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."
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 fishesas in marine mammals, as in birdshas many similarities. So you have to ask the question, Is this important?" Popper said.
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES