Middle Pleistocene – recent
Adult blue whale
Size compared to an average human
|Blue whale range (in blue)|
The blue whale (Balaenoptera musculus) is a marine mammal belonging to the baleen whale parvorder Mysticeti. Reaching a maximum confirmed length of 29.9 metres (98 ft) and weighing up to 199 tonnes (196 long tons; 219 short tons), it is the largest animal known to have ever existed. The blue whale's long and slender body can be various shades of greyish-blue dorsally and somewhat lighter underneath.
The Society for Marine Mammalogy's Committee on Taxonomy currently recognizes four subspecies: B. m. musculus in the North Atlantic and North Pacific, B. m. intermedia in the Southern Ocean, B. m. brevicauda (the pygmy blue whale) in the Indian Ocean and South Pacific Ocean, B. m. indica in the Northern Indian Ocean. There is also a population in the waters off Chile that may constitute a fifth subspecies.
Blue whales are filter feeders; their diet consists almost exclusively of euphausiids (krill). They are generally solitary or gather in small groups and have no well-defined social structure other than mother-calf bonds. The fundamental frequency for blue whale vocalizations ranges from 8 to 25 Hz and the production of vocalizations may vary by region, season, behavior, and time of day.
The blue whale was once abundant in nearly all the Earth's oceans until the end of the 19th century. It was hunted almost to the point of extinction by whalers until the International Whaling Commission banned all blue whale hunting in 1966. The International Union for Conservation of Nature has listed blue whales as endangered as of 2018. It continues to face numerous threats, both man-made (ship strikes, pollution, ocean noise and climate change), and natural (killer whale predation).
Blue whales have long, slender mottled grayish-blue bodies, although they appear blue underwater. The mottling pattern is highly variable. Individuals have a unique pigmentation pattern along the back in the region of the dorsal fin which can be used for the purpose of identification.
Additional distinguishing features of the blue whale include a broad, flat head, which appears U shaped from above; 270–395 entirely black baleen plates on each side of their upper jaw; 60–88 expandable throat pleats; long, slender flippers; a small—up to 33 centimetres (13 in)— falcate dorsal fin positioned far back toward the tail; a thick tail stock; and a massive, slender fluke.
Their pale underside can accumulate a yellowish coat of diatoms, which historically earned them the nickname "sulphur bottom". The blue whale's two blowholes create a tall, columnar spray, which can be seen 9.1–12.2 metres (30–40 ft) above the water's surface.
The blue whale is the largest known animal to have ever existed. The International Whaling Commission (IWC) whaling database reports 88 individuals longer than 30 metres (98 ft), including one of 33 metres (108 ft), but problems with how the measurements were taken suggest that any longer than 30.5 metres (100 ft) are suspect. The Discovery Committee reported lengths up to 31 metres (102 ft); however, the longest scientifically measured individual blue whale was 30 metres (98 ft) from rostrum tip to tail notch. Female blue whales are larger than males. Hydrodynamic models suggest a blue whale could not exceed 108 ft (33 m) because of metabolic and energy constraints. The highest recorded weight for the species is 199 tonnes (196 long tons; 219 short tons).
The average length of sexually mature female blue whales is 22.0 metres (72.1 ft) for Eastern North Pacific blue whales, 24 metres (79 ft) for central and western North Pacific blue whales, 21–24 metres (68–78 ft) for North Atlantic blue whales, 25.4–26.3 metres (83.4–86.3 ft) for Antarctic blue whales, 23.5 metres (77.1 ft) for Chilean blue whales, and 21.3 metres (69.9 ft) for pygmy blue whales.
In the Northern Hemisphere, males weigh an average 100 tonnes (220,000 lb) and females 112 tonnes (247,000 lb). Eastern North Pacific blue whale males average 88.5 tonnes (195,000 lb) and females 100 tonnes (220,000 lb). Antarctic males average 112 tonnes (247,000 lb) and females 130 tonnes (290,000 lb). Pygmy blue whale males average 83.5 tonnes (184,000 lb) to 99 tonnes (218,000 lb). The weight measured of the heart from a stranded North Atlantic blue whale was 0.1985 tonnes (438 lb), the largest known in any animal.
