The Indo-Pacific bottlenose dolphin (Tursiops aduncus) is a marine mammal inhabiting warm coastal waters across a vast geographical expanse. Recognized as a distinct species from the common bottlenose dolphin (Tursiops truncatus) in recent decades, it possesses unique characteristics.
Distinctive Physical Characteristics
The Indo-Pacific bottlenose dolphin has a robust body build, yet is generally more slender than its common bottlenose counterpart. Adults measure between 1.75 and 4 meters in length and can weigh up to 230 kilograms. Their coloration is a dark grey back that fades into a lighter, sometimes pink-tinted, belly.
A distinguishing feature of adult Indo-Pacific bottlenose dolphins is the presence of black spots or flecks on their undersides, though not all individuals display this ventral spotting. They possess a longer and more slender beak compared to the common bottlenose dolphin, and their dorsal fin is falcate or curved. These dolphins also have a greater number of teeth, ranging from 23 to 29 on each side of both jaws, which are more slender than those of the common bottlenose dolphin.
Habitat and Geographic Distribution
These dolphins prefer shallow, warm, coastal waters, often found in estuaries, bays, lagoons, and river mouths. Their distribution spans the Indo-Pacific region, from the coastlines of East Africa and the Red Sea, across the Indian Ocean, through Southeast Asia, and to the northern and eastern coasts of Australia.
While found throughout this extensive region, their distribution is often discontinuous, with populations tending to be relatively small and localized. They are primarily found over the continental shelf, in depths of less than 300 meters. Their ability to adapt to varying conditions, from busy ports to seagrass bays and coral reefs, underscores their resilience.
Behavior and Social Dynamics
Indo-Pacific bottlenose dolphins are highly social animals, often forming groups of 5 to 15 individuals, though larger aggregations are observed. Their social structure is a “fission-fusion” society, meaning group sizes and compositions can change frequently. Females often establish extensive networks with other females, while males form lasting alliances that may persist for a lifetime.
Communication and Hunting
Communication involves whistles and clicks, and they use echolocation for navigation and locating prey. Their hunting strategies include cooperative behaviors, such as herding fish into “bait balls” where dolphins take turns feeding. Some populations in Shark Bay, Australia, use marine sponges as tools to protect their rostrums while foraging on the seafloor, a behavior passed down through generations, primarily by females to their calves. Other hunting techniques include “shelling,” where dolphins trap prey in large snail shells and then shake the prey out at the surface.
Diet and Reproduction
Their diet is varied, consisting mainly of bony fish, but also includes cephalopods such as squid and crustaceans. Reproduction involves a gestation period of about 12 months, resulting in a single calf that weighs between 9 and 21 kilograms at birth and measures 0.8 to 1.1 meters in length. Calves are weaned between 1.5 and 2 years of age, but they may remain with their mothers for up to 5 years, with females breeding every 4 to 6 years.
Conservation Status and Threats
The Indo-Pacific bottlenose dolphin is classified as “Near Threatened” on the IUCN Red List. This status reflects their vulnerability to various human-induced threats. Habitat degradation is a significant threat, arising from coastal development, pollution stemming from industrial and agricultural runoff, and plastic waste.
Threats
Accidental entanglement in fishing gear, known as bycatch, represents a major cause of mortality for these dolphins across their range. Overfishing also contributes to the depletion of their prey species, impacting their food availability. Noise pollution from shipping and seismic surveys can disrupt their communication and echolocation abilities. In some regions, direct hunting of these dolphins still occurs, contributing to population declines.
Conservation Efforts
Conservation efforts include marine protected areas, such as the Adelaide Dolphin Sanctuary in Australia, to safeguard resident populations and their habitats. International agreements, like the Convention on the Conservation of Migratory Species of Wild Animals (CMS) Appendix II, also cover Indo-Pacific bottlenose dolphins, aiming for international cooperation in their conservation.