Blue whale

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.

Blue whales occur worldwide in all major oceans except the Arctic^1,2. 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.

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

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 California^5,6.

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 Lanka^5,6,8,9.  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 20^th 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.

Blue whales and whale watching

Please see the IWC Whalewatching Handbook