A blue whale's age is most reliably measured using ear plugs. Blue whales secrete earwax (cerumen) throughout their lives, forming long, multilayered plugs. Each deposited light and dark layer (lamina) indicates a switch between fasting during migration and feeding. As one set is laid down per year, the number of layers is an indicator of age. The maximum age of a pygmy blue whale determined this way is 73 years. Before the ear plug aging method, layers in baleen plates were used, however, these wear down and are not as reliable. The blue whale's ovaries form a permanent record of the number of ovulations (or perhaps pregnancies), in the form of corpora albicantia—fibrous masses that are permanent scars and were once used as an indication of age. In a female pygmy blue whale, one corpus albicans is formed on average every 2.6 years.
The genus name, Balaenoptera, means winged whale while the species name, musculus, could mean "muscle" or a diminutive form of "mouse", possibly a pun by Carl Linnaeus when he named the species in Systema Naturae. One of the first published descriptions of a blue whale comes from Robert Sibbald's Phalainologia Nova, after Sibbald found a stranded whale in the estuary of the Firth of Forth, Scotland, in 1692. The name "blue whale" was derived from the Norwegian "blåhval", coined by Svend Foyn shortly after he had perfected the harpoon gun. The Norwegian scientist G. O. Sars adopted it as the common name in 1874.
Blue whales were referred to as 'Sibbald's rorqual', after Robert Sibbald, who first described the species. Herman Melville called the blue whale "sulphur bottom" in his novel Moby Dick because of the accumulation of diatoms creating a yellowish appearance on their pale underside.
Blue whales are rorquals, in the family Balaenopteridae whose extant members include the fin whale (Balaenoptera physalus), sei whale (Balaenoptera borealis), Bryde's whale (Balaenoptera brydei), Eden's whale (Balaenoptera edeni), common minke whale (Balaenoptera acutorostrata), Antarctic minke whale (Balaenoptera bonaerensis), Omura's whale (Balaenoptera omurai), and humpback whale (Megaptera novaeangliae). A 2018 analysis estimates that the Balaenopteridae family diverged from other families in the late Miocene, between 10.48 and 4.98 million years ago. The earliest discovered anatomically modern blue whale is a partial skull fossil found in southern Italy, dating to the Middle Pleistocene, roughly 1.5–1.25 million years ago. The Australian pygmy blue whale diverged during the Last Glacial Maximum. Their more recent divergence has resulted in the subspecies having a relatively low genetic diversity, and New Zealand blue whales have an even lower genetic diversity.
Whole genome sequencing of blue whales and other rorqual species suggests that blue whales are most closely related to sei whales with gray whales as a sister group. This study also found significant gene flow between minke whales and the ancestors of the blue and sei whale. Blue whales also displayed a high degree of genetic variability (i.e., heterozygosity).
There is reference to a "well-documented" humpback-blue whale hybrid in the South Pacific, attributed to marine biologist Dr. Michael Poole, however no published proof exists. Hybridization between blue and fin whales has been documented across multiple ocean basins. The earliest description of a possible hybrid between a blue and fin whale was a 20-metre (65 ft) anomalous female whale with the features of both the blue and the fin whales taken in the North Pacific. In 1983, a 20-metre (65 ft) sexually immature male specimen was taken. Based upon the number of layers in the earwax, the animal's age was determined to be seven years. In 1984, whalers caught a female hybrid between a fin and a blue whale off northwestern Spain. Molecular analyses revealed a blue whale mother and a fin whale father.
In 1986, a 21-metre (70 ft) pregnant female whale was caught. Molecular analyses of the whale showed it was a hybrid between a female blue whale and a male fin whale, and that the fetus had a blue whale father. It was the first example of any cetacean hybridization giving rise to a fertile offspring. Two live blue-fin whale hybrids have since been documented in the Gulf of St. Lawrence, (Canada), and in the Azores, (Portugal). DNA tests done in Iceland on a blue whale killed 7 July 2018 by the Icelandic whaling company Hvalur hf, found that the whale was a hybrid of a fin whale father and a blue whale mother; however, the results are pending independent testing and verification of the samples. Because the International Whaling Commission classified blue whales as a "Protection Stock", trading their meat is illegal, and the kill is an infraction that must be reported. Blue-fin hybrids have been detected from genetic analysis of whale meat samples taken from Japanese markets.
