The Indo-Pacific bottlenose dolphin (Tursiops aduncus)
The Indo-Pacific bottlenose
dolphin (Tursiops aduncus) is a species of bottlenose dolphin. This
dolphin grows to 2.6 metres (8.5 ft) long, and weighs up to 230 kilograms (510
lb).It lives in the waters around India, northern Australia, South China, the
Red Sea, and the eastern coast of Africa. Its back is dark grey and its belly is
lighter grey or nearly white with grey spots.
Until 1998, all bottlenose dolphins were considered members of the single species T. truncatus. In that year, the Indo-Pacific bottlenose dolphin was recognized as a separate species. The Indo-Pacific bottlenose dolphin is generally smaller than the common bottlenose dolphin, has a proportionately longer rostrum, and has spots on its belly and lower sides. It also has more teeth than the common bottlenose dolphin — 23 to 29 teeth on each side of each jaw compared to 21 to 24 for the common bottlenose dolphin. There is evidence the Indo-Pacific bottlenose dolphin may actually be more closely related to certain dolphin species in the genera Stenella and Delphinus, especially the Atlantic spotted dolphin (S. frontalis), than it is to the common bottlenose dolphin.
Much of the old scientific data in the field combine data about the Indo-Pacific bottlenose dolphin and the common bottlenose dolphin into a single group, making it effectively useless in determining the structural differences between the two species. The IUCN lists both species as data deficient in their Red List of endangered species because of this issue.
The Indo-Pacific Bottlenose Dolphin has a discontinuous
distribution in the warm temperate to tropical Indo-Pacific, from South Africa
in the west, along the rim of the Indian Ocean (including the Red Sea, Persian
Gulf and Indo-Malay Archipelago as far east as the Solomon Islands and possibly
New Caledonia) to the southern half of Japan and southeast Australia in the east
(Wells and Scott 2002; Möller and Beheregaray 2001). It is also found around
oceanic islands distant from major land masses within this range.
Australia; Bahrain; Bangladesh; Brunei Darussalam; Cambodia; China; Comoros; Egypt; Eritrea; India; Indonesia; Iran, Islamic Republic of; Japan; Kenya; Korea, Republic of; Madagascar; Malaysia; Mayotte; Mozambique; Myanmar; New Caledonia; Oman; Pakistan; Papua New Guinea; Philippines; Saudi Arabia; Singapore; Solomon Islands; Somalia; South Africa; Sri Lanka; Taiwan, Province of China; Tanzania, Thailand; Timor-Leste; United Arab Emirates; Yemen
Few estimates of abundance have been made. There are
estimated to be 520-530 bottlenose dolphins off KwaZulu-Natal, South Africa,
most of which are probably T. aduncus (the rest T. truncatus –
Cockcroft et al. 1992). Between 136 and 179 (95% Cis = 124-212) are known off
Zanzibar, Tanzania (Stensland et al. 2006); at least 1,200 in the Persian Gulf
(Preen 2004); at least 400 photo-identified along the rim of the
Swatch-of-No-Ground, Bangladesh (Rubaiyat Mansur Mowgli and Brian D. Smith pers.
comm.); about 218 off western Kyushu, Japan (Shirakihara et al. 2002); 169 off
Mikura Island, Japan (Kogi et al. 2004); low tens at the southern tip of Taiwan
and about 50 from the Northeastern Philippines (J. Y. Wang pers. comm.); 1,099
off Queensland, eastern Australia (Chilvers and Corkeron 2003) and at least
2,000-3,000 in Shark Bay western Australia (Preen et al. 1997).
Population Trend: Unknown
Habitat and Ecology:
Indo-Pacific bottlenose dolphins generally occur over shallow
coastal waters on the continental shelf or around oceanic islands. They
sometimes occur in mixed groups with common bottlenose dolphins and other
delphinid species. They feed on a wide variety of schooling, demersal and reef
fishes, as well as cephalopods (Ross 1984; J.Y. Wang, unpubl. data).
Systems: Marine. Diet
Indo-Pacific bottlenose dolphins feed on a wide variety of fish and cephalopods (particularly squid).
In a recent study conducted by Amir et al. (2005), researchers looked at the feeding ecology of Indo-Pacific bottlenose dolphins by analyzing the stomach contents of ones that got caught in the gillnet fisheries off Zanzibar, Tanzania. The prey items found in the stomach contents included 50 species of bony fish and three species of squid. From their results, the researchers concluded the most important prey group was fish, which accounted for 87% of the total number of prey items consumed and occurred in 24 of 26 stomachs examined. Cephalopods comprised the other 13% of prey items and were found in 13 of the 26 stomachs. The remains of some crustaceans were also found; they hypothesize, however, they were consumed secondarily, since a number were found intact in the fish prey stomachs and therefore were not included in the diet analysis.
Indo-Pacific bottlenose dolphins live in groups that can number in the hundreds, but groups of five to 15 dolphins are most common. In some parts of their range, they associate with the common bottlenose dolphin and other dolphin species, such as the humpback dolphin.
