The common dolphin can be found in all of the world's temperate and tropical waters, probably making it the second most widely distributed cetacean, right behind the killer whale, which can also be found at the poles. This dolphin is found on the coasts of North America as far north as British Columbia, on European coasts and in the Mediterranean, Black, and Azov Seas. It can be found on the coasts of Africa, southeast Asia, Australia, New Zealand, Japan, and in the Indian Ocean. Although it prefers mostly warm, and temperate waters, it is very occasionally reported in Nova Scotia, Iceland, and Greenland. Strandings have been reported at Tierra del Fuego.
The common dolphin is found both along the coasts and offshore, although it is mostly found in depths of greater than 180 meters. More pelagic than the bottlenose dolphin, it is rarely seen close to the shore. There does exist an inshore form in the eastern tropical Pacific. There are several different forms of the common dolphin, which are sometimes regarded as subspecies, racial groups, or even different species. The common dolphin prefers water of a temperature between 10 and 28° C.
The most detailed distribution and taxonomy studies have been performed in the eastern tropical Pacific. Four stocks have been recognized in the area: a northern form, a central form, a southern form, and a Baja neritic form. The stocks are divided by areas of relatively low population density. Although there is much observer effort in these areas, there are relatively few sightings. The central stock extends the farthest west of the four, to 140° W. The Baja stock differs from the northern stock by modal length, color pattern and relative beak length. It extends from the Gulf of California to 185 kilometers off the Pacific coast of Baja California. There also appears to be a stock off of southern Mexico in the central stock area; it is called "Guerrero." The research performed by W. E. Evans similarly identified four population stocks off of southern California and Baja California, differentiated by the lengths of their sounts. The Pacific, Baja California, and eastern tropical Pacific forms each have short snouts, while the inshore neritic form has a long snout. In fact, Baja California is the only area where the long-snouted variety may be found. The short-snouted and long-snouted varieties are not considered separate species. The large differentiation in the eastern tropical Pacific also occurs in other areas. There are distinct forms in the Mediterranean and Black Seas, along Europe, Africa, and Japan, and in the Indian Ocean.
In Californian waters, there is a discontinuous distribution, possibly indicating competition with resident species of the genus Stenella, or that different stocks prefer different water temperatures. A similar phenomenon is seen in the eastern Mediterranean. In nearly all areas, seasonal movements follow changes in the water temperature or changes in prey migrations. Compared to the white-sided dolphin, the common dolphin is found in warmer and more saline waters. Both species are found in areas of high sea floor relief. These phenomena are related to the abundance of their prefered prey.
It is reported that 53% of dolphin odontocetes in southern Californian waters were common dolphins, out of 654 observations made over 10 years. The second most abundant species in the Pacific is the Pacific White Sided Dolphin, at 14%. The movements appeared to follow food sources, and the specimens fed on whatever prey was most abundant. Significant declines in this species' population in the Pacific have occured as a result of the eastern tropical Pacific tuna purse seine fishery.
In the North Atlantic and Mediterranean Sea, amateur reports indicate that the common dolphin is the most common species. Atlantic sightings are most frequent between July and December. A decline in population in the North Sea is suggested from stranding records, but the IWC Cetacean Committee Working Group indicates that measuring strandings is not an accurate method of determining abundance, since other factors could conspire to reduce strandings. Most sightings in the northern part of the eastern North Atlantic were in August. The most northerly record is of a group of six individuals at 78° 34' N. The sighting was in august, and the water temperature was 10.7° C. It was the only sighting north of 61° N. The highest stranding frequency occurs west of Scotland, and on the westard approaches to the English Channel, Straits of Gibraltar, and the western and central Mediterranean.
Sightings in the Mediterranean are most frequent between January and June. Individuals are reported in every area except the northern Aegean Sea. In the North Sea, records east of 0° are relatively sparse, with only nine sightings between June and October. There appears to be a decline in the population of the western Mediterranean Sea, specifically Italy, France, and northern Spain. In some areas where the species was once abundant, few sightings are recorded. Between 1980 and 1984, French strandings records were at 1/9 of their level between 1970 and 1974. In northern Spain, there have been no strandings or sightings since 1978. Interviews and strandings records in Italy also show a trend of decline. Until recently, the species was frequently confused with the striped dolphin, so estimates of past abundance are probably not accurate.