Subspecies and stocks
There are four subspecies of blue whale, recognized by the Society for Marine Mammalogy's Committee on Taxonomy. They are distributed in all major ocean basins, except the Bering Sea and the Arctic Ocean (although they have been sighted near the ice edge in the North Atlantic).
Some of these blue whale subspecies have been further divided, resulting in nine recognized management units:
- Northern subspecies (B. m. musculus) – Eastern North Pacific population
- Northern subspecies (B. m. musculus) – Central and Western North Pacific population
- Northern subspecies (B. m. musculus) – North Atlantic population
- Antarctic subspecies (B. m. intermedia)
- Pygmy subspecies (B. m. brevicauda) – Madagascar population
- Pygmy subspecies (B. m. brevicauda) – Western Australia/Indonesia population
- Pygmy subspecies (B. m. brevicauda) – Eastern Australia/New Zealand population
- Northern Indian Ocean subspecies (B. m. indica)
- Chilean subspecies (B. m. unnamed subsp.)
There are three populations in the Northern subspecies B. m. musculus. It was previously thought that blue whales in the North Pacific belonged to at least five separate populations, however, evidence from movement data derived from satellite tags, photograph-identification, and acoustic data supports two populations in the North Pacific—the Eastern and Central and Western North Pacific populations, with divisions according to acoustic calls being estimated by Monnahan et al.
The third population, the western North Atlantic population, is the only population currently recognized in the North Atlantic. However, it is thought that these whales should be split into eastern and western North Atlantic populations based on photo-identification data.
Despite having the greatest haplotype diversity of any subspecies, the Antarctic subspecies of blue whales is recognized as one stock for management purposes. Additionally, only one blue whale call type has been recorded in the Southern Ocean, and mark-recapture data suggests movements of individuals entirely around the Antarctic.
The pygmy blue whale subspecies, B. m. brevicauda, has three populations corresponding with acoustic populations, including a Madagascar population, an Eastern Australia/New Zealand population, and Western Australia/Indonesia population. Although the Western Australia/Indonesia population and the Eastern Australia/New Zealand population are morphologically similar and not genetically separated, there are no photograph-identification matches between the two populations, and mitochondrial DNA haplotype frequencies suggest a high degree of genetic isolation of the New Zealand population. An acoustic boundary between the Western Australia/Indonesia population and the Eastern Australia/New Zealand population has been identified as the junction of the Indian and Pacific Oceans.
Northern Indian Ocean subspecies
B. m. indica is currently considered a blue whale subspecies. Evidence includes a breeding season asynchronous with Southern Hemisphere blue whales, a distinct Sri Lanka call type, a slightly smaller total length at maturity, and potential year-round residency. However, the Sri Lanka call has not been detected west of the island, and there is another call type in the western North Indian Ocean, off Oman and north-western Madagascar, termed the Oman call suggesting a central Indian Ocean population.
Evidence suggesting that blue whales off the Chilean coast are a separate subspecies includes discrete geographic separation (latitudinally from Antarctic blue whales and longitudinally from pygmy blue whales), a difference in the mean length of mature females, significant genetic differentiation, and unique song types. Chilean blue whales may overlap in the Eastern Tropical Pacific with Antarctica blue whales and Eastern North Pacific blue whales. Chilean blue whales are genetically differentiated from Antarctica blue whales and are unlikely to be interbreeding; however, the genetic differentiation is less with the Eastern North Pacific blue whale; there may be gene flow between hemispheres.
The mechanism behind modern whale migration is debated. Migration may function to reduce parasitism, pathogens, and competition, improve access to prey in the spring and summer, reduce calf predation by orcas and optimize thermoregulation for growth in the winter. For many baleen whales, such as humpback and gray whales, a general migration pattern can be defined as to-and-fro migration between feeding grounds at higher latitudes and breeding habitats at lower latitudes annually. In contrast, blue whales have less specific movement patterns, with substantial evidence of alternative strategies, such as year-round residency, partial or differential migration, and anomalous habits such as feeding on breeding grounds.
