| Pilot whales | |
|---|---|
| Size comparison between a short-finned pilot whale and an average human. | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Cetacea |
| Suborder: | Odontoceti |
| Family: | Delphinidae |
| Genus: | Globicephala |
| Species | |
| Green: Long-finned range; Blue: Short-finned. | |
Pilot whales are cetaceans belonging to the genus Globicephala. There are two extant species, the long-finned pilot whale (G. melas) and the short-finned pilot whale (G. macrorhynchus). The two are not readily distinguished at sea and analysis of the skulls is the best way to tell the difference between them. Between the two species, pilot whales range in waters nearly worldwide with long-finned pilot whales living in cold waters and short-finned pilot whales living in tropical and subtropical waters. Pilot whales are among the largest of the oceanic dolphins, exceeded in size only by the killer whale. They and other large members of the dolphin family are also known as blackfish.
Pilot whales are primarily squid eaters, but will feed on fish as well. They are also highly social and studies suggest that both males and females remain in their mothers’ pods, an unusual trait among mammals, also found in certain killer whale communities. Short-finned pilot whales are also one of the few mammal species where females go through menopause and post-reproductive females may contribute to the survival of younger members of their pods. Pilot whales are notorious for stranding themselves on beaches, and are among the most common cetacean stranders. Several theories have been proposed to account for this behavior. The status of both species is not understood and they have been subject to direct and indirect catches by fisheries. Whalers in a few countries continue to hunt pilot whales.
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Pilot whales are classified into two species:
The short-finned pilot whale was described, from skeletal materials only, by John Edward Gray in 1846. He presumed from the skeleton that the whale had a large beak. A number of species classifications have been proposed for the Globicephala but only two species are recognized.[1] The long-finned pilot whale was first classified by Thomas Stewart Traill in 1809 as Delphinus melas.[2] It's scientific name was eventually changed to Globicephala melaena. Since 1986, the specific name of the long-finned pilot whale was changed to its original form melas.[3] There exist geographic forms of short-finned pilot whales in the northwest Pacific Ocean off the coast of Japan.[4] Both forms are segregated geographically and thermally making them genetically isolated stocks.[5]
Fossils of an extinct relative Globicephala baerreckii have been found in Florida and dated to the Pleistocene period.[6] Another Globicephala dolphin was uncovered in Pliocene strata in Tuscany, Italy and was named G. eturia.[6] The pilot whales were also close relatives of the extinct blunt-snouted dolphin. Close living relatives of the pilot whales are the melon-headed whale, the pygmy killer whale, the false killer whale and the Risso's dolphin.
The name "pilot whale" originated with an early theory that a pod is "piloted" by a leader.[6][7] They are also called pothead whales and blackfish. The genus name is derived from the Latin words globus ("round ball" or "globe") and kephale ("head").[6][7] Melas is Greek for "black" and macrorhynchus comes from the Greek words macro ("enlarged") and rhynchus ("snout" or "beak").