Estimates of population in the Black Sea are complicated by the fact that three species are involved in the fisheries. The authors of analyses do not state whether the figures are for only one species or all three. In the USSR fishery, before 1964, the common dolphin represented approximately 80-90% of the catches. Between 1964 and 1966, Phocoena phocoena was the dominant catch. In the early 1980s, the common dolphin was 15-16% of the Turkish fishery's catch. Estimates made between 1967 and 1973 vary from 28,000 to 258,000, with no obvious trend.
The common dolphin can grow to 2.55 meters and weigh as much as 200 lbs. A diagnostic feature is a V-shaped grove that separates the forehead from the beak. The dorsal surface is purplish-brown when the specimen is alive, and it fades to a grayish black after death. The thoracic patch has a unique bright coloration. Only the Atlantic white-sided dolphin, and sometimes young killer whales have similar colors. Tuna fisherman sometimes call this species the "white-bellied porpoise."
The many small teeth of the common dolphin are very sharp and interlock perfectly. This makes holding slippery fish very easy. The Californian and Mediterranean forms have approximately 50 teeth on each side of each jaw. Specimens in the Indian Ocean typically have a longer rostrum and more teeth: up to 60 in each side of each jaw, more than any other mammal.
At similar ages, males are slightly larger than females. The tail stock is narrow, with no dorsal or ventral keels. A distinguishing factor between specimens of Delphinus and similar long-snouted small dolphins, such as Stenella is the presence of groves on the palate.
The common dolphin strands alone relatively often. On the British coasts, it is second to the harbor porpoise in stranding frequency. The bond between the mother and calf is very strong. Either stays until the death of the other if one is captured. Similar bonds are present between males and females, and between adults of the same sex. In some parts of the Mediterranean, it is reported that they help in fishing, while in other areas, they are seen as pests.
It is the most proficient of all cetaceans at bow-riding. While most bow-riders need to leave the boat every so often, the common dolphin can continue for hours. Distances of up to 60-70 miles have been reported. Individuals climb to the bow wave, leap out of the crest of the wave at top speed, and then fall back into the water with a large splash. The fact that they follow boats and ride bow waves makes for easier capture. With the help of a moving vessel, the common dolphin can reach speeds of up to 110 kilometers per hour.
Strong socal bonds have been seen between individuals. In one case, a captive healthy male befriended a very shy female, also in captivity. When she died, he died within three days, for no other explanation than of sorrow. Often common dolphins carry wounded individuals to the surface, supporting them with their flippers. In addition, they often help harpooned individuals who cannot surface. These behavior patterns are often considered a sign of intelligence, but since the responses are not very complicated, it is very possibly just a natural instinctive reaction coming from evolution.
The common dolphin is also an extremely fast swimmer. It was reported that a destroyer travelling at 50 kilometers per hour could not overtake a group of this species. More conservative estimates place the maximum speed at no more than 45 kph. Other species can only attain 39-40 kph, so it is highly unlikely that the common dolphin can exceed this speed by such a great amount. In addition, individuals are very good at leaping. They jump often, in all configurations including headfirst entries and bellyflops. In Moby Dick, Melville called the species the "huzza porpoise" due to its frequent leaps. It was reported that an individual leaped and did a 360° roll before reentry. Wild specimens have been seen leaping up to 4-6 meters, while trained individuals can reach 6.6 meters.
The prey of the common dolphin consists of anchovies and other small fish, of which they eat much. It was reported that the stomach of one individual contained 7,596 ear bones (otoliths) of small fish. Other reports tell of individuals each with more than 15,000 otoliths. The actual method of catch by the common dolphin is not known, although surface and sometimes mid-air feeding have been reported.
They do aggregate in enormous groups, sometimes exceeding 1000 individuals. In the Black Sea, groups of more than 300,000 for feeding have been reported. Various formations have also been reported, including a line of a thickness between two and five individuals and a width extending for miles, and a column of a thickness of two to three individuals and a length also extending for miles.