Data from individual satellite tagged Eastern North Pacific blue whales suggest leisurely traveling rates of 4 knots (7.4 km/h; 4.6 mph), with a minimum average speed of 1.55 ± 1.68 miles per hour (1.35 ± 1.46 kn) (2.49 ± 2.7 km/h). With additional satellite tagged animals reported mean swim speeds of 108 ± 33.3 kilometres (67.1 ± 20.7 mi)/day. Lagerquist et al. reported mean swim speeds of 108 ± 33.3 kilometres (67.1 ± 20.7 mi)/day, ranging 58–172 kilometres (36–107 mi)/day. A pygmy blue whale tagged off Perth Canyon, Western Australia traveled at mean speeds of 1.7 ± 1.4 miles per hour (2.8 ± 2.2 km/h). The maximum speed of a blue whale while being chased or harassed has been reported at 12–30 miles per hour (20–48 km/h).
Diet and feeding
The blue whale's diet consists almost exclusively of euphausiids (krill). Blue whales have been observed near Magdalena Bay (along the western coast of Baja California, Mexico) feeding on pelagic red crabs. However, recent observations or fecal samples have not confirmed this. Other accidental or opportunistic consumption of copepods and amphipods have been documented. Blue whales feed on krill at the surface and depths greater than 100 metres (328 ft), following their prey's diel (24 hour) vertical migration through the water column.
Blue whales capture krill through lunge feeding, a bulk filter-feeding strategy that involves accelerating toward a prey patch at high speeds, opening the mouth 80–90°, and inverting the tongue, creating a large sac. This allows them to engulf a large volume of krill-laden water, up to 220 tonnes (220 long tons; 240 short tons) of water at one time. They squeeze the water out through their baleen plates with pressure from the ventral pouch and tongue, and they swallow the remaining krill. Blue whales have been recorded making 180° rolls during lunge-feeding, allowing them to engulf krill patches while inverted. They rolled while searching for prey between lunges, which has been hypothesized as allowing them to process the prey field visually to find the densest prey patches.
The Eastern North Pacific population of blue whales has been well studied. The greatest dive depth reported from tagged blue whales was 315 metres (1,033 ft). Their theoretical aerobic dive limit was estimated at 31.2 minutes, however, the longest dive duration measured was 15.2 minutes. The deepest confirmed dive from a pygmy blue whale was 510 metres (1,660 ft).
Blue whales maximize the intake of energy by increasing the number of lunges they make during a dive while targeting dense krill patches. This allows them to acquire the energy necessary for sustaining basic metabolic maintenance costs while storing additional energy necessary for migration and reproduction. Because of the high cost of lunge feeding, it has been estimated that blue whales must target densities greater than 100 krill/m3. They can consume 34,776–1,912,680 kilojoules (8,312–457,141 kcal) from one mouthful of krill, which can provide up to 240 times more energy than used in a single lunge. Energetic models have estimated the daily prey biomass requirement for an average-sized blue whale is 1,120 ± 359 kilograms (2,469 ± 791 lb) krill. Blue whales may accidentally eat small fish that are in the swarms of krill (although this is rare).
Parasites and epibiotics
Except diatoms (Cocconeis ceticola), which can create a yellowish sheen on individuals and remoras (which feed on sloughed skin), external parasites and epibiotics are rare on blue whales. They include the stalked barnacle Conchoderma auritum and acorn barnacle Coronula reginae; the former often attaches to the latter or to the baleen plates; the pseudo-stalked barnacle Xenobalanus globicipitis; the whale louse Cyamus balaenopterae; and the ciliate Haematophagus, which is very common in the baleen plates.
Reproduction and birth
Using the number of earwax lamina deposited in the earplug and the development of sexual organs from dead whales, it has been determined blue whales reach sexual maturity at about 10 years of age and at an average length of 23.5 metres (77 ft) for female Antarctic blue whales. Another method for determining age at sexual maturity involves measurements of testosterone from the baleen of male blue whales. Testosterone concentrations measured from baleen suggest the age at sexual maturity for one blue whale was nine years. Male pygmy blue whales averaged 18.7 metres (61.4 ft) at sexual maturity. Female pygmy blue whales are 21.0–21.7 metres (68.9–71.2 ft) in length and roughly 10 years old at the age of sexual maturity.
