Dolphin

Dolphins are mammalian toothed whales, or odontocetes, belonging to the family Delphinidae, the oceanic dolphins, or to the river dolphin families Platanistidae and Iniidae. They are swift swimmers with spindle-shaped bodies and beaklike snouts, and most measure less than 3 metres (10 feet) in length. Their distribution spans marine environments worldwide, from equatorial to subpolar waters, as well as several major river systems.

Dolphins are toothed whales, or odontocetes, and the term covers the oceanic dolphins of the family Delphinidae together with the river dolphin families Platanistidae and Iniidae. Delphinidae contains nearly 40 species, six of which are commonly called whales, among them the killer whale and the pilot whales. The killer whale is the largest member of Delphinidae, reaching a length of up to 32 feet and a weight of up to 11 tons, with a lifespan of 30 to 90 years.

Two species of bottlenose dolphin are recognized: the common bottlenose dolphin (Tursiops truncatus) and the Indo-Pacific bottlenose dolphin (Tursiops aduncus), and both can occur in the same area. A third possible species, the Burrunan dolphin, has been proposed for inshore populations of bottlenose dolphins in Australia.

Dolphins first appear in the fossil record in the Early Miocene Epoch, between 23 million and 16 million years ago.

Dolphins are spindle-shaped animals with beaklike snouts, and the common bottlenose dolphin takes its name from its short, thick rostrum. The common bottlenose dolphin measures 6 to 13 feet in length and weighs 300 to 1,400 pounds, while the Indo-Pacific bottlenose dolphin tends to be smaller and more slender, reaching a maximum adult length of 2.6 m and a maximum weight of 230 kg for adult males. Spinner dolphins are relatively small, with slender bodies, thin recurved flippers, and dorsal fins usually ranging from slightly curved to erect and triangular.

Among the smallest dolphins are the Māui and Hector's dolphins, whose adults are about 1.5 m long. Hector's dolphins are one of the world's smallest dolphins and are mostly found around the coast of New Zealand's South Island, while Māui dolphins are one of the rarest and smallest dolphins in the world.

Coloration and anatomy vary considerably across species. Pantropical spotted dolphins have no spots at birth but accumulate them with age until almost completely covered with overlapping spots. The body color of Inia geoffrensis varies with age: young individuals are dark gray, while adults have a solid or blotched pink hue, and males are significantly pinker than females. None of the cervical vertebrae of Inia geoffrensis are fused, which allows it to move its head in all directions. A killer whale's ears are small openings behind the eyes without outer flaps, and it hears whistles and clicks through an auditory bulla, an earbone complex in its lower jaw.

Dolphins occur worldwide in tropical and temperate inshore, coastal, shelf and oceanic waters, and in several major river systems, ranging from equatorial to subpolar waters. Bottlenose dolphins inhabit harbors, bays, gulfs and estuaries, nearshore coastal waters, deeper waters over the continental shelf, and the open ocean far offshore. Killer whales are most abundant in colder waters such as Antarctica, the north Atlantic, and the north Pacific, but also occur in tropical and subtropical waters. The pantropical spotted dolphin occurs in the Pacific, Atlantic and Indian Oceans as well as the Persian Gulf and the Red Sea, while the Atlantic spotted dolphin is limited to warm-water regions of the Atlantic.

River and coastal populations show pronounced specializations. Botos (Inia geoffrensis) during the wet season navigate smaller tributaries and venture into river floodplains and flooded forests. The Hawaiian population of spinner dolphins spends daylight hours in coastal waters, generally in calm bays, used to rest, socialize, care for young, and avoid predators. Māui dolphins are only found on the west coast of the North Island in New Zealand, while Hector's dolphins are mostly found around the coast of New Zealand's South Island and are endemic to New Zealand.

Spinner dolphins follow a fission-fusion social pattern, fusing into large schools of hundreds of animals when feeding at night and splitting into much smaller groups, sometimes of only a dozen individuals, when socializing and resting during the day.

