Blue whale

View of a blue whale and its bow wave, showing the blowhole

The blow of a blue whale

The small dorsal fin of this blue whale is just visible on the far left.

The blue whale is the largest known animal ever.[13][3][14][15] In the International Whaling Commission (IWC) whaling database, 88 individuals longer than 30 m were reported, including one up to 33.0 m, but problems with how the measurements were made suggest that measurements longer than 30.5 m are somewhat suspect.[16] The Discovery Committee reported lengths up to 102 ft (31.1 m);[17] however, the longest scientifically measured (e.g., from rostrum tip to tail notch) individual blue whale was 98 ft (29.9 m).[18] Female blue whales are larger than males.[5][19] Hydrodynamic models suggest that a blue whale could not exceed 108 ft (33 m) due to metabolic and energy constraints.[20]

The average length of sexually mature female blue whales is 72.1 ft (22.1 m) for Eastern North Pacific blue whales, 79 ft (24.1 m) for central and western North Pacific blue whales, 92 ft (28.1 m) for North Atlantic blue whales, 83.4–86.3 ft (25.4–26.6 m) for Antarctic blue whales, 77.1 ft (23.5 m) for Chilean blue whales, and 69.9 ft (21.3 m) for pygmy blue whales.[5][21][22]

In the Northern Hemisphere, males weigh an average 100 tons (200,000 lb) and females 112 tons (224,000 lb). Eastern North Pacific blue whale males average 88.5 tons (177,000 lb) and females 100 tons (200,000 lb). Antarctic males averaged 112 tons (224,000 lb) and females 130 tons (260,000 lb). Pygmy blue whale males average 83.5 (167,000 lb) and 99 tons (198,000 lb).[23] The largest heart weight measured from a stranded North Atlantic blue whale was 0.1985 tons (397 lb), the largest known in any animal.[24]

A blue whale skull measuring 5.8 meters (19 ft)

The most reliable age estimations are from ear plugs. Blue whales secrete earwax (cerumen) throughout their lives forming long, multilayered plugs. Each chronologically deposited light and dark layer (lamina) indicate a switch between fasting during migration and feeding, and one set is laid down per year, and thus the number of these layers can be used as an indicator of age.[25][26][27] The maximum age determined from earplug laminae for a pygmy whale is 73 years (n=1133).[28] Prior to the development of ear plugs as an ageing method, layers in baleen plates were used, but these wear down and are not a reliable measure. In addition, the ovaries of female blue whales form a permanent record of the number of ovulations (or perhaps pregnancies), in the form of corpora albicantia. These fibrous masses are permanent scars that have been used in the past as an indication of age.[29] In pygmy blue whales, one corpus albicans is formed every 2.6 years on average,[28] which matches the two- to three-year interval between calves.

In the North Pacific, blue whales can be seen in large numbers in Monterey Bay, CA from July through October. They may also be seen off San Diego, CA in July and August, and off Baja California Sur, Mexico and in the Sea of Cortez from January through March. In the North Atlantic, blue whales can be seen in the Gulf of St. Lawrence June through September, especially in the Saguenay–St. Lawrence Marine Park in Canada. Blue whales can best be found off Reykjavík and Húsavík in Iceland from May to August. Around the Azores, the best chance to see blue whales is February to March. In the Indian Ocean, blue whales can be seen off Mirissa, Sri Lanka in March and April. In the Southern Hemisphere, blue whales can be seen in Gulf of Corcovado between Guaitecas Archipelago, Chiloé Island, and the Chilean mainland, and off the west coast of Chiloe Island from December through March.[90][91] In Australia, blue whales and pygmy blue whales can be seen from March through May in the Perth Canyon,[92] and in Bass Strait between Victoria and Tasmania from November to May.[93] In New Zealand, blue whales and pygmy blue whales may be seen in Kaikōura on the South Island between July and August.[94]

The International Whaling Commission’s whale watching guidelines,[95] mirror these recommendations to minimize risk and adverse impacts on whales, including noise disturbance. They also provide a whale watch handbook,[96] providing more detailed guidelines for managers, regulators, operators, and anyone interested in whale watching.

In the United States, National Oceanic and Atmospheric Administration’s National Marine Fisheries Service[97] suggests that while whale watching, be alert and avoid disturbing whales from changing their normal behaviors and stay at least 100 yards (the length of a football field) away from a whale. Operate at a no-wake speeds and do not move into the path of a whale, move faster than a whale, make erratic speed or directional changes unless to avoid collision, get between two whales, chase, feed, or touch the whales.

The mechanism behind modern whale migration is debated.[98][99][100] Migration may function to reduce parasitism, pathogens, and competition,[101][102] provide greater access to prey in the spring and summer, reduce calf predation from orcas, and optimize thermoregulation for growth in the winter.[98][99][103][104] For many baleen whales, such as humpback and grey 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 on an annual basis.[105] However, blue whales are not as specific in their movement patterns, and there is substantial evidence of alternative strategies, such as year-round residency, partial or differential migration, and anomalous habits such as feeding on breeding grounds.[106] For Antarctic blue whales, for example, some remain year-round in the Antarctic, some remain year-round in northerly grounds, and most disperse throughout the Southern Hemisphere in the austral winter months.

