Indo-Pacific Bottlenose Dolphin, Tursiops aduncus

The Indo-Pacific bottlenose dolphin (Tursiops aduncus) is one of three recognized species of bottlenose dolphin that can be found in the waters near southern Australia, South China, and India. Its range also includes the Red Sea and the eastern coastal areas of Africa. All bottlenose dolphins were classified as one species, the common bottlenose dolphin or T. truncates, until 1998 when the Indo-Pacific bottlenose dolphin was found to be distinct. This species is distinct from other bottlenose dolphins, and it is though that it may be more closely related dolphins in the Delphinus or Stenella genera.

The Indo-Pacific bottlenose dolphin can reach an average body length of about 8.5 feet and a weight of up to 530 pounds, although its size can vary depending upon its location. Its body is slender and its beak is longer than that of the common bottlenose dolphin. It is light blue in color with a lighter “cape” occurring on the back. Many adults bear small black spots along the underbelly, which helps distinguish this species from the common bottlenose dolphin.

Like other species of dolphins, the Indo-Pacific bottlenose dolphin is sociable. It typically gathers in smaller groups between five and fifteen individuals, but groups numbering in the hundreds are not uncommon. In some areas of its range, it will swim alongside other species of dolphin, including the humpback dolphin and the common bottlenose dolphin. Members of this species located in Shark Bay, Australia display a unique behavior, which is thought to show a symbiotic relationship with sponges. This behavior, known as “sponging,” involves the dolphin swims to the sea floor, picks a sea sponge from the floor, and places it on its nose. It is thought that this behavior may help it catch fish, but it may be a form of play.

The breeding season for the Indo-Pacific bottlenose dolphin occurs year round in some areas of its range, but in most areas, it occurs in the spring and summer seasons. After a pregnancy period of about twelve months, one calf is born weighing between twenty and forty-six pounds. Calves are weaned at up to two years of age, but may remain with their mothers for up to five years. After calving, females will not typically breed for another four to six years.

The diet of the Indo-Pacific bottlenose dolphin consists of cephalopods, like squid, and many types of fish. One study conducted in 2005 focused on the stomach content of individuals found in fisheries near   Zanzibar, Tanzania, which were accidentally caught in gillnet fishing gear. It was found these individuals fed on at least fifty different species of bony fish and three species of squid. This suggested a preference for fish, which comprised 87 percent of the stomach contents studied. It has a typical lifespan of about forty years or more.

In 2005, studies were conducted on three different populations of Indo-Pacific bottlenose dolphins in Japanese waters. This study showed that the ambient noise within the dolphin’s area is important to the types of calls the dolphins use to communicate. It was found that calls occurred more frequently with more variations in areas with less ambient environmental noise than areas with more ambient noise. This shows that the dolphins will modify their communication structure to adequately compensate for variations in environmental noise.

Although the effects of whale watching on the Indo-Pacific bottlenose dolphin are not known, studies are being conducted to identify and threats that it may cause. In 2006, a study conducted in Shark Bay, Australia found that dolphins located in low traffic areas were less likely to undergo behavioral changes than dolphins located in high traffic areas. This showed that the dolphins in this area are affected by ship travel, although the biological effects of this travel are unknown. Another study conducted in 2006, in Jervis Bay, Australia, showed that small boat traffic affected this species’ normal movement patterns. When boats were in the same area as the dolphins, within 328 feet, the dolphins would change their direction to swim away from the boat. The speed of the dolphins also changed from active and playful to slow. Once the boats were out of the area, the dolphins returned to their normal behavior and directional movements.

The main threat to this species is habitat destruction, although this is not a major threat. It is also vulnerable to human encroachment and encounters with gillnet fishing gear. It is hunted by sharks, orcas, stingrays, and humans. These threats have taken many individuals, but they are not thought to be a cause of concern.

The Indo-Pacific bottlenose dolphin is often found in captivity, which causes come concern regarding their conservation. There are risks involved when transporting wild individuals into captivity, sustaining the dolphins in an unnatural habitat, and accidentally introducing foreign disease and species into wild populations. Before 1980, there were more than 1,500 individuals taken to the United States, the Bahamas, and Mexico. The United States stopped capturing this species in the late 1980’s, and the numbers of births within aquariums increased from six percent in 1976 to forty-four percent in 1996.

The Indo-Pacific bottlenose dolphin as a whole appears on the IUCN Red List with a conservation status of “Data Deficient.” More research is needed to properly give the species a status that would support conservation efforts. However, it is found in the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region (Pacific Cetaceans MoU). The populations found in the Timor and Arafura Seas are listed in Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS). This grants them an unfavorable status that requires more research and possibly conservation efforts.

Image Caption: Indo-Pacific Bottlenose Dolphin. Credit: Laaude/Wikipedia (CC BY-SA 1.0)