Blue whale. Image credit: NOAA.
Blue whale distribution.
The blue whale, Balaenoptera musculus (Linnaeus 1758), is a widely distributed species of baleen whale. Blue whales in the Northern Hemisphere are generally smaller than those in the Southern Ocean. Maximum body length in the North Atlantic was about 88.5 ft (27 m) and the largest blue whale reported from the North Pacific was about 88 ft (26.8 m). Adults in the Antarctic can reach a maximum body length of about 108 ft (33 m) and can weigh more than 330,000 lb (150,000 kg).
As is true of other baleen whale species, female blue whales are somewhat larger than males. Blue whales are identified by the following characteristics: a long-body and comparatively slender shape; a broad, flat "rostrum" when viewed from above; a proportionately smaller dorsal fin than other baleen whales; and a mottled gray color pattern that appears light blue when seen through the water.
Subspecies of the blue whale include:
The primary and preferred diet of blue whales is krill (euphausiids). In the North Atlantic, blue whales feed on two main euphausiid species: Thysanoëssa inermis and Meganyctiphanes norvegica. In addition, T. raschii and M. norvegica have been recorded as important food sources of blue whales in the Gulf of St. Lawrence. In the North Pacific, blue whales prey mainly on Euphausia pacifica and secondarily on T. spinifera. While other prey species, including fish and copepods, have been mentioned in the scientific literature, these are not likely to contribute significantly to the diet of blue whales.
Scientists have yet to discern many details regarding the life history of the blue whale. The best available science suggests the gestation period is approximately 10–12 months and that blue whale calves are nursed for about 6–7 months. Most reproductive activity, including births and mating, takes place during the winter. Weaning probably occurs on, or en route to, summer feeding areas. The average calving interval is probably two to three years. The age of sexual maturity is thought to be 5–15 years. There are no known differences in the reproductive biology of blue whales in the North Pacific and North Atlantic oceans.
Blue whales inhabit sub-polar to sub-tropical latitudes. Poleward movements in spring allow the whales to take advantage of high zooplankton production in summer. Movement towards the subtropics in the fall allows blue whales to reduce their energy expenditure while fasting, avoid ice entrapment in some areas, and engage in reproductive activities in warmer waters of lower latitudes. Although the species is often found in coastal waters, blue whales are thought to occur generally more offshore than humpback whales, for example.
Blue whales are found in oceans worldwide and are separated into populations by ocean basin in the North Atlantic, North Pacific, and Southern Hemisphere. They follow a seasonal migration pattern between summering and wintering areas, but some evidence suggests that individuals remain in certain areas year-round. The extent of knowledge concerning distribution and movement varies with area and migratory routes are not well known but, in general, distribution is driven largely by food requirements. For example, in the Gulf of St. Lawrence blue whales prefer deep waters where krill is concentrated.
The overall distribution of blue whales in the North Atlantic extends from the subtropics to Baffin Bay and the Greenland Sea. Blue whales are most frequently sighted in the waters off eastern Canada, with the majority of recent records from the Gulf of St. Lawrence, where they are present throughout most of the year. They are most common during the summer and fall feeding seasons and typically leave by early winter to avoid ice entrapment. Although they are rare in the shelf waters of the eastern US, occasional sightings of blue whales have been made off Cape Cod, Massachusetts. It is believed this region may represent the current southern limit of the blue whales' feeding range. In addition, there is some evidence to suggest that blue whales occasionally stray into the Gulf of Mexico and the Caribbean, but they are less common in these waters. Some scientists believe blue whales in the North Atlantic occur in relatively discrete feeding populations whereas other evidence suggests blue whales may comprise one "panmictic" population.
The blue whale's range is known to encompass much of the North Pacific Ocean, from Kamchatka to southern Japan in the west, and from the Gulf of Alaska and California south to at least Costa Rica in the east. The species is found primarily south of the Aleutian Islands and the Bering Sea. Whaling and sighting data suggest the existence of at least five subpopulations of blue whales, with an unknown degree of mixing among them:
The International Whaling Commission (IWC), however, formally considers only one management stock for blue whales in the North Pacific.
Northern Indian Ocean
There is a "resident" population of unknown taxonomic status present in the northern Indian Ocean. Blue whales sightings have been reported from the Gulf of Aden, Persian Gulf, Arabian Sea, and across the Bay of Bengal to Burma and the Strait of Malacca. The migratory movements of these whales are unknown.
In the Southern Hemisphere distributions of B. m. intermedia and B. m. brevicauda may be segregated. B. m. intermedia occurs mainly in relatively high latitudes south of the "Antarctic Convergence" and close to the ice edge. B. m. brevicada is typically distributed north of the Antarctic Convergence.
