Noisy reefs preferred by young fish

Travel brochures often use coral reef imagery to lure tourists to seemingly tranquil locales. Don't be fooled: Reefs are anything but quiet. And that's a welcome fact if you're a reef fish looking for a place to settle, scientists say.

Many reef fish coordinate their egg laying with the tides. That way, their baby fish, or larvae, drift out to sea upon hatching, explained Stephen Simpson, a tropical reef ecologist at the University of Edinburgh in Scotland.

"They do their development at sea, because it's a far safer place to be," Simpson said. The open waters are relatively predator free, allowing the larvae to grow and become strong swimmers before they brave life on the reef.

Once the larval fish are agile swimmers, the reef noise guides them back home, Simpson and his colleagues report in tomorrow's issue of the journal Science.

Stephen Swearer is a marine ecologist at the University of Melbourne in Australia. He said the finding adds to the growing body of evidence that "larval fish are far from the passive particles we used to think they were."

Until recently, scientists believed larval fish—typically under an inch (two centimeters) long—had little control over their environments and were at the whims of currents and tides.

Now research is showing that, as the larval fish develop, they become adept swimmers and can smell and hear reefs. By the time they reach the settlement stage, larval fish are aware of their environment, can react to predators, and are choosy real estate shoppers.

"What this study shows is that larval fish are using auditory information to make decisions about where to settle," Swearer said. "What we now need to determine is whether they use such information throughout the larval period."

Such knowledge, Swearer added, will provide insight to the question of whether larval fish tend to settle close to where they were born or venture to far-off reefs.

Artificial Reefs
To find out whether the reef ruckus might help guide larval fish, Simpson and his colleagues used dead coral to build 24 artificial reefs around Australia's Great Barrier Reef. On half of them, the researchers placed loudspeakers broadcasting reef noises.

The noises included the "frying bacon" sound of snapping shrimp. Other audio cues included grinding and popping sounds. Fish make the popping noises by sending air through different chambers of their swim bladders, the inflatable sacs that help fish float.

The researchers then counted the numbers of both damselfish and cardinalfish that settled on the artificial reefs.

Damselfish and cardinalfish are the most populous families of reef fish. Up to 25 percent of all fish on reefs are cardinalfish, and damselfish compose up to 50 percent of the total fish biomass (weight of all the fish) on reefs, the study says.

"So, they have a huge influence on how the reef functions, and due to their numbers, they are probably pretty important in the food chain for bigger fish," Simpson said.

In the experiment, both cardinalfish and damselfish settled on the noisy reefs in far greater numbers than on the quiet reefs.

Further experiments showed that cardinalfish showed no preference for the higher-pitched sounds of snapping shrimp or the lower-pitched fish sounds. Damselfish, however, were drawn to the shrimps' frying-bacon sounds.

"We think the sound gives the fish an indication of the quality of the reef, [which] the fish might be using as a means to pick the right place to come settle," Simpson said.

Some fish settled, at least for a day, on the silent reefs, Simpson added. Larval fish tend to time their settlement to dark, moonless nights to enhance their chances of evading predatory fish. If day dawned and a fish found itself on a big sand flat with only a nearby pile of silent rubble for shelter, the fish "would go swim to it—or die," Simpson said.

Conservation Implications
The reef ecologist said his team's findings raise the question of how noises from human activities like drilling for oil and gas or boating might affect fish navigation. Drilling-rig noise, for example, may "mask any natural cues the fish might be using," Simpson said.

On the other hand, fisheries managers may be able to use recorded reef noises to beckon fish to newly established marine reserves or to restock depleted fishing grounds.

According to Swearer, the University of Melbourne marine ecologist, whether this will work largely depends on from how far away fish can be attracted and whether larval fish are looking to settle near their native reefs or in new areas.

If the larval fish are attracted to new reefs from a wide area, "then yes, I would agree that [underwater audio] has the potential to be a method for ameliorating declining reef-fish populations," he said.