Northern subspecies (B. m. musculus) – Eastern North Pacific population: This population migrates annually to the U.S. West Coast in the summer and fall to feed.[7][38][107][108] Their documented migration to secondary feeding areas off Oregon, Washington,[109] the Alaska Gyre, and the Aleutian Islands.[48][110] The northern extent can seasonally overlap with the Central North Pacific population in the Gulf of Alaska.[44][48] The Eastern North Pacific population migrates in the winter to their breeding grounds in the Gulf of California[7][40][111] and the Costa Rica Dome in the Eastern Tropical Pacific.[39][40] Acoustic recordings suggest that some individuals may remain on their feeding grounds[112][113][114] and on their breeding grounds year-round.[39][41][42][48]

Northern subspecies (B. m. musculus) – Western and Central North Pacific population: Little is known about the migration of this population. In the summer, this population migrates to their feeding grounds southwest of Kamchatka, south of the Aleutian Islands, to the Gulf of Alaska,[43][44] and to waters off Vancouver Island, Canada.[89][115][116][117] In the winter, they migrate to low-latitude waters, including Hawaii.[45][46][48][49] Acoustic data suggests that some individuals may remain on their feeding grounds year-round.[43][44][118]

Northern subspecies (B. m. musculus) – North Atlantic population: Little is known about the winter distribution and migration patterns of this population in the North Atlantic.[119] In the Western North Atlantic, blue whales migrate to the Gulf of St. Lawrence in the summer to feed, but some may remain year-round. Sightings in the Gulf of St. Lawrence peak late August-early September[120] and tagging suggests they migrate as far south as the Mid-Atlantic Bight coastal from Delaware to North Carolina and South Carolina.[121] In the Eastern North Atlantic, blue whales have been documented south of the Azores[122] and off northwest Africa in the winter,[123] in the Azores in late spring,[124] and in Iceland in the summer.[125] There has been one photograph-identification match between an individual blue whale in Iceland and Mauritania[126] and one match between an individual blue whale in the Gulf of St. Lawrence and the Azores,[126] suggesting connectivity among blue whales in the region.

Antarctic subspecies (B. m. intermedia): Acoustic recordings indicate that this population is distributed around Antarctica and south of the Antarctic Convergence Zone in the summer, and then move into mid- and low-latitude habitats in fall and winter.[42][58][59][60][61][62][63][64][65][66][67][68][127] Antarctic blue whale calls have been acoustically detected in the Eastern Tropical Pacific in May–September.[60] Most migrate annually; however, year-round acoustic detections near the West Antarctic Peninsula,[59][128] the Weddell Sea and along the Greenwich meridian,[129] eastern Antarctica,[61][128] and year-round catches around South Georgia[130][131] suggest a small portion may remain in Antarctica year-round.[74][132]

Pygmy subspecies (B. m. brevicauda) – Madagascar population: This population is found off the Seychelles and Amirante Islands, through the Mozambique Channel to the Crozet Islands and Prince Edward Islands in the spring and summer, with a nearly continuous distribution in sub-Antarctic waters in the Indian Ocean in the summer.[74] In the fall they were caught on the Madagascar Plateau[79] and sighted there in December in substantial numbers in recent decades.[133] Vocalizations have been recorded north near Diego Garcia in May–July,[134] in the Mozambique Channel in November–December[135] and Madagascar Basin south of La Reunion Island in March–June,[68] southwest of Amsterdam Island in December–May,[68] off Crozet Island in December–June.[78] Pygmy blue whales typically remain north of 52°S,[74][136] except on rare occasions, e.g. one was acoustically detected at 61.5° S near the Antarctic continental shelf.[80]

Pygmy subspecies (B. m. brevicauda) – Western Australia/Indonesia population: This population feeds in Perth Canyon off Western Australia[75] and between the Great Australian Bight and Bass Strait off South Australia and Victoria[73][137] in the summer before migrating to waters off Indonesia to breed in the winter.[73][74][75][76][77] Acoustic data indicate that they are distributed in the sub-Antarctic waters of the southern Indian Ocean in summer and fall, including near the Crozet Islands and Amsterdam Island.[68][78] Like the Madagascar population, this population’s distribution remains north of 52°S.[74][136]

Pygmy subspecies (B. m. brevicauda) – Eastern Australia/New Zealand population: Data from strandings, sightings, and acoustic detections suggest that this population is present in New Zealand waters nearly year-round.[138] They have also been detected as far south as 52°S in summer[82] and in the Tasman Sea and Lau Basin near Tonga in winter,[84] and likely range farther afield in the south-western Pacific. Major feeding grounds have been identified in the South Taranaki Bight between the North and South Islands, along the east coast of Northland (North Island), and off the east and west coasts of the South Island and the Hauraki Gulf.[81][83]

