Sound of home draws tiny fish to reef safety

By SIMON COLLINS, Science reporter

Baby fish are using sound to find their way to coastal reefs on which to live, say Auckland University researchers.

Professor John Montgomery of the university's Leigh marine laboratory says the 2cm semi-transparent larval fish, less than three months old, sometimes swim hundreds of kilometres to find suitable reefs.

He and Dr Andrew Jeffs of the National Institute of Water and Atmospheric Research (Niwa) set four traps in Omaha Bay with lights to attract the tiny fish.

Two of the four traps also played recordings of sound from a nearby reef through a speaker suspended two metres underwater.

The two speaker-equipped traps attracted four times as many baby triplefin fish as those without sound.

The experiment has excited worldwide interest partly because it may offer a means to repopulate fished-out reefs.

Scientists once thought the baby fish floated aimlessly on ocean currents until they hit suitable reefs.

Dr Montgomery said in December that his results showed the larvae "seem to know where they're going".

Their sound-based navigation adds to mounting evidence from other sources that sound plays a big role in fish mating and spawning.

Fish make noises by moving muscles near their "swim bladders" - pockets of gas just under the skin that can be 10cm to 12cm long in a 30cm snapper.

The fish can rise or sink in the sea by taking in or letting out air.

Dr Montgomery believes swim bladders help most fish to hear. Although they have no human-like outer ears, they can pick up sound with their inner ears, and sound waves pulsating through their swim bladders may tell them from which direction sound is coming.

"Sound travels very well under water," Dr Montgomery told the ifHeraldnf.

David Snell, a student doing his doctorate on how crab larvae use sound to navigate, said some sounds from ships and whales had been traced for hundreds of kilometres.

But Dr Montgomery said triplefins, a common species on rocky reefs around New Zealand, did not have swim bladders.

"So it's still a mystery as to how they might locate the sound."

He believes triplefins and other species without swim bladders, such as sharks, may be able to tell where noise is coming from by staying close to the surface.

While some sound waves travel directly from underwater reefs to approaching fish, other waves travel upwards to the surface and bounce back into the water, giving the fish a second "fix" on where the sound is coming from.

"That may be why we have the classic image of sharks swimming just below the surface with their fins sticking out of the water," Dr Montgomery said.

They needed to be near the surface to navigate.

After being blown sometimes hundreds of kilometres away as babies, triplefins find a good spot on a reef and usually keep within a few metres of that spot for the rest of their lives.

Dr Montgomery said there was "an active controversy" among scientists on whether baby fish returned to the same spot where their parents lived, or just found the nearest suitable reef.

The research also shows that baby fish are affected by the phases of the moon.

They are more likely to return to reefs on the weak tides of quarter-moons, rather than battling through the strong new moon and full moon tides.