At night pantropical spotted dolphins dive into deeper waters to search for prey, feeding primarily on mesopelagic cephalopods and fish.

Shark Bay's dolphin population has incredibly rich social lives and an intriguing repertoire of culturally transmitted foraging innovations that involve tool use.

Common bottlenose dolphins produce individually distinctive signature whistles that are learned early in life and that help animals recognize and maintain contact with conspecifics. By the operational definition used in this field, a signature whistle is the predominant whistle type produced when an animal is isolated from conspecifics. Beecher's Information Statistic, calculated to quantify the individual distinctiveness of signature whistles, was found to be higher than for any other animal signal studied so far. The Sarasota Dolphin Whistle Database now contains 926 recording sessions of 293 individual dolphins, most of known age, sex, and matrilineal relatedness, and the longest time span over which an individual has been recorded is 43 years.

Killer whales eat a wide variety of large prey, including seals, sea lions, smaller whales and dolphins, fish, sharks, squid, sea turtles, sea birds, and sea otters. One ecotype of killer whales in the U.S. Pacific Northwest called Residents exclusively eats fish, mainly salmon, while another ecotype, called Transients or Bigg's killer whales, primarily eats marine mammals and squid.

The most common predators of bottlenose dolphins are larger sharks such as bull, tiger, and dusky sharks, which prey on smaller dolphins, calves, and females more than on larger dolphins.

Female bottlenose dolphins typically reach sexual maturity between 5 and 10 years of age, while males mature between 8 and 13 years old.

Common dolphin gestation usually lasts 10 to 12 months, and the calf is born tail first at 3 feet long and 25 to 35 pounds. Females produce a calf once every two to three years.

The common bottlenose dolphin is assessed as Least Concern on the IUCN Red List of Threatened Species, under version 3.1, in an assessment made in 2019 that retained the listing from 2008. It ranks among the most common cetacean species worldwide, and the sum of available abundance estimates is around 750,000, though most of the species' range has not been surveyed, so actual abundance is considerably higher. No evidence indicates that threats are producing a global population decline. Several populations nonetheless give cause for concern: the Fiordland subpopulation in New Zealand is Critically Endangered, the Mediterranean subpopulation is Vulnerable, and the Black Sea subspecies is Endangered. Mediterranean bottlenose dolphins are considered Vulnerable to extinction, and the Black Sea population carries an IUCN Endangered status. Bycatch in fishing gear occurs throughout the species' range, but quantitative data on bycatch rates are lacking in most areas. In the United States, bottlenose dolphins are not endangered or threatened under the Endangered Species Act, but they are protected under the Marine Mammal Protection Act.

The Indo-Pacific bottlenose dolphin is designated as Data Deficient in the IUCN Red List of Threatened Species because it occurs in fragmented coastal populations and its range includes countries where little research has taken place.

Southern Resident killer whales are the only endangered population of killer whales in the United States, ranging from central California to southeast Alaska.

Māui dolphins are among the rarest and smallest dolphins in the world, and number between 48 and 64 individuals older than one year. They are classified as Nationally critical and face threats including fishing, toxoplasmosis, oil and gas exploration and boat strike. Hector's dolphins, also among the world's smallest dolphins, number about 15,000 individuals older than one year and are classified as Nationally vulnerable.

The Chinese river dolphin faces threats including fisheries interactions, habitat degradation, habitat impediments, and pollution. During November and December 2006 an international research team conducted an intensive six-week multi-vessel visual and acoustic survey covering the entire historical range. By the end of the expedition the team failed to find any evidence that the species survives and concluded that the Chinese river dolphin may now be extinct. Periodic, undocumented sightings have occurred but none have been confirmed.

