In an effort to hear what two pods of wayward dolphins hear, federal scientists placed underwater microphones in a river near the Jersey Shore to record sounds that might be scaring them from heading back out to sea.
The recordings will be analyzed this week as scientist continue to monitor about 12 bottlenose dolphins that they have decided to leave in the Shrewsbury and Navesink rivers unless they appear to be in imminent danger.
Teri Frady, a spokeswoman for the National Oceanic and Atmospheric Administration, said the agency placed underwater microphones near the Route 36 bridge between Sea Bright and Highlands, and another one about a mile way in the Shrewsbury River on Oct. 30.
The first round of data was removed from the devices last week and will be studied this week.
The goal is to see if sounds on the tape correspond to behavior the dolphins exhibited during three days last week when NOAA researchers were on the water observing the dolphins.
A key question is whether ongoing construction work on the Route 36 bridge -- a major thoroughfare leading to and from the northern Monmouth County oceanfront -- is scaring the dolphins or discouraging them from heading back out to sea.
The dolphins passed underneath the bridge on their way into the river earlier this summer, and must go back out the same way in order to reach Sandy Hook Bay and the ocean.
Since early June, the dolphins have been feeding and frolicking in the Shrewsbury and Navesink rivers.
Animal advocates have wanted for months to either coax or shoo the animals back out to sea, citing several previous instances in which dolphins took a wrong turn, ended up in the river, and died when weather got too cold.
They worry that waiting too long could invite a replay of a scenario that resulted in the deaths of four dolphins who lingered in the Shrewsbury River in 1993. Ice eventually closed in on them and they drowned.
Already this year, two of the dolphins -- whose numbers initially were put at about 15 -- have died.
On Tuesday, a group of six dolphins was seen in the Shrewsbury between Sea Bright and Rumson, in a spot where they have spent lots of time since first appearing this year. Several were seen leaping straight up out of the water, then splashing down horizontally. One was seen slapping its tail on the surface of the water.
Bob Schoelkopf, co-director of the Marine Mammal Stranding Center, said the leaping could represent playful behavior. Or it could be related to feeding activity, or the animals trying to shake loose parasites, he added.
The tail slapping is more worrisome because it shows distress, he said.
"They're not happy when they slap their tails," he said. "That's a sign of annoyance."
The stranding center and NOAA have been on opposite sides of a debate on what to do with the dolphins. But the federal agency, which has jurisdiction over the animals and control over the center's animal rescue permit, has decided that to try to move the dolphins would be too risky.
The agency is continuing to monitor the situation and only plans an intervention if the animals are in imminent danger, such as stranding themselves or showing signs of serious illness.
Schoelkopf said water temperatures in the river are dropping. The temperature was 52 degrees on Monday, down from more than 58 degrees on Oct. 27.
Federal officials say they hope that when water temperatures fall, the bait fish the dolphins survive on will leave the river, and the dolphins will, too.
Frady said the observers found the dolphins behaving normally, feeding on bay anchovies and menhaden. She said they counted the same 12 dolphins that were observed at the end of October, when the decision not to intervene was made.