Blue whales exhibit no well-defined social structure other than mother-calf bonds from birth until weaning. They are generally solitary or found in small groups. Little is known about mating behavior, or breeding and birthing areas. As a traveling pair, a male blue whale typically trails a female, and is generally successful at repelling an intruder male after a short and vigorous battle. Blue whale anatomy, specifically a small testis-to-body weight ratio and documented visual observations of a second male joining the traveling pair, suggest a polygynous, antagonistic male-male competition strategy. Mating is thought to occur in the fall and throughout winter.
Female blue whales give birth every two to three years, depending on body condition and lactation period. Pygmy blue whales were estimated to give birth every 2.6 years (95% CI=2.2–3.0). Pregnant females gain roughly four percent of their body weight daily, amounting to 60% of their overall body weight throughout summer foraging periods. Gestation lasts 10–11 months. No records of natural births are known, although a blue whale that ended up in Trincomalee harbor gave birth to a calf before being towed back to sea the following day.
For Antarctic blue whales, a single calf is born at 7.0 metres (23 ft) in length and weighs 2.8–3 tonnes (2.8–3.0 long tons; 3.1–3.3 short tons). There is a six to eight month weaning period until the calf is 16 metres (53 ft) in length.
Blue whale milk is roughly (g/100 g) 45–48 water, 39–41 fat, 11–12 protein, 7.4 carbohydrates and 1.3 sugar, thus containing 12 times more fat than whole milk from cows. The amount of milk transferred from mother to calf has not been measured. Blue whale milk contains 18 megajoules (MJ) per kg, which is roughly 4,302 Kilocalorie/kg. Blue whale calves gain roughly 37,500 pounds (17,000 kg) during the weaning period. Estimates suggest that because calves require 2–4 kilograms (4.4–8.8 lb) milk per kg of mass gain, blue whales likely produce 220 kilograms (490 lb) of milk per day (ranging from 110 to 320 kilograms (240 to 710 lb) of milk per day). The first video of a calf thought to be nursing was filmed in New Zealand in 2016.
There are no direct measurements of the hearing sensitivity of blue whales. Hearing predictions are inferred from anatomical studies, vocalization ranges, and behavioral responses to sound. Blue whale inner ears appear well adapted for detecting low-frequency sounds. Their vocalizations are also predominantly low frequency; thus, their hearing is presumably best at detecting those frequency ranges. Southall et al. estimated the hearing range of cetaceans to extend from approximately 7 Hz to 22 kHz.
Blue whale vocalizations are among the loudest and lowest frequency sounds made by any animal. The source level of blue whales off Chile in the 14 to 222 Hz band were estimated to be 188 dB re 1 μPa at 1 m, 189 dB re 1 μPa at 1 m for Antarctic blue whales, and 174 dB re 1 μPa at 1 m for pygmy blue whales.
The fundamental frequency for blue whale vocalizations ranges from 8 to 25 Hz. Blue whale song types were divided initially into nine song types, although ongoing research suggests there are at least 13 song types. The correlation between song types and genetic subdivisions is unknown, but song types are used currently as the primary method of separating blue whale populations because they are stable in shape over multiple decades for each region. The characteristics of specific call types vary with respect to fundamental frequency, bandwidth, and duration, among others. The production of vocalizations may vary by region, season, behavior, and time of day. The purpose of vocalization is unknown, but songs produced only by males appear to have a sexually related purpose, while both sexes produce "D-calls" and other non-repeating calls during feeding.
Possible reasons for calling include:
- maintenance of inter-individual distance
- species and individual recognition
- contextual information transmission (for example, feeding, alarm, courtship)
- maintenance of social organization (for example, contact calls between females and males)
- location of topographic features,
- location of prey resources.