Northern Indian Ocean subspecies (B. m. indica): Although its taxonomic status is in flux, historical catches were taken from waters off Somalia and southern Arabia from May–October, and they are thought to then disperse to the east coast of Sri Lanka, west of the Maldives, the Indus Canyon, and the southern Indian Ocean in December–March.[86] Sighting and stranding data[47][74][86][139] suggest that most remain in the central northern Indian Ocean year-round[140][141] Acoustic evidence of Sri Lanka song types suggest some travel to sub-Antarctic waters around Crozet Islands in late summer and early fall.[78] Recent evidence of a new song type off Oman and north-west Madagascar (and the absence of the Sri Lanka song type there) suggests there might be a separate north-western Indian Ocean population making "Oman" calls and a central Indian Ocean population making "Sri Lanka" calls.[142]

Chilean population In summer and fall, Chilean blue whales feed along the west coast of South America, particularly the Chiloense Ecoregion, including the Corcovado Gulf, Pacific and northwest coasts of Chiloé Island, and inner sea of Chiloe Archipelago.[91][90][143][144] The whales then migrate to lower latitude areas including the Galapagos Islands and the Eastern Tropical Pacific.[91][145][146] Acoustic recordings in the Eastern Tropical Pacific may be year-round but generally peak in June[147] and are infrequently detected from September to March.[41][148]

Data from individual satellite tagged Eastern North Pacific blue whales suggest leisurely traveling rates of 4 knots, or 4.6 mph (7.41 km/h),[149] with a minimum average speed of 1.55 ± 1.68 mph (2.49 ± 2.7 km/h).[150] With additional satellite tagged animals (n=10),[40] reported mean swim speeds of 108 ± 33.3 km/day, ranging 58–172 km/day. That would translate to 2.79 ± 0.86 mph (4.5 ± 1.39 km/h), ranging from 1.5 to 4.45 mph (2.42–7.17 km/h). Lagerquist et al.[151] reported mean swim speed using only high-quality satellite locations as 2.55 ± 1.43 mph (4.1 ± 2.3 km/h) from 11 tagged blue whales. A pygmy blue whale tagged off Perth Canyon, Western Australia traveled at mean speeds of 2.8 ± 2.2 km/h. There are regular reports of much faster migration speeds and rapid sustained bursts of speeds; notably their swimming speeds were too fast for whalers until the modern era of whaling introduced steam-powered boats and explosive-tipped harpoons. The maximum speed of a blue whale while being chased or harassed has been reported at 20 to 48 km/h (10–25 knots).[47]

The blue whale diet consists almost exclusively of euphausiids (krill) except off Sri Lanka. Blue whales have been observed near Magdalena Bay (along the western coast of Baja California, Mexico) feeding on pelagic red crabs.[152][153][154] However, this has not been confirmed by recent observations or fecal samples. Other accidental or opportunistic consumption of copepods and amphipods have been documented.[155][156] Blue whales feed on krill at the surface and at depths greater than 328 ft (100 m), following their prey’s diel vertical migration through the water column.[5][157][158][159] The main prey species of krill targeted by blue whales varies among habitat in the different ocean basins.

• North Pacific (B. m. musculus): Analyses of fecal samples have revealed that the dominant prey for blue whales feeding off central and southern California are Thysanoessa spinifera and Euphausia pacifica.[47][153][157][160][161][162][163][164] Blue whales have also been observed feeding on Nyctiphanes simplex in the Gulf of California.[165][166]
• North Atlantic (B. m. musculus): Stomach content analysis suggest that blue whales in the North Atlantic feed on large euphausiid species.[50][167][168] Based on observations of feeding whales and concurrent water sampling, the two most important species are believed to be Thysanoessa raschii and Meganyctiphanes norvegica.[37]
• Antarctic subspecies (B. m. intermedia): This subspecies feeds on dense patches of patches of Antarctic krill (Euphausia superba) during austral summer.[169] In the Eastern Tropical Pacific, they may feed on Euphausia vallentini and Euphausia frigida, as well as lanternfish (Myctophum punctatum).[78][170]
• Pygmy subspecies (B. m. brevicauda): Stomach content analysis of pygmy blue whales revealed a range of krill and other zooplankton, with a large proportion of Euphausia frigida, Euphausia vallentini, and Myctophum punctatum.[171] Off Australia the pygmy subspecies of blue whale feeds on Euphausia recurva and Nyctiphanes australis,[73][75] and off New Zealand, they feed on Nyctiphanes australis.[172]
• Northern Indian Ocean subspecies (B. m. indica): DNA metabarcoding of fecal samples suggests that Dendrobranchiata (prawns) and Euphausiacea (krill) were the two most ingested prey groups (87% and 8%, respectively) of the Northern Indian Ocean subspecies of blue whale, followed by Amphipods and Cephalopods.[173]
• Chilean population: Euphausia vallentini dominates the mesoplankton in the Chiloense Ecoregion,[174] doubling from winter to spring.[175] Although there is no visual observation or other data (e.g., stomach content, fecal samples) to inform the diet of the Chilean subspecies, Euphausia vallentini coincides with the seasonal peak in acoustic detections of the blue whale subspecies.[147]

