Federal wildlife authorities, however, are preparing for the worst - that the group may freeze to death in the frigid waters.
On Saturday, a group of the dolphins was seen swimming near the Route 36 bridge between Sea Bright and Highlands. Bob Schoelkopf, co-director of the Marine Mammal Stranding Center, said the animals looked like they wanted to go under the bridge and back out to the open waters of Sandy Hook Bay.
But he said some of the young dolphins appeared hesitant to go under the span, where noisy repair work is going on above.
"It looked like they were really trying to get out," Schoelkopf said. "I have a feeling it might be a problem with the juveniles afraid to go through" the channels under the bridge.
"They were moving around erratically, and there was some tail-slapping, which is a sign of aggravation," he said.
Authorities and rescuers have long thought that construction work on the bridge may be serving as an acoustic barrier to the dolphins getting back out to sea. The span is one of the main gateways to and from the popular northern Monmouth County shore area, which includes the Gateway National Recreation Area at Sandy Hook and several wealthy riverfront towns.
The dolphins, who originally numbered about 15, 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 the bay and the ocean. Two have died so far.
On Monday, four dolphins were spotted near the Oceanic Bridge in the Navesink River between Rumson and Middletown. None were spotted in the Shrewsbury River, leading Schoelkopf to voice cautious optimism that at least some of the dolphins who were swimming near the Route 36 bridge on Saturday had successfully made it underneath and out to sea.
"I really hope they did make it," he said.
Since early June, the dolphins have been feeding and frolicking in the two rivers. Back then, water temperatures were in the high 60s.
On Monday, it was 35 degrees, and several smaller tributaries of the rivers were frozen over. Air temperatures Sunday night into Monday morning were in the teens, and a wind chill made it feel like single digits.
Animal advocates have wanted for months to either coax or shoo the dozen or so remaining animals back out to sea, citing several previous instances in which dolphins took a wrong turn, ended up in the Shrewsbury 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 in 1993. Ice eventually closed in on them and they drowned.
Later this week, the stranding center and the National Oceanic and Atmospheric Administration will confer on response plans for a mass stranding, in which most or all the dolphins become distressed and beach themselves on the riverbank or in shallows.
"We're pretty confident about what to do if it's just one," said Teri Frady, a spokeswoman for the agency, which has jurisdiction over the animals. "But a mass stranding requires more preparation and resources."
The agency decided in late October that it will not intervene to try to shoo or coax the dolphins out of the river unless they appear to be in imminent danger.
Schoelkopf said there may be little rescuers could do in the case of a mass stranding.
"If they end up like that, they're probably past the point of recovery," he said.
His facility in Brigantine can't take injured or sick dolphins for any length of time because they require warm water (the center's tanks are not heated), and the center is already caring for stranded seals in those tanks.
Area aquariums can't take more than one or two dolphins, even if they survive a stranding, he added.
