The common dolphin throughout history has often been recorded in art and literature. It was recently proposed that two forms of this species, the short- beaked (delphis) and long-beaked (capensis) common dolphin, represent two distinct species.
Common dolphins are colorful, with a complex crisscross or hourglass color pattern on the side; the long-beaked common dolphin being more muted in color. When looking at the profile of the two common dolphin species, the short-beaked common dolphin has a more rounded melon that meets the beak at a sharp angle, as compared to the long-beaked common dolphin that has a flatter melon that meets the beak at a more gradual angle.
Color patterns on the common dolphin are the most elaborate of any cetacean. The back is dark gray-to-black from the top of the head to the tail dipping to a V on the sides below the dorsal fin. The flanks are light gray behind the dorsal fin and yellowish-tan forward of the dorsal fin, forming an hourglass pattern. Its belly is white. There are large dark circles around the eyes connected by a dark line that runs across the head behind the beak and a black stripe runs from the jaw to the flippers.
FINS AND FLUKES
The dorsal fin is triangular-to-falcate (curved). It is pointed and located near the middle of the back and is black-to-light gray in color with a black border. The flippers are long and thin and slightly curved or pointed depending on geographical location. Flukes are thin and pointed at the tips with a slight notch in the center.
LENGTH AND WEIGHT
Common dolphins can reach lengths of 7.5 to 8.5 feet (2.3-2.6 m) and weigh as much as 297 lb. (135kg). The short-beaked common dolphin is relatively heavier, and has a larger dorsal fin and flippers than the long-beaked one.
Delphinus delphis feeds on squid and small schooling fish. In some parts of the world, they feed at night on the deep scattering layer, which moves towards the water’s surface during that time. Common dolphins have been seen working together to herd fish into tight balls. Like many other dolphin species, the common dolphin will sometimes take advantage of human fishing activities (such as trawling), feeding on fish escaping from the nets or discarded by the fishermen.
MATING AND BREEDING
Sexual maturity is reached at 3 to 4 years of age or when they reach 6 to 7 feet in length (1.8 to 2.1 m). Calves are 30 to 34 inches at birth (76 to 86 cm ); gestation period is 10 to 11 months.
DISTRIBUTION AND MIGRATION
The common dolphin may be one of the most widely distributed species of cetaceans, as it is found world-wide in temperate, tropical, and subtropical seas. The long-beaked common dolphin is found more in coastal waters; the short-beaked common dolphin is found in offshore waters, including the Eastern Atlantic Ocean as far south as Florida.
Like all mammals, dolphins are warm blooded, breathe air, give birth to live babies, feed their new born milk, and are born with hair. Being warm, blooded, or homeothermic, dolphins maintain a constant body temperature regardless of the surrounding water temperature. Unlike terrestrial mammals, including humans, dolphins are conscious breathers, meaning they must be aware of their breathing to avoid involuntarily taking a breath while underwater. Common dolphins can dive for as long as 15-20 minutes but typically hold their breath for only a few minutes.
Common dolphins may live for 35 years or more, with females generally living longer than males. Common dolphins are often found in large herds of hundreds or even thousands. They are extremely active, fast moving, and engage in spectacular aerial behavior. They are noted for stampeding in these extremely large groups across the ocean, riding bow and stern waves of boats, often changing course to bow ride the pressure waves of fast-moving vessels and even large whales. Common dolphins can be frequently seen in association with other marine mammal species.
Traditionally, hundreds of thousands of common dolphins have been taken incidentally, along with spinner and pan-tropical spotted dolphins, in purse seine nets used during tuna fishing operations in the eastern tropical Pacific although these numbers have improved. Common dolphins also may be caught accidentally in other fishing gear, such as midwater trawls. Turkish and Russian fishermen used to catch large numbers of common dolphins in the Black Sea for meat (to be used for fish meal) and oil. The fishery stopped after the common dolphin numbers became and remain severely depleted; there are several reports suggesting that the Turkish fishery may have recently resumed. Many common dolphins are taken in a Japanese small cetacean fishery and directly caught in the Mediterranean. Some common dolphins have been taken in Peru for human consumption.
Studies suggest that the immune system of these animals can be severely affected by heavy metals, PCBs and other pollutants. The status of common dolphins, relative to OSP, in the U.S. Atlantic Eastern Economic Zone (EEZ) is unknown. The species is not listed as threatened or endangered under the Endangered Species Act. There are insufficient data to determine the population trends for this species. The total fishery-related mortality and serious injury for this stock is not less than 10% of the calculated PBR and, therefore, cannot be considered to be insignificant and approaching zero mortality and serious injury rate. This is not a strategic stock because the 1999-2003 average annual fishery-related mortality and serious injury does not exceed PBR. However, the average annual mortality does not include the North Atlantic Bottom Trawl fishery, which is under analysis.
BIBLIOGRAPHY: For further details about common dolphins you may want to consult the following literature:
• Evans, W.E. 1994. Common dolphin, white-bellied porpoise Delphinus delphis Linnaeus, 1758. Pages 191-224 in Handbook of
Marine Mammals, vol. 5: The first book of dolphins (S.H. Ridgway and R. Harrison, eds.). San Diego: Academic Press.
• Marine Mammals of the World. FAO Species Identification Guide. Rome: FAO. Klinowska, M. 1991. Dolphins, Porpoises and
Whales of the World: The IUCN Red Data Book. Gland, Switzerland: IUCN (World Conservation Union).
• Whales and Dolphins. Leatherwood, S. and R. Reeves. 1983. San Francisco: Sierra Club Books.
ACKNOWLEDGMENTS: The information contained in this document was gathered from various sources, including NOAA, ACS and our own publications.