The average school size in the Black Sea surveys conducted between 1978 and 1980 was 10.4 individuals. In a 1981 survey, it was 36.6 individuals. In the eastern North Atlantic and Bay of Biscay south of 50° N, 32% of groups had more than 20 individuals. Towards the north, 24% had more than 20 individuals, while in the south, 35% had more than 20 individuals. In the Mediterranean, larger groups were found to the west. Near Gibraltar, 37% of groups had more than 20 individuals, while in the farthest eastern sector, no groups had more than 20 individuals. In the eastern tropical Pacific, the average school size is very large. The northern stock has an average group size of more than 1000 individuals, while the central stock had between 400 and 600 individuals per group and the southern stock had an average group size of between 400 and more than 1400. There is possibly some segregation by sex and age, or possibly each group represents one limited gene pool. The large schools makes catching easier, especially in the seine nets of the Black Sea. Often the common dolphin associates with other species, including the striped dolphin in the Mediterranean and the Pacific white sided dolphin in the North Pacific. It is also sometimes found in tuna schools in the eastern tropical Pacific and eastern tropical Atlantic, leading to much death by the eastern tropical Pacific tuna fishery.
A study performed by W. E. Evans used radiotelemetry to track common dolphins in the wild. Although it could not resolve issues of population size and home range, it could answer questions regarding diurnal and nocturnal behavior, diving behavior, and the individual movements over time and distance. Some radio packs included pressure transducers to measure the depth of dives. Eight specimens were equiped, and aerial observations were also used to correlate the findings. According to the study's findings, a herd may travel 120 kilometers in a single day, while some herds of 200-300 individuals travel 270 kilometers in ten days. The movements seemed to follow the sea-bottom topography. Some individuals orient to the sea mounts, canyons, and escarpments. These areas are thought to produce small planktonic animals fed on by small fish, which form much of the common dolphin's diet. The diving behavior is also correlated with the vertical movement of the DSL. The deepest dives occur after sunset and before dawn, when the DSL is high in the ocean. During night-time deep diving, dives may reach 200 feet, with one report of 846 feet. The longest dive was approximately four minutes, while most are less than one minute. Since the dolphins collect near escarpments and seamounts of depths of around 2000 meters, they probably make heavy use of echolocation.
The echolocation capabilities of Delphinus are similar to but less advanced than those of the bottlenose dolphin. In addition to echolocation, they use passive sonar, also called silent navigating, and sight to navigate. The former is always used in good weather and calm sea, where the visibility is up to 70-80 meters deep. It has been observed that they travel according to the sun. They never go north to south, but always east to west or west to east, depending on the time of day. This is probably a result of the movements of the prey species.
19 different whistle sounds have been recorded, more than the bottlenose dolphin (17) and the pilot whale. They are used for communication and to inidicate stress. They seem to correlate somewhat with behavior, but the sounds have not been identified as a structured language. In one case, a captive female died, and the male tankmate circled her, whistling continuously.
Although inidividuals have been reported with shark-bite scars, the primary enemy of the common dolphin appears to be the killer whale. In one case, a group of killer whales was circling a crowd of common dolphins. One killer whale left the circle and rushed the crowd, causing the common dolphins to become hysterical. The killer whales then started killing and consuming the dolphins. It was reported that the water turned red with blood. In addition, killer whales have been reported to scream to scare common dolphins. The dolphins travel in all directions, always running into a wall of sound. They become so terrified that they start swimming frantically in circles.
The name "dolphin" comes from the Greek word delphys, meaning womb. In ancient Greece, the term applied to all dolphins, as they made no distiction between different species. In fact, one species of fish (Coryphaena hippurus, now known as the dolphin fish, the mahi-mahi, or the dorado, was also called a dolphin by the Greeks, further adding to the confusion. In ancient times, the common dolphin was considered the most pure representative of the dolphin family, appearing on cups, coins, and friezes. Only relatively recently has the species been recognised as individual rather than an all-purpose "dolphin" classification. There have been thousands of years of confusion over what does and does not constitute a dolphin, and over what the different species of dolphin are.