Blue whales capture krill through lunge feeding, a bulk filter-feeding strategy that involves accelerating toward a prey patch at high speeds,[176][177][178] opening the mouth 80–90°,[179][180] and inverting the tongue, creating a large sac.[181] This allows them to engulf a large volume of krill-laden water,[169][182][183] up to 220 tons of water at one time.[184] The water is then squeezed out through their baleen plates with pressure from the ventral pouch and tongue, and the remaining krill are swallowed.[179][182][185] Blue whales have been recorded making 180° rolls during lunge-feeding, allowing them to engulf krill patches while inverted; and they rolled while searching for prey between lunges, which has been hypothesized as allowing them to visually process the prey field to find the densest prey patches.[186]

The Eastern North Pacific population of blue whales have been well studied. The greatest dive depth reported from tagged blue whales was 315 meters.[185] Their theoretical aerobic dive limit was estimated at 31.2 min,[187] however, the longest dive duration measured was 15.2 minutes.[185] The deepest confirmed dive from a pygmy blue whale was 1,660 ft (506 m).[188]

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.[185][189] They can consume 34,776–1,912,680 kJ (~480,000 kilocalories) from one mouthful of krill, which can provide up to 240 times more energy than used in a single lunge.[185] Energetic models have estimated that the daily prey biomass requirement for an average-sized blue whale is 1,120 ± 359 kg krill.[190][191] Blue whales may accidentally eat small fish that happen to be in the swarms of krill (although this is rare).[192][193]

A blue whale calf with its mother

Using the number of earwax lamina deposited in the earplug and development of sexual organs from dead whales,[47] it has been determined blue whales reach sexual maturity at about 10 years old and at an average length of 23.5 m for female Antarctic blue whales.[15][47][89][194][195] Another method for determining age at sexual maturity, involves measurements of testosterone from the baleen of male blue whales.[196][197] Testosterone concentrations measured from baleen suggest that the age at sexual maturity for one blue whale was 9 years.[196] Male pygmy blue whales averaged 61.4 ft (18.7 m) at sexual maturity.[198][199] Female pygmy blue whales are 68.9–71.2 ft (21.0–21.7 m) in length[21] and roughly 10 years old at age of sexual maturity.[21][89][22]

Blue whales exhibit no well-defined social structure[200] other than mother-calf bonds from birth until weaning.[201] They are generally solitary or found in small groups.[202] Little is known about mating behavior, or breeding and birthing areas.[15][195] As a traveling pair, a male blue whale typically trails a female,[159][203] and is generally successful at repelling an intruder male after a short and vigorous battle.[204] Blue whale anatomy, specifically a small testis-to-body weight ratio[205] and documented visual observations of a second male joining the traveling pair,[206] suggest a polygynous, antagonistic male-male competition strategy.[195][206] Mating is thought to occur fall through winter.[15][195]

Female blue whales give birth every two to three years, depending on body condition and lactation period.[28][47][194][206][207][208] Pygmy blue whales were estimated to give birth every 2.6 years (95% CI=2.2–3.0).[28] Pregnant females gain roughly 4% of their body weight daily,[167] amounting to 60% of their overall body weight throughout summer foraging periods.[194] Gestation lasts 10–11 months. No records of natural births are known,[206] although a blue whale that ended up in Trincomalee harbor gave birth to a calf before being towed back to sea the following day.[209]

For Antarctic blue whales, a single calf is born at 23 ft (7 m) in length and weighs 2.8-3 tons (2540–2722 kg).[15][105] There is a 6–8 month weaning period until the calf is 53 ft (16 m) in length.[15][5][105][195][206][210][211]

Blue whale milk is roughly (g/100 g) 45–48 water, 39–41 fat, 11–12 protein, 7.4 carbohydrates and 1.3 sugar,[212][213] 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.[214] Blue whale calves gain roughly 37,500 lb (17,000 kg) during the weaning period.[14] Estimates suggest that because calves require 2–4 kg milk per kg of mass gain, blue whales likely produce 220 kg milk/d (ranging from 110 to 320 kg milk/d).[215] The first video of a calf thought to be nursing was filmed in New Zealand in 2016.[216] Additional videos of blue whale calves nursing have been captured by drones, including off Dana Point, CA and off the South Coast of Sri Lanka.

There is reference to a "well-documented" humpback-blue whale hybrid in the South Pacific, attributed to marine biologist Dr. Michael Poole,[10][217] 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 65 ft anomalous female whale with the features of both the blue and the fin whales taken in the North Pacific.[218] In 1983 a 65 ft (19.8 m) long male specimen taken was 65 ft long and sexually immature. Based upon the number of layers in the earwax, the age of the animal was determined to be seven years. In 1984, a female hybrid between a fin and a blue whale was caught by whalers off northwestern Spain. Molecular analyses revealed a blue whale mother and a fin whale father.[219] In 1986, a 70 ft (21.3 m) pregnant female whale was caught. Molecular analyses of the whale showed that 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.[220] Two live blue-fin whale hybrids have since been documented in the Gulf of St. Lawrence, Canada and in the Azores (Portugal).[221] DNA tests done in Iceland of a blue whale killed July 7, 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;[222] however, the results are pending independent testing and verification of the samples. Because blue whales are classed as a "Protection Stock" by the International Whaling Commission, trading their meat is illegal, and the kill is an infraction that must be reported.[223] Blue-fin hybrids have also been detected from genetic analysis of whale meat samples taken from Japanese markets.[224]

