The researchers placed underwater microphones in a river near the Jersey shore to record sounds that might be scaring the dolphins from heading back out to sea.
The recordings will be analyzed this week as scientist continue to monitor the estimated 12 bottlenose dolphins that have been feeding and frolicking in the Shrewsbury and Navesink rivers since the beginning of summer.
A key question is whether 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 Shrewsbury River in early June and must go back out the same way in order to reach Sandy Hook Bay and the ocean.
Animal advocates have wanted for months to 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 that lingered in the 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 spotted in the Shrewsbury. 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 mammal stranding center and the National Oceanic and Atmospheric Administration have been on opposite sides of a debate over what to do with the dolphins. The federal agency, which has jurisdiction over the animals and control over the center's animal rescue permit, has decided that trying 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.