Fish communicate with each other in a secret language of grunts, growls, chirps and pops, researchers in New Zealand have discovered.
Goldfish were found to be good listeners Photo: IAN JONES
Far from being a place of deep silence, the underwater world is abuzz with the sound of fish sweet-talking the opposite sex, warning others of danger, giving directions, and general background chatter.
Predators may even hunt out prey by intercepting fish talk, researcher Shahriman Ghazali of Auckland University said.
"All fish can hear but not all can make sound -- pops and other sounds made by vibrating their swim bladder, a muscle they can contract," he said.
Mr Ghazali, who is presenting a paper on his research to fellow marine scientists in Wellington this week, hopes to decipher the contexts for different types of communication.
"This is the next step. We are 99 per cent sure they are fish sounds.
"Now we want to find out what the sounds mean," he told the New Zealand Herald.
The main reasons appear to be attracting mates, scaring off predators, and orientating themselves around reefs, he said.
He placed groups of fish into tanks in a laboratory, gave them a few weeks to settle in, and monitored them using an underwater microphone and instruments that detect water movement.
It emerged that gurnard are among the most talkative, making distinctive grunts and keeping up a pattern of chatter throughout the day.
Cod, on the other hand, stay mostly silent, except while spawning when they become very vocal.
"The hypothesis is that they are using the sound as a synchronisation so that the male and female release their eggs at the same time for fertilisation," Mr Ghazali said.
"Outside spawning season, you won't hear a sound from them."
Fish known as bigeyes produce a popping sound, which appears to operate as a sort of Morse code.
Mr Ghazali debunked a commonly held belief that crayfish in New Zealand waters make a similar popping sound when divers approach.
"I didn't get any sound from any of them," he said.
He also advised pet owners who tap the bowls of goldfish not to hold their breath for a reply.
"Goldfish have excellent hearing but they don't make any sound whatsoever," he said.
Fish are far cleverer than scientists have previously believed
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