There are no direct measurements of the hearing sensitivity of blue whales. Hearing predictions are inferred from anatomical studies,[218][225][226][227] vocalization ranges,[228][229][230] and behavioral responses to sound.[231][232] Blue whale inner ears appear well adapted for detecting low-frequency sounds.[233][234][235][236] Their vocalizations are also predominantly low frequency; thus, their hearing is presumably best at detecting those frequency ranges.[228] Southall et al.[237] 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.[32][238] 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,[238] 189 dB re 1 μPa at 1 m for Antarctic blue whales,[239] and 174 dB re 1 μPa at 1 m for pygmy blue whales.[78]

The fundamental frequency for blue whale vocalizations ranges from 8 to 25 Hz.[240] Blue whale song types were initially divided into nine song types,[65] although ongoing research suggests at least 13 song types.[241] The correlation between song types and genetic subdivisions is unknown, but song types are currently used 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 "D-calls" and other non-repeating calls are produced during feeding by both sexes.

Possible reasons for calling include:

1. maintenance of inter-individual distance
2. species and individual recognition
3. contextual information transmission (for example feeding, alarm, courtship)
4. maintenance of social organization (for example contact calls between females and males)
5. location of topographic features,
6. location of prey resources.[228]

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.[242][243][244][245] A and B calls are often produced in repeated co-occurring sequences as song only by males, suggesting a reproductive function.[245][158] D calls are produced by both sexes during social interactions while foraging and may considered multi-purpose contact calls.[158][246][247] Because the calls have also been recorded from blue whale trios from in a putative reproductive context, it has been recently suggested that this call has different functions.[204] The blue whale call recorded off Sri Lanka is a three‐unit phrase. The first unit is a pulsive call ranging 19.8 to 43.5 Hz, lasting 17.9 ± 5.2 s. The second unit is a FM upsweep 55.9 to 72.4 Hz lasting 13.8 ± 1.1 s. The final unit is a long (28.5 ± 1.6 s) tone that sweeps from 108 to 104.7 Hz.[248] The blue whale call recorded off Madagascar, a two‐unit phrase,[249] starts with 5–7 pulses with a center frequency of 35.1 ± 0.7 Hz and duration of 4.4 ± 0.5 s followed by a 35 ± 0 Hz tone lasting 10.9 ± 1.1 s.[248] In the Southern Ocean, blue whales calls last roughly 18 seconds and consist of a 9-s-long, 27 Hz tone, followed by a 1-s downsweep to 19 Hz, and another downsweep to 18 Hz.[59][62] They also produce short, 1–4 s duration, frequency-modulated calls ranging in frequency between 80 and 38 Hz.[62][250]

At least seven blue whale song types have been shifting linearly downward in tonal frequency over time, though at different rates.[251][252][253]

The Eastern North Pacific blue whale tonal frequency is 31% lower than it was in the early 1960s.[251][252] The frequency of pygmy blue whales in the Antarctic has steadily decreased at a rate of a few tenths of hertz per year since 2002.[253] One hypothesis is that as blue whale populations recover from whaling, this is increasing sexual selection pressure (i.e., lower frequency indicates larger body size),[252] although given the difficulties in measuring length from living whales, there is little evidence for changes in body size since whaling ended.

The genus name, Balaenoptera, means winged whale[10] while the species name, musculus, could mean "muscle" or a diminutive form of "mouse", potentially a clever pun by Carl Linnaeus,[4][10] who named the species in Systema Naturae.[35] One of the first published descriptions of a blue whale comes from Robert Sibbald’s Phalainologia Nova,[254] after Sibbald found a stranded whale in the estuary of the Firth of Forth 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.[255]

Blue whales were referred to as ‘Sibbald’s rorqual’, after Robert Sibbald, who first described the species.[254] Herman Melville called the blue whale "sulphur bottom" in his novel Moby Dick[11] due to the accumulation of diatoms creating a yellowish appearance on their pale underside.[4][5]

The name rorqual comes from the Norwegian word rørhval, a reference to the whale’s throat grooves, which are an elastic structure of blubber and muscle also known as the ventral grove blubber[181][256][257][258] extending from the chin to the umbilicus.[259][260]

Blue whales are rorquals, in the family Balaenopteridae[261] 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).[262]

Molecular evidence places blue whales in the Superorder Cetartiodactyla, which includes the Orders Cetacea (under which blue whales are classified) and Artiodactyla, even-toed ungulates. This classification is supported by evidence of morphological homology between cetaceans and artiodactyls in two described archaic whales.[263][264]

The phylogeny of the blue whales is still debated because their placement varies depending on the molecular markers and phylogenetic analysis used.[265][266][267][268][269][270] The most recent analysis estimates that the Balaenopteridae family diverged from other families in the late Miocene, between 10.48 and 4.98 million years ago.[270]

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.[271] The Australian pygmy blue whale diverged during the Last Glacial Maximum. Their more recent divergence has resulted the subspecies to have a relatively low genetic diversity,[272] and New Zealand blue whales have even lower genetic diversity.[138]