Changes in distribution
There is some evidence which suggests the distribution and migratory patterns of blue whales may have changed in at least four areas: northern Norway, southern Japan, eastern Aleutian Islands, and northern California.
In northern Norway the paucity of sightings during recent surveys along the coast where blue whales were common in the late 1800s and early 1900s, may suggest that the historic distribution has changed. However, it could also indicate depletion of the population by whaling.
In the western North Pacific, the lack of blue whales off southern Japan today may also suggest that the distribution of these animals has changed or that the animals of this region have been extirpated. South of the eastern Aleutian Islands (160°W and 180°W), relatively large concentrations of blue whales were documented in the 1970s but the species appears rare there today, suggesting that illegal and unreported whaling depleted the population.
Off northern California (e.g., Farallon Islands, Moss Landing, and Trinidad), the recent appearance of numerous blue whales is noteworthy in light of their rarity in these regions prior to the late 1970s. Such changes in distribution may reflect a shift in feeding from the more offshore euphausiid, Euphausia pacifica, to the primarily "neritic" euphausiid, Thysanoëssa spinifera.
Blue whales were drastically depleted by commercial whaling activities worldwide, in the 19th and first half of the 20th centuries. In the Southern Hemisphere, pre-exploitation population estimates range from 150,000 to 210,000 whales; recent abundance estimates range between 400 and 1,400 whales. In the North Pacific, pre-exploitation population size is estimated as approximately 4,900 blue whales, whereas the current population estimate is a minimum of 3,300 blue whales. In the North Atlantic, estimates for the entire basin are considered unreliable, but range from 1,100 to 1,500 blue whales pre-exploitation, and 100 to 555 whales currently.
|Global abundance estimates for blue whales by ocean basin (IWC, 2007)|
|Southern Hemisphere||North Pacific||North Atlantic|
Western North Atlantic
Based on data from individuals found only in the Gulf of St. Lawrence, the current minimum population estimate for the western North Atlantic stock is 308 whales. It has estimated that the blue whale population in the western North Atlantic during the late 1960s and early 1970s was in the very low hundreds at most, and according to its most recent stock assessment, NMFS has no evidence to refute this estimate. More than 320 individual blue whales have been photo-identified in the Gulf of St. Lawrence and the total number of photo-identified individuals for eastern Canada and New England is 352 as of Fall 1997.
According to the most recent NMFS stock assessment, there is insufficient data to determine population trends for this species.
There are no accurate estimates of pre-exploitation population sizes available for the western North Atlantic; however, several rough estimates have been made by scientists. It has been approximated that there were more than 1,100 and as many as 1,500 blue whales in eastern Canadian waters.
Eastern North Atlantic
There is no current estimate for the number of blue whales in eastern North Atlantic waters. However, some data have been collected for blue whales in Icelandic waters. As of autumn 1997, 32 individuals had been photo-identified in Icelandic waters. Additional studies have suggested that the population in Icelandic and neighboring waters may be in the high hundreds or greater than 1,000.
Sightings data off the west and southwest coasts of Iceland suggest the population has been increasing at about five percent per year since the late 1960s.
Despite these differences in pre-exploitation estimates and the lack of estimates for current population abundance, it is clear that blue whale stocks in the western, eastern, and central North Atlantic were severely depleted by the time that legal protection was introduced in 1955.
Western North Pacific
There is insufficient data to evaluate the current abundance or population trends of blue whale stocks in the western North Pacific, but blue whales in Hawaiian waters are considered extremely rare.
Eastern North Pacific
Blue whale abundance in the eastern Pacific is about 1,700. Along the California coast blue whale abundance has been increasing during the past two decades. The magnitude of this apparent increase is too large to be accounted for by population growth alone and, therefore, it is assumed that a shift in distribution has occurred. Although the population in the North Pacific is expected to have grown since protection began in 1966, the possibility of continued unauthorized takes, incidental ship strikes and mortality, and serious injury in fishing gear makes this uncertain. Also, the evident scarcity of blue whales in areas of former abundance (e.g., Gulf of Alaska and near the Aleutian Islands) suggests that the increasing trend does not apply to the species' entire range in the eastern North Pacific.
The primary threats currently facing blue whales are:
Additional threats that have not been documented but could potentially affect these populations include:
Although whaling substantially reduced blue whale populations worldwide during the first half of the 20th century, whaling is no longer considered a threat today.
