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Bottlenose dolphins, Tursiops truncatus, have a huge global distribution and are present in many inshore and offshore waters. Inshore animals are often the most commonly seen cetacean in an area, even though their actual numbers may be small. Many inshore populations have disappeared in the last 100 years. In the UK, conservation measures are in place for the two largest remaining inshore groups in Cardigan Bay, Wales and in the Moray Firth, Scotland.
Bottlenose dolphins, like most other dolphins, produce fewer tonal echolocation clicks than porpoises and can vary the pitch of the loudest tone. Clicks can have a bimodal frequency spectrum and can be varied by the animal during the course of a train of clicks. The spectral content of the click varies substantially as measured at increasing angles from the centre of the sound beam, with lower frequencies becoming more predominant away from the axis of the click. Wide bandwidth clicks are excluded by the T-POD and studies of Tursiops truncatus have generally shown that a percentage of groups, up to 40%, are missed. The wider bandwidth capabilities of the C-POD enable a wealth of click and train information to be collected from these same encounters. This is actually a feature of any simple directional source that is small in relation to the wavelength of the sounds produced. But in cetaceans, the sound source is complex and includes acoustic lensing achieved by layered specialised 'acoustic' fats that conduct sound at different speeds. Some cetaceans can modify the physical shape of the melon which is the large acoustic lens that appears to be the dolphin's, or whale's, forehead. The waveform and spectrum shown below illustrate a click from a bottlenose dolphin, Tursiops aduncus. The peak frequency is at 130 kHz with a lower mode around 30 kHz. More typically, bottlenose dolphin clicks have their energy peak at around 50 kHz. The energy peak of porpoises is never so low and this sharp distinction is useful in distinguishing dolphins from porpoises in T-POD surveys. A more helpful distinction is the bandwidth, which is smaller in porpoise clicks. When looking for porpoises, the T-POD can be set to exclude the wider bandwidth clicks from dolphins and, when looking for dolphins, the detection of clicks at 50 kHz excludes porpoises.
For more information on this species, please visit the Convention of Migratory Species web site.
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