Purrs of Fish Could Be Sounds of Survival

Using a boat, an underwater microphone and a tape recorder, scientists say they can capture the “purring” and “knocking” sounds of fish.

Then, they believe, they can develop spawning maps that fishery managers can use to protect overfished species and their habitats.

Certain species of fish--many of them of high commercial value--make unique sounds during their reproductive phases, allowing scientists to identify them.

And where there are sounds of courting fish, fish eggs are not far behind, according to researchers who gathered in Seattle recently at the Joint 16th International Congress on Acoustics and the 135th meeting of the Acoustical Society of America.

The correlation between identifiable fish sounds and egg presence is significant, because fishery managers cannot protect spawning fish if they cannot first locate them, said Joseph Luczkovich, an associate professor of biology at East Carolina University.

“It’s very important that we know which fish we’re hearing, so that we can set up spawning grounds,” Luczkovich said. “Almost every fisherperson worldwide understands that if you over-harvest spawning aggregations, you’re not going to have a future.”

Luczkovich and fellow researcher Mark Sprague, an assistant professor of physics also at East Carolina University, found large volumes of sea trout eggs in parts of North Carolina’s Pamlico Sound.

At the same time, they recorded and analyzed choruses of sea trout that purred so loudly they could be heard from the boat. They found the same correlation for red drum fish, which produce a loud knocking sound when courting.

Sea trout, less commonly known as weakfish or corvina, and red drum are members of the drum and croaker family of fish. The researchers recorded underwater sounds measuring more than 145 decibels--a volume they said is close to that of loud rock music. They also found a strong correlation between the loudness of the sounds and egg production.

The fish make the sound with their swim bladder--a gas-filled sack in their bodies. The fish also emit sounds during aggressive encounters and to alert other fish, Luczkovich said.

Sprague said the existing method of locating fish by catching them in gill nets is inefficient and unreliable.

Paul Brouha, executive director of the American Fisheries Assn. in Bethesda, Md., said that sonar techniques also used to map fish populations can identify some fish species based on size and numbers, but “it’s definitely much more hit and miss.”

Luczkovich said it was hard to estimate what percentage of overfished species produce sounds that could be recorded using the techniques his team employed.

But he said other commercially important fish also make sounds, among them cod, mackerel and grouper. “Of the top 20 or so commercially harvested fish species . . . perhaps half of them produce sounds,” he said.

Brouha called the research “very impressive,” but he noted that it was a relatively new study that needs to be repeated over a period of years to verify that gatherings of vocalizing fish in spawning areas are not “chance aggregations.”

The researchers’ conclusions are reasonable in light of the habits of some land creatures--such as prairie chickens--that return to the same spot every year to mate, Brouha said.