The delicate "ping" of a submerged transmitter is music to Jason Moak's ears.
"It's exciting when you hear it," said Moak, who is monitoring the annual shortnose sturgeon migration in the Savannah River.
About 20 of the endangered fish were fitted with tiny radios as part of an effort to learn more about their spawning habits.
Moak, the senior research scientist for the Southeastern Natural Sciences Academy, is trying to keep track of them -- a task easier said than done.
"Closer to the coast, in the estuaries, sometimes you'll hear four or five of them at a time," he said. "Up here closer to Augusta, we aren't hearing much."
The coastal fish typically move to upstream spawning grounds as water temperatures warm.
In the Savannah and many Eastern rivers, the creation of dams and lakes has eliminated most of the sturgeon's ancestral spawning areas. Scientists want to know whether the few suitable spawning areas left are sufficient to sustain the imperiled species.
Moak uses listening stations scattered among 200 meandering miles of river in efforts to find the tagged fish.
Although a tagged, egg-laden female spent six days below Augusta's New Savannah Bluff Lock & Dam in April 2009, none has been detected that far inland this year.
"Because the water was so high, and it stayed colder longer, there may be a delayed migration this year," he said. "That's all part of what we're trying to find out."
So far, Moak has detected nine tagged female sturgeon that had moved upstream above the Interstate 95 bridge, presumably headed toward Augusta.
He uses a boat to check monitoring stations in hopes of hearing the coded series of pings that will identify a tagged sturgeon.
"We've gotten several fish that weren't ours," he said. "One of them turned out to be a striped bass -- all the way from New Jersey."
Scientists believe between 1,000 and 2,000 shortnose sturgeon remain in the Savannah River, along with unknown numbers of its much larger cousin, the Atlantic sturgeon, which can reproduce in coastal estuaries and does not require inland spawning habitat.
The study, funded by The Nature Conservancy using a grant from the National Oceanic and Atmospheric Administration, has been under way several years.