"Bigeyes are producing something like a popping sound but they organise them temporarily so it's like morse code."

To discover which fish was making each noise, Mr Ghazali brought groups of individual species from the sea to a tank at the laboratory. Using a hydrophone, he continually recorded crayfish to test if there was any basis to the commonly held belief they made sounds when divers approached.

"Funnily enough, I didn't get any sound from any of them," he stated. Instead, when the test was repeated with bigeye, an endemic nocturnal fish, it was discovered they were making the noises. It was possible sounds were made in response to divers approaching, and that other fish used sound for functions including communicating and orienting themselves around reefs.

Getting any fish to start making the sounds had not been easy while they were held in the tanks. They only made sounds in groups, and also took some time to adapt to their new environment. "Goldfish have excellent hearing, but excellent hearing doesn't associate with vocalisation – they don't make any sound whatsoever."