Pilot whales are predominantly dark grey, brown or black, but have some light areas such as a grey saddle pitch behind the dorsal fin.[7] Other light areas are an anchor-shaped patch under the chin, a faint blaze marking behind the eye, a large marking on the belly and a genital patch.[7] The dorsal fin is set forward on the back and sweeps backwards. A pilot whale has a more robust body shape than most dolphins and has a distinctive large, bulbous melon.[7] Pilot whales have long, sickle-shaped flippers and tail stocks that are flattened from side to side and are deep dorso-ventrally.[7] Male long-finned pilot whales develop more bulbous melons than females,[7] although this does not seems to be the case for short-finned pilot whales off the Pacific coast of Japan.[8]
There are subtle differences between long and short-finned pilot whales and it is difficult to tell the species apart at sea in areas where their distributions overlap.[6] The length of the pectoral flippers relative to total body length, and the number of teeth have been used to distinguish the two species.[1] It was traditionally thought that the long-finned pilot whale has 9-12 teeth in each row and flippers 1/5 of total body length, while the short finned pilot whale has 7-9 teeth in each row and flippers 1/6 of total body length.[7] More recent information from a large series of Atlantic specimens of both species show considerable overlap in these features and there appear to be clines rather than distinct groupings as has been suggested.[7] Thus, the most reliable characteristics for distinguishing between the two species are skull differences.[6][7] The long-finned pilot whale's skull is narrower and the premaxilla leaves 1 cm of the lateral borders of the maxillae uncovered.[6] The short-finned pilot whale's skull is shorter and broader with the premaxillae covering the maxillae.[6]
The size and weight depend on the species as long-finned pilot whales are generally larger than short-finned pilot whales.[8][9] Their life span is about 45 years in males and 60 years in females for both species. Both species exhibit sexual dimorphism. Adult long-finned pilot whales reach a body length of approximately 6.5 m, with males being 1 m longer than females.[10] Their body mass reaches up to 1,300 kg in females and up to 2,300 kg in males.[11] For short-finned pilot whales, adult females reach a body length of approximately 5.5 m, while males reach 7.2 m and may weigh up to 3,200 kg.[11]
Pilot whales can be found in oceans nearly worldwide. The long-finned pilot whale prefers slightly cooler waters than the short-finned and is divided into two populations. The larger group is found in a circumpolar band in the Southern Ocean running from approximately 20° S to 65° S. It may be sighted off the coasts of Chile, Argentina, South Africa, Australia and New Zealand.[12] There are estimated to be in excess of 200,000 individuals in this population. The second population is much smaller and inhabits the North Atlantic Ocean, in a band that runs from South Carolina in the United States across to the Azores and Morocco at its southern edge and from Newfoundland to Greenland, Iceland and northern Norway at its northern. It is also present in the western half of the Mediterranean Sea.[12]
The short-finned pilot whale is more populous. It is found in temperate and tropical waters of the Indian, Atlantic and Pacific Oceans.[12] Its population overlaps slightly with the long-finned pilot whale in the temperate waters of the North Atlantic and Southern Oceans.[6] There are 150,000 individuals in the eastern tropical Pacific Ocean. There are estimated to be more than 30,000 animals in the western Pacific, off the coast of Japan. Pilot whales are generally nomadic, but resident populations exist in certain places such as coastal California, Hawaii, and the Canary Islands.[6] They prefer continental shelf breaks, slop waters and areas of high topographic relief.[6]
Both species eat primarily squid.[12] The whales will make seasonal inshore and offshore movements in relation to the distribution of their favored prey.[6] Fish prey include Atlantic cod, Greenland turbot, Atlantic mackerel, Atlantic herring, hake, and spiny dogfish.[6] In the North Atlantic, Atlantic mackerel may be an important food item in their diet during winter. In the Faroe Islands, whales mostly eat squid but will also eat fish species such as greater argentine and blue whiting. However, Faroe whales do not seem to feed on cod, herring or mackerel despite them being periodically abundant.[13]
Pilot whales generally take several breaths before diving for a few minutes. Feeding dives may last over ten minutes. They are capable of diving to depths of 600 meters, but most dives are to a depth of 30–60 meters. Shallow dives tend to take place during the day while deeper ones take place at night. When making deep dives, pilot whales often make fast sprints to catch fast moving prey like squid.[14] Short-finned pilot whales foraging in the same depth range as sperm whales and beaked whales perform the more energetic hunting tactics in comparison. They make a high speed energetic burst when swimming near the deep part of the dive and then make one or two buzzes, possibly in an attempt to capture prey.[14] This is unusual considering deep-diving breath-holding animals would be expected to swim slow and steadily to conserve oxygen. It is possible that if pilot whales have a higher metabolic rate than other deep diving mammals to enable sprinting, this would be one factor causing the relatively short diving period of pilot whales compared to other marine mammals of similar sizes.[14] This apparent lower diving capability has also been recorded for long-finned pilot whales.[15]
