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Ballena azul Balaenoptera musculus

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The largest animals ever to have lived on the planet (surpassing even dinosaurs), blue whales inspire awe and wonder with all the records they break:  The largest blue whale ever recorded was 33 m long; a blue whale’s heart is the size of a small car; a child could crawl through a blue whale’s arteries; and blue whales produce the loudest sound on earth – even if it is too low in frequency for humans to hear it.  There are at least five recognized sub-species of blue whale that occur in different ocean basins. These are

  • B. m. musculus, Northern blue whale
  • B. m. intermedia, Antarctic blue whale
  • B. m. indica,  Northern Indian Ocean blue whale
  • B. m. brevicauda, Pygmy blue whale
  • B. m. un-named subsp., Chilean blue whale.

Pygmy blue whales are smaller and are generally restricted to the Southern Hemisphere including the Indian Ocean. “True” blue whales refer to the larger musculus, intermedia, and indica subspecies. Antarctic blue whales (intermedia) are the largest of the species, but have been severely depleted after decades of whaling.  Blue whales are usually found offshore, and their seasonal migrations and breeding and feeding grounds are generally poorly understood.  However, there are a few places where they can be seen with some regularity during whale watching trips, such as the Gulf of California in Mexico, the Gulf of St. Lawrence in Canada, the California coast of the United States, and Sri Lanka.

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Not to be confused with

Blue whales have the sleek and slender body shape of other rorqual whales like Bryde’s, sei and fin whales.  However, their great size and unique blue mottled colouration should make them easily distinguishable from any other species.  

Distribution

Blue whales occur worldwide in all major oceans except the Arctic1,2. They are also absent from some regional seas such as the Mediterranean, Okhotsk and Bering seas. They are almost never seen off of eastern South America or eastern Australia.  Despite their wide distribution the blue whales are not often encountered, partly due to their reduced numbers, and party because they generally occur in offshore waters, with only a few known coastal breeding and feeding areas. 

Native to the following countries: Angola; Argentina; Australia; Bahamas; Bangladesh; Benin; Bermuda; Brazil; Cameroon; Canada; Cape Verde; Chile; China; Cocos (Keeling) Islands; Colombia; Comoros; Cook Islands; Costa Rica; Côte d'Ivoire; Djibouti; Ecuador; El Salvador; Equatorial Guinea; Eritrea; Falkland Islands (Malvinas); Faroe Islands; France; French Southern Territories; Gabon; Ghana; Gibraltar; Greece; Greenland; Grenada; Guatemala; Iceland; India; Indonesia; Iran, Islamic Republic of; Iraq; Ireland; Japan; Kenya; Madagascar; Malaysia; Maldives; Marshall Islands; Mauritania; Mauritius; Mexico; Morocco; Mozambique; Myanmar; Namibia; New Caledonia; New Zealand; Nicaragua; Nigeria; Northern Mariana Islands; Norway; Oman; Pakistan; Palau; Panama; Peru; Philippines; Pitcairn; Portugal; Russian Federation; Saint Helena, Ascension and Tristan da Cunha (Tristan da Cunha); Saint Pierre and Miquelon; Sao Tomé and Principe; Senegal; Seychelles; Somalia; South Africa; South Georgia and the South Sandwich Islands; Spain; Sri Lanka; Taiwan, Province of China; Tanzania, United Republic of; Thailand; Timor-Leste; Togo; United Kingdom; United States; Uruguay; Western Sahara; Yemen

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Biology and Ecology

Feeding

Throughout their range, blue whales feed predominantly on small shrimp-like crustaceans called krill. While the species of krill may differ from one ocean basin to another, the manner in which the whales feed is the same:  usually lunge feeding  through large swarms of prey, either by coming up directly underneath them with open mouths and throat pleats expanded, or by swimming on their sides with open mouths.  While doing this, blue whales can engulf more than 100 tons of water and krill, then close their mouths and contract their throat pleats, thus forcing water out of their mouths and straining the krill through their baleen plates. 

Social structure, reproduction and growth

Very little is known about blue whale mating and calving, although a few nursing grounds have been identified, for example in the Gulf of California.  Blue whale calves are generally born in the winter after a 10- 12-month gestation period, and remain with their mothers for roughly 8 months, during which they can double in size, gaining up to 90 kg per day from drinking the mother’s rich milk (whale milk is almost more the consistency of cottage cheese with a 30-40%  fat content).  Blue whales are usually seen singly or in pairs, and do not form large mating aggregations like other species, but they may gather in larger numbers where good feeding opportunities are available.  With such a sparse distribution throughout the world’s oceans, blue whales use powerful vocalizations to communicate with each other across long distances.  Blue whales’ calls are generally infrasonic (17-20 Hz), and too low for humans to hear, but at 188 decibels, they are one of the loudest and lowest sounds produced by any animal.1

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Research, threats and conservation status

Individual blue whales can be recognized by the pattern of  mottling on their flanks or sides.  Researchers build up catalogues of individual whales, like those in Loreto Bay, Mexico, that have been monitored for over 25 years3, and to generate population estimates as has been done for blue whales off the coast of California4 and Chile.  Blue whales are also studied through the attachment of satellite tags to track their movements over time and determine which areas are important for them. This approach helped to identify an overlap in blue whale habitat and shipping lanes off the coast of California5,6.

