There are 32 species of dolphins, members of the cetacean family Delphinidae, that are distributed in all of the oceans of the world. These marine mammals are usually found in relatively shallow waters of coastal zones, but some may be found in open ocean. Dolphins are a relatively modern group; they evolved about 10 million years ago during the late Miocene. The Delphinidae represents the most diverse group, as well as the most abundant, of all cetaceans. Among the delphinids are the bottlenose dolphins (Tursiops truncatus ), best known for their performances in oceanaria; the spinner dolphin (Stenella longirostris ), which have had their numbers decline due to tuna fishermen's nets; and the orca or the killer whale (Orcinus orca ), the largest of the dolphins. Dolphins are distinguished from their close relatives, the porpoises, by the presence of a beak.
Dolphins are intelligent, social creatures, and social structure is variously exhibited in dolphins. Inshore species usually form small herds of two to 12 individuals. Dolphins of more open waters have herds comprised of up to 1,000 or more individuals. Dolphins communicate by means of echolocation, ranging from a series of clicks to ultrasonic sounds, which may also be used to stun its prey. By acting cooperatively, dolphins can locate and herd their food using this ability. Aggregations of dolphins also have a negative aspect, however. Mass strandings of dolphins, a behavior in which whole herds beach themselves and die en mass, is a well-known phenomenon but little understood by biologists. Theories for this seemingly suicidal behavior include nematode parasite infections of the inner ears, which upsets their balance, orientation, or echolocation abilities; simple disorientation due to unfamiliar waters; or even perhaps magnetic disturbances.
Because of their tendency to congregate in large herds, particularly in feeding areas, dolphins have become vulnerable to large nets of commercial fishermen. Gill nets , laid down to catch oceanic salmon and capelin, also catch numerous non-target species, including dolphins and inshore species of porpoises. In the eastern Pacific Ocean, especially during the 1960s and 1970s, dolphins have been trapped and drowned in the purse seines of the tuna fishing fleets. This industry was responsible for the deaths of an average of 113,000 dolphins annually and in 1974 alone, killed over half a million dolphins in their nets. Tuna fishermen have recently adopted special nets and different fishing procedures to protect the dolphins. A panel of netting with a finer mesh, the Medina panel, is part of the net furthest from the fishing vessel. Inflatable power boats herd the tuna as the net is pulled under and around the school of fish. As the net is pulled toward the vessel many dolphins are able to escape by jumping over the floats of the Medina panel, but others are assisted by hand from the inflatable boats or by divers. The finer mesh prevents the dolphins from getting tangled in the net, unlike the large mesh which previously snared the dolphins as they sought escape. Consumer pressure and tuna boycotts were major factors behind this shift in techniques on the part of the tuna fishing industry. To advertise this new method of tuna fishing and to try to regain consumer confidence, the tuna fishing industry has begun labeling their products "dolphin safe." This campaign has been successful in that slumping sales from the boycotts have picked up over the last few years.
[Eugene C. Beckham ]
Dolphins, Porpoises and Whales of the World. Gland, Switzerland, IUCN—The World Conservation Union, 1991.
Evans, P. The Natural History of Whales & Dolphins. New York: Facts on File, 1987.