Vocalizations produced by the Eastern North Pacific population have been well studied. This population produces long-duration, low frequency pulses ("A") and tonal calls ("B"), upswept tones that precede type B calls ("C"), moderate-duration downswept tones ("D"), and variable amplitude-modulated and frequency-modulated sounds. A and B calls are often produced in repeated co-occurring sequences as song only by males, suggesting a reproductive function. D calls are produced by both sexes during social interactions while foraging and may be considered multi-purpose contact calls. Because the calls have also been recorded from blue whale trios from in a what was considered to be a reproductive context, it has been suggested recently that this call has different functions. The blue whale call recorded off Sri Lanka is a three‐unit phrase. The first unit is a pulsive call ranging from 19.8 to 43.5 Hz, lasting 17.9 ± 5.2 seconds. The second unit is an FM upsweep from 55.9 to 72.4 Hz lasting 13.8 ± 1.1 seconds. The final unit is a long (28.5 ± 1.6 s) tone that sweeps from 108 to 104.7 Hz. The blue whale call recorded off Madagascar, a two‐unit phrase, starts with 5–7 pulses with a center frequency of 35.1 ± 0.7 Hz and duration of 4.4 ± 0.5 seconds followed by a 35 ± 0 Hz tone lasting 10.9 ± 1.1 seconds. In the Southern Ocean, blue whales' calls last roughly 18 seconds and consist of a 9-second-long, 27 Hz tone, followed by a 1-second downsweep to 19 Hz, and another downsweep to 18 Hz. They also produce short, 1–4 second-duration, frequency-modulated calls ranging in frequency between 80 and 38 Hz.
At least seven blue whale song types have been shifting linearly downward in tonal frequency over time, though at different rates. The Eastern North Pacific blue whale tonal frequency is 31% lower than it was in the early 1960s. The frequency of pygmy blue whales in the Antarctic has decreased at a rate of a few tenths of a hertz per year starting in 2002. One hypothesis is that as blue whale populations recover from whaling, this is increasing sexual selection pressure (i.e., a lower frequency indicates a larger body size).
Anthropogenic (man-made) threats
- Ship strikes: Ship strikes are a significant mortality factor for blue whales, especially off the U.S. West Coast, which has some of the greatest densities of commercial ship traffic in the world. Nine blue whales were known to have been killed and one seriously injured by ship strikes between 2007 and 2010 off California. The five deaths in 2007 were considered an unusual mortality event, as defined under the Marine Mammal Protection Act. Lethal ship strikes are also a problem in Sri Lankan waters, where their habitat overlaps with one of the world's busiest shipping lanes. Ship strikes killed eleven blue whales between 2010 and 2012 around Sri Lanka, and at least two in 2014. Ship strikes killed two Chilean blue whales in recent years off southern Chile. Possible measures for reducing future ship strikes include better predictive models of whale distribution, changes in shipping lanes, vessel speed reductions, and seasonal and dynamic management of shipping lanes.
- Entanglement: Few cases of blue whale entanglement in commercial fishing gear have been documented. The first report in the U.S. occurred off California in 2015, reportedly some type of deep-water trap/pot fishery. Three more entanglement cases were reported in 2016. In Sri Lanka, a blue whale was documented with a net wrapped through its mouth, along the sides of its body, and wound around its tail. There are also non-lethal effects from entanglements, including stress, which decreases an individual's reproductive success or reduces life span. Injuries from entanglements may weaken individuals, making them more vulnerable to other causes of mortality.
- Ocean noise: Increasing anthropogenic underwater noise changes the acoustic environment and impacts blue whales. In addition to masking blue whale communication ranges, exposure to anthropogenic sound can cause a number of behavioral responses. For baleen whales, these responses range from mild to severe, and have been shown to cause immediate changes in swimming and dive behavior intervals, disruptions of foraging, resting, and socializing, habitat displacement, hearing loss; and habituation, in addition to exposure to noise from commercial shipping and seismic surveys as a part of oil and gas exploration. Blue whales in the Southern California Bight decreased calling in the presence of mid-frequency active (MFA) sonar. Exposure to simulated MFA sonar interrupted blue whale feeding, especially when animals were in the deep feeding mode, although the responses depended heavily on the animal's behavioral state at the time of exposure.
- Pollutants: The potential impacts of pollutants on blue whales is unknown. However, because blue whales feed low on the food chain, there is a lesser chance for bioaccumulation of organic chemical contaminants. Analysis of the earwax of a male blue whale killed by a collision with a ship off the coast of California showed contaminants like pesticides, flame retardants, and mercury. Reconstructed persistent organic pollutant (POP) profiles suggested that a substantial maternal transfer occurred during gestation and/or lactation. Male blue whales in the Gulf of St. Lawrence, Canada were found to have higher concentrations of PCBs, dichlorodiphenyltrichloroethane (DDT), metabolites, and several other organochlorine compounds relative to females, reflecting maternal transfer of these persistent contaminants from females into young.