Common parasites of pilot whales include whale lice, cestodes and nematodes.[7] They also can be hosts to various pathogenic bacteria and viruses such as Streptococcus, Pseudomonas, Escherichia, Staphylococcus and influenza.[7] In a sample of 55 fishery-killed long-finned pilot whales off Newfoundland, the most common disease was infestation with a parasitic worm of the upper respiratory tract.[16]
Both species live in groups of 10-30 but some groups may number 100 or more. Data suggests that the social structures of pilot whale pods are similar to those of "resident" killer whales. The pods are highly stable and the members have close matrilineal relationships.[6] Pod members are of various age and sex classes, although adult females tend to outnumber adult males. They have been been observed making various kin-directed behaviors have such as providing food.[17] Numerous pods will gather together and form loose aggregations. These aggregations likely serve to exchange individuals or to provide opportunities for mating.[7] It is also possible that they serve a protective function as they become more cohesive in certain situations.[9]
Both species are loosely polygynous.[9] Data suggests that offspring of both sexes reach sexual maturity in their mother’s pod and usually remain there for life; despite this, there does not seem to be any inbreeding within a pod.[7] It is believed that male pilot whales remain with their female relatives while mating elsewhere, a rare trait among mammals.[18] During aggregations, male pilot whales will temporarily leave their pods to mate with females from other pods.[18] There does not seem to be any indication of male reproductive dominance or competition for mates.[19] After mating, a male pilot whale may spend only a few months with the female and individual males may sire several fetuses within a pod.[20] Males return to their own pods when the aggregations disband, and their presence may contribute to the survival of their related pod members.[18] There is no evidence of social segregation of "bachelor" groups.[9][18]
Pilot whale pods off southern California have been observed in three different groups: traveling/hunting groups, feeding groups and loafing groups.[21] In traveling/hunting groups, individuals are oriented in a broad rank of up to two miles in width but only a few individuals deep, which have been described as "chorus lines".[22] Sexual and age-class segregation has also been observed in chorus lines.[21] In feeding groups, individuals tend to remain fairly independent of one another although is sometimes a general movement of the whales in a given direction.[21] Loafing groups are nearly stationary aggregations of around 12-30 individuals floating at the surface that touch or come close to each other. Different types of behaviors take place in these groups, including mating.[21]
Pilot whales have one of the longest birth intervals of the cetaceans,[6] calving once every 3–5 years. Mating and calving in long-finned pilot whales peaks in the summer, though some calving occurs throughout the year.[23] For short-finned pilot whales calving tends to peak in spring and autumn in the Southern Hemisphere and varies by stock in the Northern Hemisphere.[23] Gestation is estimated to be 12–16 months for long-finned pilot whales and around 15 months for short-finned pilot whales.[6] Short-finned pilot whale calves are generally 1.4-1.85 meters (4 feet 6 inches to six feet) at birth, and weigh about 55 kilograms (120 pounds). Long-finned pilot whale calves are usually 1.4 meters (4 feet 6 inches) at birth, and generally weigh about 55-85 kilograms (120-190 pounds).[6] The calf nurses for up to 3 years, with some evidence for longer lactation and extensive mother calf bonds.[6] Short-finned pilot whale females will go though menopause,[24] something not as common in females of long-finned pilot whales.[25] It is possible that older, post-reproductive females play important roles in their pods and contribute to the survival of younger members.[17][26] Post-reproductive females will continue to lactate and nurse young which indicates that females will invest more energy in present offspring as they lose the ability to bear offspring of their own.[6] Short-finned pilot whales have a slower growth rate than long-finned pilot whales. For the short-finned pilot whale, females become sexually mature at nine years old and males at about 13–16 years.[6] For the long-finned pilot whale, females reach maturity at around eight years and males at around 12 years.[6]