Natural Predators

The only known natural predator of blue whales is the killer whale7. A National Geographic documentary in 1978 showed the hunt and predation of a blue whale calf, but such events are rare: blue whales can usually outswim killer whales and escape any danger.

Human induced threats

While accidental entanglement in fishing gear poses the greatest threat to most other species of whale and dolphin, blue whales, with their great size and strength may be able to break free of gear more easily than other species.  Reports of lethal entanglements for this species are rare, although 12% of blue whales found in eastern Canadian waters carry scarring consistent with fishing gear interaction1. Ship strike is thought to present a greater risk to blue whales, especially in areas where their habitat overlaps with shipping lanes, as it does off the coast of California and Sri Lanka5,6,8,9.  Commercial exploitation of krill and climate change affecting the distribution of krill in various ocean basins could also have a negative impact on blue whales10.

Conservation status

Because a single blue whale yielded so much oil, it was highly prized by whalers, but until the advent of mechanized harpoons and factory ships that were fast enough and large enough to chase down and process blue whales, they had been relatively inaccessible.    The greatest number of blue whales was taken in the first half of the 20th century, with nearly 30,000 whales killed in the 1930-31 season alone.   Over 300,000 blue whales were killed in the Southern Hemisphere alone, and a further 20,000 in the North Atlantic and North Pacific combined.

Blue whales have been protected from hunting by the IWC since 1966, and today some populations appear to be recovering at rates of up to 7%  per year11.  But many populations appear to still be small, and others are difficult to study due to their diffuse distribution in offshore waters.  Globally, the species is listed as Endangered on the IUCN Red List, and under Appendix 1 on the Convention on Migratory Species CMS.  The Antarctic subspecies is listed as Critically Endangered due to the fact that the current estimated population is still less than 1% of its original pre-whaling size12. Pygmy blue whales are considered data deficient on the IUCN Red List, and Northern Indian Ocean blue whales have not been separately assessed, but would most likely also be considered data deficient.

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Blue whales and whale watching

When blue whales can be found, they offer a breath-taking spectacle. Even if they do not engage in the leaps and displays that are typical for humpback or right whales, their sheer size and grace will impress viewers. They are not often the main target of whale watching, and as such, few studies have focused on the potential impacts of whale watching on this species.  However, researchers in Loreto Bay, Mexico have worked closely with national park authorities and whale watch tour operators to promote and implement ‘passive’ boat-based whale watching, a method where boats maintain a distance of at least 100m from whales and turn off their engines while watching them.  This has been proven to have less impact on whales’ behaviour and can result in more prolonged and rewarding views of the whales.  Read more about the Loreto Bay case study here.

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Referencias

Mostrar/Ocultar referencias
  1. Sears, R. & Perrin, W. F. in Encyclopedia of Marine Mammals   (eds W. Perrin, B. Wursig, & J.G.M. Thewissen)  121-124 (Elsevier, 2009).
  2. Branch, T. A. et al. Past and present distribution, densities and movements of blue whales Balaenoptera musculus in the Southern Hemisphere and northern Indian Ocean. Mammal Review 37, 116-175 (2007).
  3. Gendron, D. Population Ecology of the Blue Whales, Balaenoptera musculus, of the Baja California Peninsula PhD thesis, (2002).
  4. Calambokidis, J. & Barlow, J. Abundance of blue and humpback whales in the Eastern North Pacific estimated by capture-recpature and line-transect methods. Marine Mammal Science 20, 63-85, doi:10.1111/j.1748-7692.2004.tb01141.x (2004).
  5. Redfern, J. et al. Assessing the Risk of Ships Striking Large Whales in Marine Spatial Planning. Conservation Biology 27, 292-302 (2013).
  6. Redfern, J. V. et al. Predicting cetacean distributions in data-poor marine ecosystems. Diversity and Distributions, n/a-n/a, doi:10.1111/ddi.12537 (2017).
  7. Jefferson, T. A., Webber, M. A. & Pitman, R. L. Marine Mammals of the World: a Comprehensive Guide to their Identification. Second Edition.  (San Diego: Academic Press, 2015).
  8. de Vos, A., Brownell, R., Tershy, B. & Croll, D. Anthropogenic Threats and Conservation Needs of Blue Whales, Balaenoptera musculus indica, around Sri Lanka. Journal of Marine Biology 2016 (2016).
  9. McKenna, M. F., Calambokidis, J., Oleson, E. M., Laist, D. W. & Goldbogen, J. A. Simultaneous tracking of blue whales and large ships demonstrates limited behavioral responses for avoiding collision. Endangered Species Research 27, 219-232 (2015).
  10. Thomas, P. O., Reeves, R. R. & Brownell, R. L. Status of the world's baleen whales. Marine Mammal Science, doi:10.1111/mms.12281 (2015).
  11. Branch, T. A., Matsuoka, K. & Miyashita, T. Evidence for increases in Antarctic blue whales based on Bayesian modelling Marine Mammal Science 20, 726-754, doi:10.1111/j.1748-7692.2004.tb01190.x (2004).
  12. Branch, T. A., Matsuoka, K. & Miyashita, T. Evidence for increases in Antarctic blue whales based on Bayesian modelling. Marine Mammal Science 20, 726-754 (2004). 

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