In the past, the common dolphin was often confused with the bottlenose dolphin, especially in stories regarding the interation with humans. However, the common dolphin is more pelagic than the bottlenose, rarely being seen close to the shore. The taxonomy has been somewhat confused, as some people believe that there exist more than one species. Most differences involve physical appearance, especially the length of the snout and number of teeth, while others involve the behavior, as different stocks show somewhat different behavior patterns. In 1889, F. W. True listed nine different species in the genus Delphinus, and then after examining the individuals found only four valid species: D. delphinus, D. longirostris, D. capensis, and D. roseiventris.
Historically, the major threat of the common dolphin has been the Black Sea fishery. The fishery is not currently in operation, having been shut down in Russia and Romania in the 1960s and banned in Turkey since 1983. However, the threat of the Turkish fishery reopening looms. A major source of concern is that the catch of anchovies in the Black Sea is increasing, and anchovies are a major prey species of the common dolphin. The effects of this increase on common dolphin population are not known.
The eastern tropical Pacific tuna purse seine fishery has represented the most significant threat to the common dolphin. In 1986, it was estimated that 25,000 individuals were killed, and on average, four times as many common dolphins are killed as spotted dolphins. The proportion went from just over 7% in 1979 to around 1% in 1982-1984, and then back to almost 7% in 1986. The reasons for these fluctuations are not known. The 1987 kill was estimated to be 20,000, approximately 5% of the population.
In many parts of the range, small numbers are taken, mostly for human consumption and anmial feed. Such areas include the Azores, Venezuela, Peru, Japan, Spain, western Africa, and possibly Israel. In the Azores, the meat of dolphins is very popular. Individuals claim that it tastes like steak. In the Mediterranean, it is said that individuals are shot for entertainment. There are probably incidental takes wherever fishing occurs.
The common dolphin is listed in CITES Appendix II and in Appendix II of the Berne Convention. Meetings of the BS, a convention between the governments of Bulgaria, Romania, and the USSR were convened to exchange information on the Black Sea stocks and to coordinate a halt to catching by those three countries. The North and Baltic Sea populations are listed in CMS Appendix II. Unfortunately, there appears to be no effective legal barrier against the reopening of the Turkish fishery. The IUCN/SSC Action Plan recognises that the northeastern Mediterranean and coastal eastern tropical Pacific populations might be at risk and proposes specific projects to obtain the status of these stocks.
At least 90 individuals are known to have been kept in captivity worldwide. At least 37 were taken from the US Pacific coast, one from Japan, 22 from the Mediterranean, two from other parts of western Europe, and 28 from New Zealand. It is said that although this species is easy to catch, it is very difficult to keep in captivity. The New Zealand dolphinaria specialized on this species, although they have not been successful at inciting breeding. Very little research has been done on common dolphins in captivity since so few individuals have been kept for long periods of time. At least one individual is recorded to have been born in captivity. It lived for 16 weeks, and then died of a staphylococcus infection. In other cases, there were no survivors.
Individuals from different areas appear to behave differently in captivity, possibly indicating different subspecies. Individuals captured from North America are described as difficult to keep in captivity among several species. They are considered very shy and delicate, and are often in shock when they are first brought into captivity. Of 22 common dolphins captured for display between 1966 and 1972, all died by 1975. Most did not live 60 days, although 15% did survive more than one year. The New Zealand dolphins, on the other hand, seemed very trainable and clever. The trainer, Frank Robson, described using very unusual techniques in which he talked to the animals and made use of psychic powers.
Baker, Mary L. Whales, Dolphins, and Porpoises of the World. New York: Doubleday & Company, Inc., 1987.
Carwardine, Mark. Eyewitness Handbooks: Whales, Dolphins, and Porpoises. New York: Dorling Kindersley Ltd., 1995.
Ellis, Richard. Dolphins and Porpoises. New York: Alfred & Knopf, Inc., 1982.
Klinowska, Margaret. Dolphins, Porpoises, and Whales of the World: The IUCN Red Data Book. Gland, Switzerland: World Conservation Union, 1991.