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Fish Croaks Like a Frog, But Why?

John Roach
for National Geographic News
November 7, 2005
 
With its staccato, drumlike call, the Atlantic croaker (Micropogonias
undulatus)
sounds more like a frog than a fish.

Abundant from Cape Cod to Mexico, the species belongs to a family of fish known as drums. The fish makes its namesake call by vibrating special muscles against its swim bladder.

Why croakers croak when and where they do remains unclear. Damon Gannon, a staff scientist at the Mote Marine Laboratory in Sarasota, Florida, hopes to uncover some answers.

Such findings would further our knowledge of the species, which is a key food source for bottlenose dolphins, and the potential effects of human noise pollution on other fish.

In recent years studies have suggested that noise from ships, seismic oil exploration (the use of sound to find deposits), military sonar, and other sources adversely impacts whales and dolphins. But whether those same effects extend to fish is little studied, Gannon said.

Arthur Popper, an expert on noise pollution and fish at the University of Maryland in College Park, said, "There is concern, a valid concern, that the oceans are getting noisy."

The biologist said there is not much research on the issue, noting that peer-reviewed scientific literature on the subject is limited to fewer than ten papers on several fish species. Most of those papers have involved research conducted by Popper and his colleagues.

"The caveat is that there are 25,000 to 30,000 species of fish," he said. "And we know a little about effects of sound on only very few."

Not Like the Others

Gannon, of the Mote Marine Laboratory, first learned about Atlantic croakers' incessant croaking as a graduate student at the Duke University Marine Laboratory in Beaufort, North Carolina. At the time, Gannon was studying the feeding habits of bottlenose dolphins.

He found that dolphins eat mostly noise-making fish, such as spots, seatrouts, and croakers.

Gannon began to study how bottlenose dolphin diets differ in different habitats and found that the marine mammals gorge on croakers inside estuaries.

This was surprising because bottlenose dolphins find their prey by listening to them, Gannon said, noting that croakers spawn during the winter and that drum fish species typically croak only during mating season.

Intrigued as to why the dolphins eat so many croakers at a time of year when the croaker should be silent, Gannon searched through the scientific literature.

He learned that male and female croakers develop their sound producing apparatus at a very young age. The muscles required for producing the croaking sound stay fit year-round.

Other drum fish, and mostly just males, cycle through seasonal phases, building up their sound-producing muscles for spawning season and allowing them to atrophy the rest of the year.

This suggests that the sound-producing mechanism for most drum fish relates to mating calls.

"All this led us to idea that [the croakers] are obviously using sound differently from the other drum fishes," Gannon said. "That's what prompted us to go and study sound use by croakers."

Why Croak?

Gannon is still uncertain as to why the croakers croak so much, especially in a manner that makes them easy targets for hungry bottlenose dolphins. But he is starting to make a few educated guesses.

"The best guess is that they use sound maybe as a low-level aggressive display when feeding in groups," he said.

Another possibility is that the sounds are contact calls to help the fish find each other in the murky, turbid, estuary waters that inhabit. For that theory to be true, however, biologists must show that groups benefit croakers.

"Contact calls have never been shown in fish before," Gannon said.

Regardless, the data on drum fish, including croakers, suggests that acoustic communication is important for the entire family, raising concerns about ocean noise pollution, Gannon said.

Popper, the University of Maryland fish acoustics expert, said that despite very limited data on the subject, it is possible that ocean noise pollution at current levels may have little effect on fish.

He notes, however, that the data come from laboratory conditions that are very different from those in the open ocean.

"We do know [that] sound, noise in the environment, affects humans. And the ear in fishes—as in marine mammals, as in birds—has many similarities. So you have to ask the question, Is this important?" Popper said.

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