Indo-Pacific Bottlenose Dolphin

The Indo-Pacific Bottlenose Dolphin (Tursiops aduncus) is a charismatic and intelligent marine mammal frequently encountered in the warm coastal waters of the Indo-Pacific. Belonging to the family Delphinidae, these highly social cetaceans are a distinct species from the more widely known common bottlenose dolphin (Tursiops truncatus), with key morphological differences that allow for identification by keen observers.

Found across a vast range from South Africa, along the Indian Ocean rim, including the Red Sea and Persian Gulf, to southern Japan and Australia, these dolphins primarily inhabit shallow, coastal environments, estuaries, bays, and areas around oceanic islands. They typically occur at depths of less than 300 m, often preferring waters shallower than 50 m. Indo-Pacific Bottlenose Dolphins are known for their active surface behaviors, including leaping, surfing, and tail-slapping. They are also frequently observed bow-riding boats, a playful behavior that offers divers and boaters a unique opportunity for interaction. These dolphins are carnivorous, preying on a variety of fish and cephalopods such as squid. They live in complex social structures, typically forming groups of 5 to 15 individuals, though larger aggregations of over 100 can occasionally be seen. The species is currently listed as "Near Threatened" by the IUCN, highlighting the importance of understanding and protecting their coastal habitats. Divers may occasionally encounter this species at dive sites such as Maamigili Beyru (Whale Shark Point).

Identification

• Size: Adults can reach a maximum length of 2.7 m (9 ft) and weigh up to 230 kg (510 lb), with males generally being slightly larger than females. Calves are born at lengths between 0.84 and 1.5 m.
• Body Shape: Compared to the common bottlenose dolphin, the Indo-Pacific Bottlenose Dolphin has a more slender body build.
• Coloration: Their dorsal surface is typically slate blue or dark grey, with darker flippers and extremities. The underside is lighter, often grey or nearly white, and can have a pinkish tint.
• Ventral Spotting: A key distinguishing feature, especially in adults, is the presence of prominent dark grey speckling or black spots on their belly and lower sides. These spots tend to increase with age and are rarely seen on common bottlenose dolphins.
• Rostrum (Beak): They possess a proportionately longer, more slender, and better-defined rostrum or beak compared to the common bottlenose dolphin, which tapers more abruptly near the base.
• Dorsal Fin: The dorsal fin is tall and triangular, often described as larger and broader-based than that of the common bottlenose dolphin, and typically less falcate (curved). It may also appear slightly hooked.
• Melon: The melon (forehead) is generally smaller than that of the common bottlenose dolphin, with a gently curving profile and a sharp crease before the beak.
• Teeth: Indo-Pacific Bottlenose Dolphins typically have more teeth than their common counterparts, with 23 to 29 conical, single-rooted teeth on each side of each jaw.
• Other Features: They may exhibit a dark ring around the eye, and their cape (the darker area on the back) is often more distinct, sometimes with a light spinal blaze extending below the dorsal fin. Their flippers are long and pointed at the tips.

The Indo-Pacific Bottlenose Dolphin (*Tursiops aduncus*) is a highly adaptable marine mammal found across a broad range of environments in the warm temperate and tropical waters of the Indo-Pacific. This species is primarily a coastal inhabitant, favoring shallow, nearshore waters, but can also be found in more open ocean settings.

Habitat and Distribution

• Geographic Range: The Indo-Pacific Bottlenose Dolphin's distribution spans the Indian Ocean and the western Pacific Ocean. This includes the waters around India, northern Australia, South China, the Red Sea, the Arabian Gulf, and the eastern coast of Africa, extending as far east as the Solomon Islands and potentially New Caledonia, and north to southern Japan.
• Preferred Environments: These dolphins are commonly observed in a variety of coastal habitats, showcasing their ecological plasticity.
  • Coastal Waters: They are frequently found in shallow coastal areas, often within 10 km of the shoreline.
  • Estuaries and Bays: Estuarine environments, bays, and lagoons are significant habitats, where their distribution can vary with season and tidal state, influencing water temperature and prey availability.
  • Coral Reefs: Indo-Pacific Bottlenose Dolphins are also associated with coral reef ecosystems.
  • Other Substrates: While they utilize diverse habitat types, including seagrass beds, sandy substrates, and reefs, some studies indicate a preference for bare sand habitats, though seagrass use may increase seasonally. Complex bottom substrates, such as mixtures of rock, sand, and seaweed, are also preferred for activities like feeding and rubbing.
• Depth Range: This species typically inhabits shallow waters, generally at depths of less than 50 m. However, they have been recorded at depths of up to 300 m, particularly near the shore. In some inshore areas, they may prefer very shallow waters, sometimes less than 3 m deep.
• Water Temperature: Indo-Pacific Bottlenose Dolphins thrive in warm temperate to tropical waters, with surface temperatures typically ranging from 20–30°C. They can tolerate a wider range, with observations in waters as cool as 12°C in some regions.
• Ecosystem Associations: As apex predators in their coastal environments, they feed on a variety of fish and cephalopods, including squid and octopus. They are also known to associate with other dolphin species, such as the common bottlenose dolphin and humpback dolphins, in parts of their range. Their presence at dive sites like Maamigili Beyru (Whale Shark Point) occasionally highlights their use of productive coastal areas frequented by other large marine life.

