Beneath New Zealand waters a secret language is being spoken, of which humans know little.

Fish have been found to grunt, growl and "pop" at each other, according to University of Auckland marine scientist Shahriman Ghazali.

He is making underwater recordings at Leigh Marine Reserve to work out which fish talk and why.

"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."

Fish are believed to speak to each other for a number of reasons, including to attract mates, scare predators, or orientate themselves.

Mr Ghazali initially placed the fish in tanks, and after giving them several weeks to acclimatise, he began making underwater recordings.

If the fish began making noises, he tried to decipher what the context for the communication was.

"This is the next step," he said. "We are 99 per cent sure they are fish sounds, now we want to find out what the sounds mean."

Gurnard were found to have a wide vocal repertoire, and maintained a constant chatter.

He also debunked the observation by divers that crayfish made "popping" sounds. Big eye fish, found in similar crevices to crayfish, are responsible for the noise.

"Some species of crayfish elsewhere do make a sound, which is like strumming a guitar. So I caught a few and listened but didn't get any sound."

Some fish spoke only on special occasions. Mr Ghazali observed cod were mostly silent, but very vocal while spawning.

"The hypothesis is that they are using sound as a synchronisation so that the male and female release their eggs at the same time for fertilisation."

"Outside spawning season, you wouldn't hear a sound from them."

He believed predators could find prey by intercepting fish talk.

And species that made a habitat around reefs, such as the demsel fish, attempted to scare off threatening fish, or even divers, with sounds.

The human ear was designed to notice changes in air pressure, not water, so instruments were used to detect water movements, or signs or fish communicating, in the tanks.

Mr Ghazani also mentioned, for those people who tap fish bowls, that goldfish don't make any noise, but have exceptional hearing.

His paper on fish vocalisation is being presented at the New Zealand Marine Sciences Society conference in Wellington this week.