented migratory journey of any mammal.

Migratory populations remain almost entirely distinct from one another, returning to the same coastal and polar regions year after year. In the northern hemisphere, migration is concentrated along the coastal waters of North America and Asia and to a much lesser extent Europe. These humpback populations summer along the Aleutian Islands, Kamchatka Peninsula, Canadian Maritime provinces and Gulf of St. Lawrence, Greenland, Iceland, and along the Norwegian fjords; and winter near California, the outlying islands of the Caribbean Sea, Japan, the Philippines, and even Hawai'i. In the southern hemisphere, humpbacks summer at Antarctica and winter around the south Pacific islands, South Africa, Brazil, Chile, Costa Rica (crossing the equator by way of the Humboldt current), as well as two separate winter populations off the east and west coasts of Australia.

Courtship, mating, and raising the young occur during the winter months. Poleward migration does not usually occur until after the young have been weaned, approximately six months after birth. During this entire time the adult humpbacks never feed, living entirely off blubber stored up during the summer polar feedings on krill and small schooling fish such as herring and mackerel. Humpback gestation lasts 11.5 months, meaning that there is an entire migratory cycle between mating and giving birth.

While short-lived social groups come together to compete for females and to feed, migration is largely undertaken individually except for mothers and calves, which may continue the weaning process during part of the migration. Although humpback adults rarely interact socially with the other mammalian species sharing their part of the ocean, humpback calves have been observed playing with dolphin calves.

Not all humpback whales migrate. At least one population of humpback whales, living in the Arabian Sea and Indian Ocean, remains there year-round. This may be due to the route poleward being blocked by the Arabian peninsula and Indian subcontinent.

Tracking migrating humpback whales has been a challenge. The earliest attempted records go back to early whaling expeditions, but these tended to focus on where and when large concentrations of humpbacks could be expected. Later tracking attempts, including an ongoing photographic catalogue of all known humpback whales in the north Atlantic, used tail fluke markings, unique as a fingerprint, to identify individual humpbacks. Still others tagged individuals with radio receivers, to be tracked with triangulation and later with satellite telemetry.

While still a species on the IUCN Red List of Threatened Species, the humpback whale is no longer technically red-listed. The International Union for the Conservation of Nature and Natural Resources has recently reclassified the humpback whale as being of least concern (LC 2.3), a category reserved for species which have been evaluated but which are not (or no longer) deemed at risk, and thus shared by such species as the rock pigeon and the common juniper. Recent conservation efforts along its entire migratory route have resulted in a significant increase in humpback numbers to at least 80,000, up from an estimated low of around 10,000 in the 1960s, with perhaps only 700 remaining in the north Atlantic. However, the population before commercial whaling is estimated to have been upward of 125,000, and the newly revised status is being used by Japan as a major argument to overturn the current moratorium on whaling.

Identifying migratory routes is of crucial importance to preserving the species. Quite apart from the threat due to hunting and even non-whaling fishing nets, many marine species are particularly vulnerable to sound pollution such as that produced by most ocean transports, and humpback whales are no exception. In fact, deafness in a humpback may as well be death to that humpback. Although echolocation, the ability to locate prey through sonar, has never been proven in a baleen whale species, whalesong is believed to play a strong role in navigation, as well as in mating rituals, territoriality, cooperative 'bubble net' feeding, and other socialization. How the various aspects of whalesong function remains as yet unknown. However, what is certain is that to disrupt whalesong is to disrupt the patterns of life itself. Thus accurate identification of migratory routes may in future affect the location of standard shipping lanes.

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