Because previous studies of bottlenose dolphins had reported exposure to plasticizing chemicals and particle loads in gastrointestinal tracts but not inhalation, the objective was to determine whether inhalation is a viable route of microplastic exposure for free-ranging dolphins. Exhalation samples were opportunistically collected from dolphins in Sarasota Bay, Florida, and Barataria Bay, Louisiana, during catch-and-release health assessments. All dolphin samples contained at least one suspected microplastic particle, and the polymer composition of a subset was determined. The study is the first to document microplastic exposure via inhalation in a free-ranging cetacean species, and further research is needed to understand whether this exposure route adds to or worsens the risk of adverse health impacts from microplastics alone or alongside other stressors.

Because of the unique association between tuna and dolphins in the eastern Pacific Ocean and the purse seine fishery it supports, protecting dolphin stocks became a priority for the United States, and the Dolphin Protection Consumer Information Act was passed in 1990 to address the issue. NOAA Fisheries' Tuna Tracking and Verification Program, established under the Dolphin Protection Consumer Information Act, monitors all domestic cannery production and imports of frozen or processed tuna and tuna products to ensure compliance with dolphin-safe certification and labeling requirements. In most fisheries, U.S. regulations require a written statement from the captain of a vessel certifying that no purse seine net or other fishing gear was intentionally deployed on or used to encircle dolphins during the fishing trip and that no dolphins were killed or seriously injured in those sets. The Marine Mammal Protection Act paved the way for domestic and international agreements that have helped reduce dolphin bycatch in the yellowfin tuna fishery by more than 99 percent.

• 1959–1972
  Eastern Tropical Pacific tuna fishery
  Over three million pantropical spotted dolphins were killed in the fishery.^{16Spotted dolphin · International Whaling Commission ↗}
• 1970
  The Sarasota study begins
  Bottlenose dolphins in Sarasota Bay, Florida, became the focus of health and population abundance studies, one of the longest-running research efforts on a wild dolphin population.^{13First evidence of microplastic inhalation among free-ranging small cetaceans · PLOS ONE ↗}
• December 21, 1972
  Marine Mammal Protection Act
  The United States enacted the Marine Mammal Protection Act, which established a moratorium on the taking of marine mammals.^{17Marine Mammal Protection Act · NOAA Fisheries (U.S. National Oceanic and Atmospheric Administration) ↗}
• 1989
  Chinese river dolphin listed as endangered
  NOAA Fisheries listed Chinese river dolphins as endangered under the Endangered Species Act, by which time the species had already declined sharply in the Yangtze.^{18Chinese River Dolphin · NOAA Fisheries (U.S. National Oceanic and Atmospheric Administration) ↗}
• 1990
  Dolphin Protection Consumer Information Act
  The Dolphin Protection Consumer Information Act was passed.^{14Dolphin-Safe Tuna · NOAA Fisheries (U.S. National Oceanic and Atmospheric Administration) ↗}
• 1994
  Marine Mammal Protection Act amendments
  Made several key changes to how bycatch of marine mammals in commercial fisheries is regulated, including a goal for reducing incidental mortality and serious injury to rates approaching zero.^{15Bycatch · NOAA Fisheries (U.S. National Oceanic and Atmospheric Administration) ↗}
• 1999
  United States joins the AIDCP
  The United States signed on as a Party to the Agreement on the International Dolphin Conservation Program.^{14Dolphin-Safe Tuna · NOAA Fisheries (U.S. National Oceanic and Atmospheric Administration) ↗}
• 2000
  Tuna Tracking and Verification Program
  The Tuna Tracking and Verification Program was implemented to monitor compliance with dolphin-safe certification and labeling requirements.^{14Dolphin-Safe Tuna · NOAA Fisheries (U.S. National Oceanic and Atmospheric Administration) ↗}
• 2001
  Last confirmed baiji
  The last confirmed occurrence of the Chinese river dolphin was recorded; every later report of the species remained unconfirmed.^{18Chinese River Dolphin · NOAA Fisheries (U.S. National Oceanic and Atmospheric Administration) ↗}