Molecular evidence has suggested that common and pygmy hippopotamus, extant members of the family Hippopotamidae, are the closest living relatives to the order Cetacea.[273][274] This monophyletic clade is nested in Cetartiodactyla, which includes the even-toed ungulates.[275] Whole genome sequencing of blue whales and other rorqual species suggests that blue whales are most closely related to sei whales with grey whales as a sister group,[270] which is curious given the most common hybrids are with fin whales. This study also found significant gene flow between minke whales and the ancestors of the blue and sei whale. Blue whale also displayed a high degree of genetic variability (i.e., heterozygosity).[270]

There are four subspecies of blue whale,[31][32] recognized by the Society for Marine Mammalogy’s Committee on Taxonomy:

1. B. m. musculus[35]
2. B. m. intermedia[57]
3. B. m. brevicauda[70]
4. B. m. indica[85]

Some of these blue whale subspecies have been further divided, resulting in nine recognized management units:[32]

1. Northern subspecies (B. m. musculus) – Eastern North Pacific population
2. Northern subspecies (B. m. musculus) – Central and Western North Pacific population
3. Northern subspecies (B. m. musculus) – North Atlantic population
4. Antarctic subspecies (B. m. intermedia)
5. Pygmy subspecies (B. m. brevicauda) – Madagascar population
6. Pygmy subspecies (B. m. brevicauda) – Western Australia/Indonesia population
7. Pygmy subspecies (B. m. brevicauda) – Eastern Australia/New Zealand population
8. Northern Indian Ocean subspecies (B. m. indica)
9. Chilean subspecies (B. m. unnamed subsp.)

Northern subspecies

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;[211] however, evidence from movement data derived from satellite tags,[40][276] photograph-identification,[7] and acoustic data[44][48][65] 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.[277]

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[50][278][279] based on photo-identification data.

Antarctic subspecies

Despite having the greatest haplotype diversity of any subspecies,[280] 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,[62][65][128][281] and mark-recapture data suggests movements of individuals entirely around the Antarctic.[74]

Pygmy subspecies

The pygmy blue whale subspecies, B. m. brevicauda, has three populations corresponding with acoustic populations, including a Madagascar population, Eastern Australia/New Zealand population, and Western Australia/Indonesia population.[282] Although the Western Australia/Indonesia population and the Eastern Australia/New Zealand population are morphologically similar and not genetically separated,[283] there are no photograph-identification matches between the two populations,[284] and mitochondrial DNA haplotype frequencies suggest a high degree of genetic isolation of the New Zealand population.[138] 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.[84][88]

Northern Indian Ocean subspecies

B. m. indica is currently considered a blue whale subspecies.[31][86][285] Evidence includes a breeding season asynchronous with Southern Hemisphere blue whales,[87] a distinct Sri Lanka call type,[65][140] a slightly smaller total length at maturity,[89] and potential year-round residency.[140][141] However, the Sri Lanka call has not been detected west of Sri Lanka, and there is another call type in the western North Indian Ocean, off Oman and north-western Madagascar, termed the Oman call[142] suggesting a central Indian Ocean population.

Chilean subspecies

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,[21][22] significant genetic differentiation,[21][145][280][286] and unique song types.[238][287][288] 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[289] and are unlikely to be interbreeding; however, the genetic differentiation is lesser with Eastern North Pacific blue whale, there may gene flow between hemispheres.[289]

Blue whale populations have declined dramatically due to commercial whaling.

Blue whales were nearly hunted to extinction in the 19th and 20th centuries by commercial whalers until the International Whaling Commission banned the taking of blue whales in 1967.[290] The International Whaling Commission catch database estimates that 382,595 blue whales were caught between 1868 and 1978, including 7,973 in the North Pacific (2.09%), 10,442 in the North Atlantic (2.73%), 5,383 in the South Pacific (1.41%), and in the Southern Ocean, 13,022 pygmy blue whales (3.40%) and 345,775 Antarctic blue whales (90.40%).[291] The Soviet Union continued to illegally hunt blue whales in the Northern and Southern Hemisphere through to 1973,[79][292][293][294] and Spanish vessels caught occasional blue whales up until 1978.[295] The global blue whale population abundance is estimated to be 10,000–25,000 blue whales, roughly 3–11% of the population size estimated in 1911.[72] Recovery and current population sizes vary regionally and by subspecies.