Pilot whales commonly vocalize to keep contact with members of their pod. With active behavior, vocalizations are more complex while less active behavior is accompanied by simple vocalizations. Differences have been found in the calls of the two pilot whales species.[27] Long-finned pilot whales calls are of a lower frequency and narrower frequency range than short-finned pilot whale calls.[27] In one study of North Atlantic long finned pilot whales, certain vocalizations were heard to accompany certain behaviors.[28] Simple whistles are produced more frequently when whales are "milling", a restful behavior type.[28] When making energetic activity at the surface, whales make many sound types, notably complex whistles and pulsed sounds, at a greater frequency.[28] Most whistle types are produced more often when whales are spread over a larger area and when more subgroups are present.[28]
A study of short-finned pilot whales off the southwest coast of Tenerife in the Canary Islands has found that each pod has its own discrete repertoire of calls which may serve as means of contact between members of the same pod.[29] Short-finned pilot whales have also been recorded producing tonal calls during deep foraging dives at depths of up to 800 m.[30] The number and length of the calls seem to decrease with depth despite being farther away from conspecifics at the surface. As such, the energy content of calls is limited by ambient pressure at these depths. Frequency content was found to be unaffected and this may provide a cue for group or species identification of diving whales. Social calls could be important for maintaining social ties between foraging animals, but may possibly be negatively impacted by noise from vessels.[30]
Pilot whales are also known to emit "shrills" or "plaintive cries", which are variations of their whistles, when stranded, captured or injured.[31]
Of the cetaceans, pilot whales are among the most common stranders. Because of their strong social bonds, whole groups of pilot whales will strand. Single stranders have been recorded and these are usually diseased.[6] Group stranding tends to be of mostly healthy individuals. It is not known what causes these animals to strand but there are different hypotheses.[6] It has been suggested that the animals may get confused or trapped in shallow water; that if the whales are using the earth's magnetic field for navigation,they may get disoriented by geomagnetic anomalies or that a sick individual gets stranded and the group follows.[6] It has also been suggested that if an individual whale with great social importance or of strong filial bonds gets stranded, for whatever reason, others in the pod may follow and get stranded as well and will keep returning to the beach once removed because of a secondary social or epimeletic response.[7] This social response has been used by researchers from New Zealand to successfully keep a pod of long-finned pilot whales from stranding repeatedly.[32] In addition, the distress calls of the beached young of the pod appeared to evoke a stranding response from the older whales and thus the younger whales were transferred offshore to buoys and this lured the older whales back out to sea.[32] It is likely that mass strandings are caused by a number of factors.[7]
The IUCN lists both species as "Data Deficient" in the Red List of Threatened Species. The North and Baltic Sea subpopulations of long-finned pilot whales have been listed in Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS). However, recent data on movements in the northwest and northeast Atlantic suggest that these subpopulations should also be included in Appendix II of CMS.[33] The short-finned pilot whale is listed on Appendix II of CITES.[34]
The long-finned pilot whale has traditionally been hunted by "driving", which involves many fishermen and boats gathering in a semicircle behind a pod of whales close to shore, and slowly drive them towards a bay where they become stranded and are then slaughtered. This practice was common in both the nineteenth and twentieth centuries. Humans have hunted pilot whales around the Faroe Islands since at least the sixteenth century.[7] This continued to the modern day with about 1,500 catches per year in the 1970s[35] and increasing to near 2,500 in the 1980s.[36] Small numbers of pilot whales have been caught in other areas of the North Atlantic, such as Norway, West Greenland,[37] Iceland,[38] the Irish coast and Cape Cod. Cape Cod’s fisheries of pilot whales took place in the late nineteenth and early twentieth centuries with one fishery taking 2,000–3,000 per year.[39] Newfoundland's long-finned pilot whale fishery had a peak annual catch of around 10,000 in 1956 but after the 1960s, went into decline[35] and is no longer operating. In the Southern Hemisphere, exploitation of long-finned pilot whales have been sporadic and low.[35] Currently long-finned pilot whales are only hunted at the Faroe Islands and Greenland.[33]
The short-finned pilot whale has also been hunted for many centuries, particularly by Japanese whalers. From 1948 to 1979, a mean annual catch of 91 (ranging from 0–781) of the northern form was taken by small-type harpoon equipped whaling vessels, off Hokkaido and Sanriku.[9] A mean annual catch of 302 (ranging from 0–781) of the southern form was harvested by drive and harpoon fisheries at Taiji, Izu and Okinawa between 1948 and 1980.[9] For both fisheries, there was a peake in the late 1940s and early 1950s and then a decline for a few exceptional years.[9] Japan took a total of 2,326 short-finned pilot whales from 1985 to 1989.[34] This had decreased to about 400 per year by the 1990s.