- Plastics and microplastics: The impact of plastic and microplastic ingestion on blue whales is unknown. Baleen whales are exposed to plastic ingestion as a result of filter-feeding activity. Microplastics can also be a significant source of persistent organic pollutants, as well as polyethylene, polypropylene and, particularly, phthalates, which are potential endocrine disruptors and can affect population viability.
- Oil: Whales can inhale, ingest, or absorb petroleum compounds or dispersants, which can injure their respiratory or gastrointestinal tracts, or affect liver or kidney functions. While ingestion is a risk for whales, a 2019 study found oil did not foul baleen, and instead was easily rinsed from baleen by flowing water.
- Climate change: Current climate change projections predict blue whale habitat will decrease significantly. Additionally, warming oceans may affect krill availability in a number of ways, including vertical distribution because of a deepening of the thermocline and increasing stratification of the water column and poleward shifts resulting from the contraction of favorable habitat and changes to coastal upwelling. Ocean acidification may adversely affect blue whale prey, as krill embryonic development, hatch rates, and post-larval metabolic physiology.
- Predation: The only known natural threat to blue whales is the killer whale (Orcinua orca), although the rate of fatal attacks by killer whales is unknown. Photograph-identification studies of blue whales have estimated that a high proportion of the individuals in the Gulf of California have rake-like scars, indicative of encounters with killer whales. Off southeastern Australia, 3.7% of blue whales photographed had rake marks and 42.1% of photographed pygmy blue whales off western Australia had rake marks. Documented predation by killer whales has been rare. A blue whale mother and calf were first observed being chased at high speeds by killer whales off southeastern Australia. The first documented attack occurred in 1977 off southwestern Baja California, Mexico, but the injured whale escaped after five hours. Four more blue whales were documented as being chased by a group of killer whales between 1982 and 2003. The first documented predation event by killer whales occurred September 2003, when a group of killer whales in the Eastern Tropical Pacific was encountered feeding on a recently killed blue whale calf. In March 2014, a commercial whale watch boat operator recorded an incident involving a group of transient killer whales harassing a blue whale in Monterey Bay. A similar incident was recorded by a drone in Monterey Bay in May 2017. In both cases the blue whale fled and escaped. A second documented kill occurred in May 2019 off the south coast of Western Australia when killer whales attacked, killed, and ate a sub-adult blue whale.
- Competition: There is little to no direct evidence of interspecific competition between blue whales and other baleen whale species. Surveys using tagging, line-transect surveys, hydroacoustic surveys, and net sampling have found that despite the overlap with blue whales and other baleen whales, there appears to be niche partitioning in space and/or time and selection of prey species. In the Southern Ocean, baleen whales have been found to preferentially feed on Antarctic krill of specific sizes, which would result in reduced interspecific competition.
Blue whales were initially difficult to hunt because of their size and speed. Large-scale takes did not begin until 1864, when the Norwegian Svend Foyn invented the exploding harpoon gun which could be used on steam and diesel-powered ships. Blue whale takes peaked in 1931 when over 29,000 blue whales were killed. The International Whaling Commission banned all hunting of blue whales in 1966 and gave them worldwide protection. However, the Soviet Union continued to illegally hunt blue whales in the Northern and Southern Hemisphere through to 1973, and the last catch was taken by a Spanish expedition in 1978.
The global blue whale population is estimated to be 5,000–15,000 mature individuals as of 2018. They were protected in areas of the Southern Hemisphere starting in 1939. In 1955 they were given complete protection in the North Atlantic under the International Convention for the Regulation of Whaling; this protection was extended to the Antarctic in 1965 and the North Pacific in 1966. The protected status of North Atlantic blue whales was not recognized by Iceland until 1960.
Blue whales are formally classified as endangered under the Endangered Species Act and considered depleted and strategic under the Marine Mammal Protection Act. The International Union for Conservation of Nature has listed blue whales as endangered.
They are also listed on Appendix I under the Convention on International Trade in Endangered Species of Wild Fauna and Flora and the Convention on the Conservation of Migratory Species of Wild Animals. Although for some populations there is not enough information on current abundance trends (e.g., Pygmy blue whales), others are critically endangered (e.g., Antarctic blue whales).
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