The Indo-Pacific Bottlenose Dolphin (Tursiops aduncus) is a highly intelligent and social marine mammal, often encountered by divers in coastal and estuarine waters across its range from eastern Africa to Australia and the western Pacific Ocean.

Underwater Encounter Behavior

Indo-Pacific Bottlenose Dolphins are known for their social nature and can display a range of behaviors when encountering vessels and divers. While generally considered more shy and less inquisitive than common bottlenose dolphins, some populations have become habituated to human presence, particularly in areas with established dolphin swim programs. They are frequently observed bow-riding vessels, a behavior where they surf the pressure wave created by a boat's bow. Divers may observe them traveling, foraging, or engaging in social interactions. When interacting with vessels, dolphins may approach, swim alongside, or even engage in playful behaviors such as chasing and splashing. However, responses can vary, with some individuals or groups choosing to avoid vessels by diving or changing their travel direction and speed. In some unique instances, dolphins have been observed initiating "object play" by bringing items like seaweed to humans.

Feeding Habits

As carnivorous and opportunistic predators, Indo-Pacific Bottlenose Dolphins primarily feed on a diverse diet of bony fish and cephalopods such as squid and octopus. Their diet can vary regionally, with a preference for certain species depending on local availability. They employ various hunting techniques, including cooperative foraging, where groups work together to herd schools of fish. In shallow waters, they may use their tails to "kick" fish into the sand or chase small fish onto the shore. A remarkable foraging strategy observed in some populations, particularly females, involves using sea sponges to protect their sensitive rostrums while digging for fish in the sand, a skill passed down through generations. Foraging behavior often involves shallow dives multiple times per minute, and they may leap from the water while feeding. Studies have also indicated that they exhibit concentrated foraging behavior at night, with females often foraging more frequently during the daytime than males.

Social Structure

Indo-Pacific Bottlenose Dolphins are highly social animals, living in complex societies with a fission-fusion grouping pattern. They typically form pods ranging from 5 to 15 individuals, though occasional aggregations of over 100 have been observed. Their social structure can be sexually segregated, with mature males often forming strong, long-lasting alliances with 1 to 3 or even 3 to 4 other males to assist with mate guarding and defense against rival alliances. Females, on the other hand, tend to live in larger groups and maintain extensive networks of associates within their home ranges. Young dolphins spend several years with their mothers, learning essential survival and social skills. Communication within pods occurs through a diverse range of sounds, including echolocation clicks for navigation and feeding, and unique "signature whistles" that help individuals identify each other.

Notable Seasonal Patterns Relevant to Divers

Indo-Pacific Bottlenose Dolphins generally inhabit shallow coastal waters, typically at depths less than 300 m, and often within 50 m depth, near the shore or around oceanic islands. Many communities exhibit site fidelity and maintain year-round residence within small home ranges along the coastal zone. However, their spatial distribution can vary seasonally and with tidal states, influenced by factors such as water temperature and prey distribution. While mating and calving can occur throughout the year in some regions, peak birth rates are often observed from October to December or during spring and summer months. Studies in certain estuaries have shown peak sightings in spring, with variations in spatial distribution and behavior (e.g., feeding, socialising, resting) influenced by season, tidal phase, and tidal range. For instance, in some areas, higher overall abundance has been noted in winter and lower in summer, potentially linked to prey availability. Divers visiting dive sites like Maamigili Beyru (Whale Shark Point) may encounter these dolphins occasionally, reflecting their wide-ranging yet often resident coastal habits.