North Pacific (B. m. musculus)

The reported take of all North Pacific blue whales by commercial whalers totaled 9,773 between 1905 and 1977.[277]

• Eastern North Pacific population: An estimated total number of Eastern North Pacific whales caught by commercial whalers from 1905 to 1971 in the North Pacific was 3,441 (95% range 2,855–3,920; with a 95% uncertainty range of 2,593–4,114), leaving an estimated 951 (95% CI=460–1730) remaining after whaling was banned.[277][296] The best abundance estimates for the Eastern North Pacific stock of blue whales from photographic mark-recapture data from 2005 to 2011 is 1,647 (CV=0.07).[297] Abundance estimates from these data suggest no evidence of an increase in this population size since the early 1990s.[298] The most recent assessment found that the population had recovered to 97% (but with wide confidence intervals of 62–99%) of pre-whaling levels.[296][299]
• Central North Pacific population: The only available abundance estimate for the Central North Pacific population comes from a 2010 shipboard line-transect survey of the Hawaiian Islands Exclusive Economic Zone, resulting in a summer/fall abundance estimate of 133 (CV=1.09) blue whales.[49] The minimum estimated population size is 63 blue whales.[300] However, these surveys do not cover the more northerly waters where most historical catches were taken, and data from the recent POWER surveys should provide a more complete estimate of their current abundance in the near future.
• Japan blue whale population: Some researchers have informally suggested that a former population of blue whales off Japan were driven to extinction by commercial whaling, mostly from the Kumanonada Sea off Wakayama, in the Gulf of Tosa, and in the Sea of Hyūga. Whaling catch records indicate a small number of takes around the Korean Peninsula and in the coastal waters of the Sea of Japan. The last recorded stranding off coastal Japan was in the 1910s.[301] In recent years, there have been 374 sightings of blue whale schools east of 157°E and north of 35°N off Japan, with abundance estimates of 38 to 958 during the years of 2008–2014,[302] but it is not clear if these are sightings of the population considered to be extirpated or an expansion westwards of the western and central North Pacific population.

North Atlantic (B. m. musculus)

Roughly 15,000 blue whales may have inhabited the North Atlantic before whaling began.[303][304][305] In the western North Atlantic, there were an estimated 1,100–1,500 prior to modern whaling,[303][304] and in the eastern North Atlantic, estimates range from a "few thousand" to 10,000 blue whales in the Denmark Strait and 2,500 from northern Norway.[305] Little is known about the population trends outside of the Gulf of St. Lawrence area. Between 1979 and 2009, 440 blue whales were photo-identified in the Gulf of St. Lawrence estuary and northwestern Gulf of St. Lawrence.[120] Researchers speculate that there may be between 400 and 1,500 individuals.[18][55][306] A summer shipboard survey in 1987 resulted in a maximum estimate of 442 blue whales off Iceland.[279] Sightings from ship-based surveys in the Central and Northeast Atlantic in 1987, 1989, 1995 and 2001 provided abundances estimates ranging from 222 (CI=115–440) in 1987 to 979 (CI=137-2,542) in 1995.[125] The most recent estimate is between 1,000 and 2,000[307] off the east coast of Greenland, Denmark Strait, Iceland, Jan Mayen, Faroe Islands, west coast of Ireland, and north of the United Kingdom.

Antarctic population (B. m. intermedia)

Catches of Antarctic blue whales (345,775 whales) reduced the population from 239,000 (95% CI=202,000–311,000) in the 1920s to a low of 360 (150–840) animals in the early 1970s, which is 0.15% (0.07–0.29%) of pre-exploitation levels.[308] The best abundance estimate for Antarctic blue whales is 2,280 individuals (CV=0.36) based on the International Whaling Commission International Decade of Cetacean Research and the Southern Ocean Whale Ecosystem Research (SOWER) annual summer surveys from 1991/92 through 2003/04, which covered 99.7% of the area between the pack ice and 60° S.[309] The population is estimated to be increasing, at a rate of 7.3% per year (95% CI=1.4–11.6%) but the most recent abundance estimate is less than 1% of pre-exploitation levels,[308] although if population growth has continued at 7.3% per year, there may now be as many as 10,000 individuals. In 2020, during a 23-day survey, scientists from the British Antarctic Survey (BAS) detected a population of 55 blue whales near a sub-Antarctic island of South Georgia. Prior to the survey, only a couple of animals have been observed in this area after mass hunting in the early 20th century.[310]

Pygmy subspecies (B. m. brevicauda)

Few reliable estimates exist for population status of pygmy blue whales. Given historical catches, pre-exploitation abundance estimates must have been at least 7,598 individuals for the Madagascar population, and 2,900 individuals for the Australia/Indonesia population.[79] There is no abundance estimate for overall pygmy blue whale abundance, which includes the Madagascar population, Western Australia/Indonesia population, and Eastern Australia/New Zealand population. Best[133] suggest a minimum abundance of 424 (CV=0.42) pygmy blue whales on the Madagascar Plateau, or 472 (CV=0.48). Kato et al.[311] estimated 671 (279–1613) pygmy blue whales from a line-transect survey of a small area off the southern coast of Australia. Acoustic monitoring during migrations of Australian pygmy whales in the southeastern Indian Ocean resulted in an estimate of 660–1,750 whales.[76][137][312] The estimate of pygmy blue whale off New Zealand from mark-recapture data is 718 (95% CI=279-1926, SD=433).[138]

Northern Indian Ocean subspecies (B. m. indica)

Roughly 1,300 blue whales were caught illegally by Soviet Union whalers from 1963 to 1966 in the Arabian Sea.[87] Models from a number of surveys in 2014 from a small area 150 km east–west and 50 km north–south just south of Sri Lanka suggest an abundance estimate of 270 blue whales (CV=0.09, 95%, CI=226–322).[313]