The peak mating and calving seasons are in the spring and summer, although mating and calving occur throughout the year in some regions. Gestation period is about 12 months. Calves are between 0.84 and 1.5 metres (2.8 and 4.9 ft) long, and weigh between 9 and 21 kilograms (20 and 46 lb). The calves are weaned between 1.5 and two years, but can remain with their mothers for up to five years. The interbirth interval for females is typically four to six years.
In some parts of its range, this dolphin is subject to predation by sharks; its life span is more than 40 years.
Indo-Pacific bottlenose dolphins located in Shark Bay, Australia are thought to have a symbiotic relationship with sponges by doing what is called “sponging”. A dolphin breaks a marine sponge off the sea floor and wears it over its rostrum, apparently to probe substrates for fish, possibly as a tool or simply for play.
The species’ near-shore distribution makes it vulnerable to
environmental degradation, direct exploitation, and fishery conflicts (Curry and
Smith 1997, Wells and Scott 1999; Reeves et al. 2003). Until hunting was
outlawed in 1990, this species was hunted in a large-scale drive fishery in
Taiwan’s Penghu Islands. Some Indo-Pacific bottlenose dolphins are taken in the
small cetacean fisheries of Sri Lanka.
Incidental catches occur in a number of fisheries throughout the range, including gillnets and purse seines. A Taiwanese shark gillnet fishery operated in northern Australian waters during the early 1980s and took up to 2,000 per year (Harwood and Hembree 1987). Incidental catch in Taiwan continues to be a serious problem. For example, multiple individuals have been seen observed in single catches there and throughout most of the species’ range (J.Y. Wang pers. comm.). A large proportion of dolphins (~40%) off Bangladesh exhibit scars and mutilations consistent with rope and net entanglement in trawl and gill-net fisheries (Rubaiyat Mansur Mowgli and Brian D. Smith pers. comm.). In South Africa and Australia, Indo-Pacific Bottlenose dolphins also suffer considerable mortality in the large-mesh nets set to protect bathers from sharks (Peddemors 1999; Reeves et al. 2003).
Live-captures for oceanarium display have taken place in Taiwan, Indonesia and the Solomon Islands in recent years from unassessed populations; their preference as a captive display species makes them vulnerable to depletion from such catches (Wang et al. 1999, Reeves et al. 2003, Kahn ).
Indo-Pacific bottlenose dolphins in coastal areas are exposed to a wide variety of threats in addition to direct and indirect takes. Threats that are cause for concern include: 1) the toxic effects of xenobiotic chemicals; 2) reduced prey availability caused by environmental degradation and overfishing (Jackson et al. 2001); 3) direct and indirect disturbance and harassment (e.g. boat traffic and commercial dolphin watching and interactive programs); 4) marine construction and demolition and 5) other forms of habitat destruction and degradation (including anthropogenic noise). Although these and other threats are technically challenging to quantify by comparison with takes, their cumulative impact is likely to result in longitudinal population declines. Lack of historical data in many cases hampers understanding of long-term trends, possibly resulting in shifting baselines.
The species is listed in Appendix II of CITES.
More research is needed to establish the range and clarify the taxonomy of the genus Tursiops. More information is also needed on population size and the extent and magnitude of direct and indirect takes so that their impact on this species can be assessed.
Effects of whale watching
Not much is known about the impact of whale watching on cetaceans, but research is being conducted at several locations.
Morisaka et al. (2005) conducted a study on three populations of Indo-Pacific bottlenose dolphins in Japan. The characteristics of acoustic signals are believed to be affected by the acoustic environments among habitats, and geographical variation in animal acoustic signals can result from differences in acoustic environments; therefore, the characteristics of the ambient noise in the dolphin's habitats and the whistles produced were compared. Ambient noise was recorded using a hydrophone located 10m below the surface and whistles were recorded by using an underwater video system.
The results showed dolphins produced whistles at varying frequencies with greater modulations when in habitats with less ambient noise, whereas habitats with greater ambient noise seem to cause dolphins to produce whistles of lower frequencies and fewer frequency modulations. Examination of the results suggest communication signals are adaptive and are selected to avoid the masking of signals and the decrease of higher-frequency signals. They concluded ambient noise has the potential to drive the variation in whistles of Indo-Pacific bottlenose dolphin populations.
Jervis Bay, Australia
Small, motorized vessels have increased as a source of anthropogenic noise due to the rise in popularity of wildlife viewing such as whale watching. Lemon et al. (2006)) showed powerboat approaches within 100m altered the dolphin surface behaviour from traveling to milling, and changed their direction to travel away from the powerboat. When the powerboat left the area and its noise ceased, the dolphins returned to their preceding behaviour in the original direction.
Shark Bay, Australia
Another study by Bejder et al. (2006) in Shark Bay, Western Australia on dolphin behavioural responses showed there were significant changes in the behaviour of targeted dolphins when compared with their behaviour before and after approaches by small watercraft. Dolphins in the low traffic site showed a stronger and longer-lasting response than dolphins in the high traffic site. These results are believed to show habituation of the dolphins to the vessels in a region of long-term vessel traffic. However, when compared to other studies in the same area, moderated responses, rather, were suggested to be because those individuals sensitive to vessel disturbance left the region before their study began.
Although these studies do show statistical significance for the effects of whale-watching boats, these results do not have biological significance and need to be researched further.