A primary threat to blue whales is mortality and serious injury caused by ship strikes. In the eastern North Pacific, ship strikes were implicated in the deaths of blues whales in 1980, 1986, 1987, 1993 and 2002. The average number of blue whale mortalities in California attributed to ship strikes was 0.2 per year from 1991–1995 and from 1998–2002. In September 2007, three blue whale mortalities were confirmed to be caused by ship strikes in the Santa Barbara Channel off Southern California. These deaths were part of a larger Unusual Mortality Event, declared by the Working Group on Marine Mammal Unusual Mortality Events. In the western North Atlantic, at least nine percent of the whales in the Gulf of St. Lawrence have injuries or scars attributed to contact with ships. This area has a relatively high risk of ship strikes because the St. Lawrence Seaway has heavy ship traffic during the time of year when blue whales are relatively abundant. In March 1998, a dead 20 m (66ft) male blue whale was brought into Rhode Island waters on the bow of a tanker. The cause of death was determined to be ship strike. There are no records of ship strikes for blue whales in the western North Pacific but mortalities caused by ship strikes have likely occurred without being reported.
Despite the lack of observed fisheries interactions in the last decade, incidental take in fisheries threaten blue whales for two reasons. First, past records of entanglements suggest that interaction with fishing gear may affect blue whales. Second, entanglement rates may be underestimated because blue whales may break through or carry away fishing gear, perhaps suffering unrecorded subsequent mortalities or serious injuries. It is also likely that stranding data underestimate the number of whales killed by fishing gear, because most whales do not drift far enough to strand on beaches or to be detected floating in the nearshore corridor. Although direct observation of mortality is rare, at least two documented cases of dead blue whales are apparently from the effects of entanglement in fishing gear (one in 1987 off Stellwagen Bank and the other in the 1990s in the Gulf of St. Lawrence).
Little is known about natural mortality of blue whales in the North Atlantic. Ice entrapment is known to injure and kill some blue whales, particularly along the southwest coast of Newfoundland during late winter and early spring. In the Gulf of St. Lawrence, scarring on the dorsal surface of some whales is thought to be from contact with ice. Also, two blue whales bore rake-like markings assumed to be from the teeth of killer whales (Orcinus orca). However, no direct evidence of predation on blue whales has been reported in this area.
A well-documented observation of killer whales attacking a blue whale off Baja California proves that blue whales are at least occasionally vulnerable to these predators. A high proportion of the blue whales in the Gulf of California bear injuries or rake-like scars that are the result of encounters with killer whales, although the extent to which such attacks are fatal is unknown. Unlike in the western North Atlantic, injury or suffocation from ice entrapment is not known to affect blue whales in the North Pacific.
Other potential non-primary threats
Anthropogenic noise, competition for prey resources, habitat degradation, and vessel disturbance are additional concerns; however, there is little evidence available to describe or quantify the impacts of these threats on blue whales. For example, while anthropogenic noise is known to be a threat for other cetaceans little is known about whether, or how, vessel noise affects blue whales. Competition for prey resources is possible between blue whales and other sympatric whale species but there is no direct evidence for interspecific competition and it is unlikely blue whales compete with humans for prey resources. Habitat degradation (e.g., chemical pollution) has occurred in some areas of the North Atlantic (e.g., St. Lawrence River) but the impacts of this degradation have not been proven to affect blue whales and are understudied. Lastly, the effect of vessel disturbance (e.g., whalewatching boats) is of concern but there is no direct evidence to demonstrate that persistent close approaches by tour boats has a negative effect on blue whales.
Blue whales were hunted as early as the 19th century but were not intensively hunted until the turn of the 20th century. From the early 1900s to the mid-1960s blue whales were hunted in all the world's oceans and their populations significantly reduced. At least 9,500 blue whales were taken by commercial whalers throughout the North Pacific between 1910 and 1965 and at least 11,000 were taken in the North Atlantic from the late 19th to mid 20th centuries.
In 1966, the IWC banned commercial whaling for blue whales and no whaling (either "aboriginal subsistence" or commercial) occurs at present.
Illegal whaling for blue whales has been documented or is likely to have occurred in the North Pacific, North Atlantic, and Southern Hemisphere. A small number of illegal kills of blue whales have been documented in the Northern Atlantic off Canada and Spain, and in the eastern North Atlantic. Blue whales were also killed in the Southern Hemisphere by the Soviet Union after 1966. Some illegal whaling by the USSR also occurred in the North Pacific; it is likely that blue whales were among the species taken by these operations, but the extent is not known.
No aboriginal whaling for blue whales is known to exist. Aboriginal hunters who target whales off the coast of the Lesser Antilles and Greenland do not target blue whales. Further, Norwegian whaling operations target only minke whales, and the commercial whaling stations in Iceland, Spain, and the Portugese islands of the Azores and Madeira remain officially closed.