Each dolphin in the Shannon estuary can be identified by its own whistle, writes Yvonne Cunningham.
Researchers are using the unique signature whistles used by communicating dolphins as a way to identify and count individual dolphins living in the Shannon estuary.
Scientists at Galway Mayo Institute of Technology (GMIT) are conducting the study in the estuary, which is a Special Area of Conservation under the EU Habitats Directive. It is home to the only known resident group of bottlenose dolphins here and is a known calving area for this highly social mammal.
"Dolphins use sound quite a bit because it carries well underwater, where it's not always that easy to see," says John Cunningham, GMIT lecturer and researcher.
Dolphins make a variety of sounds to communicate and navigate. "Dolphins use whistles for social reasons, if they want to communicate or tell other dolphins that they have located food," Cunningham explains.
"When they are foraging or they locate prey they make a sound which tends to attract other members of the dolphin community, also they have a general kind of alarm signal. These whistles are called contour whistles and are all about one or two kilohertz so they're within the human audible range," he says.
While they use whistling to communicate, dolphins use different sounds for navigation. "Beyond the human audible range you have clicks which are echolocation signals. Dolphins use these like bats use sonar to identify objects underwater," says Cunningham.
The GMIT team is studying all of the various sounds, but is focusing on the whistles that they believe establish dolphin identity. "This study is about contour whistles. The signature whistle is a subset of the contour whistles which is a characteristic of the individual dolphins," says Cunningham.
"A dolphin will emit a characteristic whistle if he is isolated away from the main group."
Dolphins in the Shannon estuary come in close to shore to feed. When they do, the sounds they make are captured by underwater microphones and travel along a cable to a computer housed in a cowshed in Carrigaholt. Joanne O'Brien of the Shannon Dolphin Wildlife Foundation oversees this sound capture activity.
"We can use this system to record the dolphins as and when we want," says Dr Simon Berrow, project manager at the Shannon Dolphin Wildlife Foundation.
THE HOURS AND hours of recorded data are then analysed by the GMIT group. The team must first get rid of the background sounds of snapping shrimp. "Snapping shrimp are generally found in warmer waters than here. They're amazing really, they make a very loud snapping sound by clamping a claw into a socket on their bodies, they've even been known to interfere with submarines," says Cunningham.
The whistle frequencies are then scanned and converted into graphs. "We scan multiple recordings, then graph them and compare the shapes to see if the whistles fall into different groups or categories. Sometimes shapes repeat themselves - these are the dolphins' signature whistles," says Cunningham.
They also examine the whistles statistically to see if they can be classified into different groups on the basis of sound frequency.In some cases they are be able to match individual dolphins to their whistles.
"Dolphins can be visually identified by markings on their dorsal fin as they do quite a bit of biting, and if we're careful we may be able to pin down and compare visual observation of the dolphin to the sound they make," says Cunningham.
The overall whistle activity will give researchers an idea of what the dolphin population is, and the estuary can be monitored round the clock.
"We can acoustically monitor the estuary night and day to pick up sounds, it's a lot easier to monitor acoustically than visually," says Cunningham.
Signature whistles can already be matched to individual dolphins in pools. "You can ascribe very definitely identities to dolphins in pools, by identifying the sounds and analysing the frequencies of their whistles," he states. "But it is harder to do this with a big group of dolphins in the wild such as the group of bottlenose dolphins in the Shannon estuary. It is difficult to record in the Shannon estuary because there are about 30 dolphins in the group."
The study is being carried out as a master's thesis by postgraduate Sabina Clarke under the supervision of John Cunningham.
The United Nations has designated 2007 Year of the Dolphin