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Dolphins have a taste for noisy fish

Dolphin dinner An analysis of dolphin faeces confirms that some dolphins prefer eating noisy prey.

The study, published today in the journal Royal Society Biology Letters, backs up a hypothesis based on the study of stomach contents from stranded dolphins.

Perth-based marine biologist Dr Glenn Dunshea says researchers do not usually analyse faeces to work out the diet of dolphins since "scooping it out of the water is pretty tricky".

But Dunshea struck it lucky with a population of bottlenose dolphins in Florida, which are regularly captured for health assessment as part of the Sarasota Dolphin Research Program.

"During that process they defecate and so you can collect it," he says.

"I was the only guy there who was excited when the dolphins pooed on the deck of the boat."

Dunshea took faecal and gastric juice samples from 19 dolphins and used a technique called "molecular prey detection" to work out what the dolphins had been eating.

This technique relies on the fact that DNA survives the digestive process so the DNA of prey will be present in faeces and gastric juices.

"Every species have DNA that's unique to that species," says Dunshea, who analysed mitochondrial DNA for his study.

Back in his laboratory in Tasmania, Dunshea extracted DNA from the samples he collected and confirmed an intriguing hypothesis, originally put forward by those at the Sarasota Dolphin Research Program, that the dolphins were showing a preference for 'noisy' prey.

"These particular animals seem to have a quite a high proportion of fish in their diet that actually make sound," says Dunshea. "They actually kind of grunt."

"The dolphins don't use their sonar to find the fish, they are actually quiet, and listen for noises that are being made by the fish and they use that as cue to hunt down their prey."

While Dunshea found the most common prey evident in dolphin faeces was a 'quiet' species -- called the pin fish -- this species is also prevalent in the environment.

By contrast, the relative amounts of 'noisy' species such as the gulf toad fish and spotted sea trout found in the dolphin diet was high compared to their relative abundance in the ecosystem.

This told Dunshea the dolphins were deliberately selecting these species.

Confirming previous studies

The results confirmed previous findings by the Sarasota Dolphin Research Program, which relied on analysing the stomach contents of dead stranded animals from the same population.

This is, in fact the usual way that people study the diet of whales and dolphins.

But, there has been concern that the stomach contents of stranded animals doesn't reflect what dolphins actually eat in the wild.

"People are worried that if the animals were sick then they were not eating what normal healthy animals eat," says Dunshea.

Either what they ate made them sick, or the dolphins were unable to catch their normal prey.

Importantly, Dunshea's research has now validated the accuracy of relying on the stomach contents of stranded animals in ecological studies of dolphins.

Dunshea carried out the research as part of PhD while at the Australian Marine Mammal Centre and the University of Tasmania.