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.
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"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.
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