Chilean population

The Chilean blue whale is found in the southeastern Pacific Ocean, especially the Chiloé-Corcovado region,[21][91] and lower latitude areas including Peru, the Galapagos Islands and the southern portions of the Eastern Tropical Pacific.[91][145][146][148]

Roughly 5,728 blue whales from the Chilean subspecies were killed by whalers in the Southeast Pacific,[314] 4,288 off Chile.[315] The minimum pre-exploitation abundance was estimated at 1,500–5,000.[314][316] The best abundance estimates from open population models off Isla Grande de Chiloé spanning 2004-2012 suggest that ~570-760 whales are feeding seasonally in this region. Closed population models for the same feeding grounds in 2012 estimate 762 (95% CI=638–933) and 570 (95% CI=475–705) for photographs of left and right sides of the animals, respectively.[317]

• Ship Strikes: Ship strikes are a significant mortality factor for blue whales, especially off the U.S. West Coast,[318] which has some of the greatest densities of commercial ship traffic in the world.[319] Nine blue whales were known to have been killed and one seriously injured by ship strikes between 2007 and 2010 off California.[320] The five deaths in 2007 were considered an Unusual Mortality Event (UME), as defined under the Marine Mammal Protection Act.[318][321] Lethal ship strikes are also a problem in Sri Lankan waters, where their habitat overlaps with one of the world’s business shipping lanes.[322][323] Eleven blue whales were killed by ship strikes between 2010 and 2012 around Sri Lanka,[324] and at least two in 2014.[325] Two Chilean blue whales were killed by ship strikes in recent years off southern Chile.[326][327] 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.[319][328][329]
• 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.[330] Three more entanglement cases were reported in 2016.[331] 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.[332] 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.[333][334]
• Ocean Noise: Increasing anthropogenic underwater noise changes the acoustic environment and impacts blue whales.[335] In addition to masking blue whale communication ranges,[336][337] exposure to anthropogenic sound can result in 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,[338][339] disruptions of foraging, resting, and socializing,[340] habitat displacement,[338][339] hearing loss;[339] and habituation.[341][342] In additional to exposure to noise from commercial shipping[343][344] and seismic surveys as a part of oil and gas exploration,[345][346] blue whales are exposed to military sonar and explosives in every ocean basin. Strandings of other cetaceans have coincided with high intensity noises used during military sonar testing.[347][348][349][350][351][352][353][340] Blue whales in the Southern California Bight decreased calling in the presence of mid-frequency active (MFA) sonar.[354] Exposure to simulated MFA sonar interrupted blue whale feeding, especially when animals were in the deep feeding mode, although the responses were heavily dependent on the animal’s behavioral state at the time of exposure.[183][335][355][356][357]
• 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.[358] Analysis of the earwax of a male blue whale killed by a collision with a ship off the coast of California showed contaminants such as pesticides, flame retardants, mercury. Reconstructed persistent organic pollutant (POP) profiles suggested that a substantial maternal transfer occurred during gestation and/or lactation.[197] 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.[359]
• 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. A high abundance of microplastics was found in Mediterranean fin whales,[360] suggesting that blue whales are also susceptible. 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.[360][361]
• 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.[362][363] While ingestion is a risk for whales, a recent study found that oil did not foul baleen, and instead was easily rinsed from baleen by flowing water.[346]
• Disease and toxins: There is no current evidence linking biotoxins to acute or chronic impacts to blue whales, and while morbillivirus and Brucella sp. infections have caused significant impacts to other cetacean species,[364][365] there is limited data in infection in blue whales.
• Climate Change: Blue whale habitat is predicted to decrease significantly with current climate change projections.[366][367] Additionally, warming oceans may affect krill availability in a number of ways, including vertical distribution due to a deepening of the thermocline and increasing stratification of the water column[368] and poleward shifts resulting from the contraction of favorable habitat and changes to coastal upwelling.[369][370][371] Ocean acidification may adversely affect blue whale prey, as krill embryonic development,[372] hatch rates,[373] and post-larval metabolic physiology[374] are likely to be affected by increasingly acidic conditions.

• 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 blue whales in the Gulf of California have rake-like scars, indicative of encounters with killer whales.[375] 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.[376] 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.[377] The first documented attack occurred in 1977 off southwestern Baja California, Mexico, but the injured whale escaped after five hours.[378] Four more blue whales were documented as being chased by a group of killer whales between 1982 and 2003.[379] 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.[380] 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.[381] A similar incident was recorded by a drone in Monterey Bay in May 2017.[382] 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.[383]
• Competition: There is little to no direct evidence for interspecific competition with blue whales and other baleen whale species.[384] 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[385][386][387] in space and/or time and selection of prey species.[388][389][184] 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.[390]

Blue whales were initially tough to hunt because of their size and speed.[32] Large-scale takes did not begin until 1864, when the Norwegian Svend Foyn invented the exploding harpoon gun and by using them with steam and diesel-powered ships.[391][392] From 1866 to 1978, more than 380,000 blue whales were taken, mostly from Antarctic waters.[291] 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 illegal whaling into the 1970s,[87][292] and the last catch was taken by a Spanish expedition in 1978.