Pilot whales have also fallen victim to by-catches. In one year an estimated 30 short-finned pilot whales have been taken by the squid round-haul fishery in southern California waters.[40] In the California drift gill net fishery between 1993 and 1995, the mean annual take of short-finned pilot whales was 20.[7] In 1988, the foreign Atlantic mackerel fishery on the east coast of the US was responsible for a take of 141 pilot whales and was suspended in early May of that year for this reason.[7]
As with other marine mammals, pilot whales are susceptible to certain pollutants. High amounts of DDT and PCB have been found in the tissues of long-finned pilot whales off the Faroes, France, the UK and the eastern US.[7] Heavy metals such as cadmium and mercury have also been found in pilot whales off the Faroes.[41] However, pilot whales examined off Newfoundland and Tasmania were found to have had very low levels DDT.[7] Short-finned pilot whales off the west coast of the US, Japan and the Antilles were found to have had high concentrations of DDT and PCB.[7]
Pilot whale meat is available for consumption in a very few areas of Japan, mainly along the central Pacific coast, and also in other areas of the world, such as the Faroe Islands. The meat is high in protein (higher than beef) and low in fat.[42] Because a whale's fat is contained in the layer of blubber beneath the skin, the meat is a dark shade of red.[42][43] In Japan, where pilot whale meat can be found in certain restaurants and izakayas, the meat is sometimes served raw, as sashimi, but just as often pilot whale steaks are marinated, cut into small chunks, and grilled.[43] When grilled, the meat is slightly flaky and quite flavorful, gamey, though similar to a quality cut of beef, with distinct yet subtle undertones recalling its marine origin.[42][43][44]
In both Japan and the Faroe Islands, the meat is contaminated with mercury and cadmium, causing a health risk for those frequently eating it, especially children and pregnant women.[45] In November 2008, an article in New Scientist reported that research done on the Faroe Islands resulted in two chief medical officers recommending against the consumption of pilot whale meat, considering it to be too toxic.[46] In 2008 the local authorities recommended that pilot whale meat should no longer be eaten due to the contamination. This has resulted in reduced consumption, according to a senior Faroese health official.[47]
Pilot whales, mostly short-finned pilot whales, have been kept in captivity. A small number of long-finned pilot whales were captured off New England before 1973. Since then, two stranded specimens from the east coast of the US have been held in captivity for a short time.[48] Short-finned pilot whales off southern California, Hawaii and Japan have been collected for aquariums and oceanariums. 33 pilot whales were taken from off the coast of southern California from 1966–1972,[49] with 17 of them being taken since 1973.[48] Two of these were placed at Sea World in San Diego. At the same time, 20 whales were captured near Hawaii.[48] Drive fisheries in Taiji took an annual mean number of six whales during 1974-84 and were used for public display.[7] Pilot whales generally have a high mortality rate in captivity. Between 1966 and 1973, the whales had an average annual survival rate of 0.51,[49] with those that survived for at least 90 days having an annual survival rate of 0.73.[49]