Two dolphins have died so far, and concern is growing among animal rescuers that the rest could die as well if they're not removed.
The leader of one such group says leaving the 12 remaining animals there is tantamount to letting them all die.
"We're already at 20 percent mortality with this group," said Bob Schoelkopf, co-director of the Marine mammal Stranding Center. "What happens when it hits 50 percent? According to them, nothing.
"They say people need to be educated that this is something that happens to animals sometimes - they die," Schoelkopf added.
The National Oceanic and Atmospheric Administration says it has been monitoring the dolphins for the past two weeks and sees no signs any of them are in distress or danger.
"These animals are in typical habitat, food is present, and we have no reason to believe they are stressed," said Teri Rowles, director of the agency's National Marine Mammal Health and Stranding Program. "We're not going to interfere in what appears to be a completely natural phenomenon, especially when doing so carries a high risk of harming healthy animals."
The agency noted that other dolphins ventured into the river in 1993 and 2000.
"NOAA did try to move them," the agency wrote in a press release. "In both cases, all the animals perished."
But Schoelkopf said that's because the agency waited far too long to get involved. He has been calling for intervention since the beginning of July. In the 1993 case, the dolphins died after getting trapped under ice.
Earlier this month, federal officials drew up contingency plans to an intervention, including trying to either coax or scare the dolphins out of the rivers and out to the open ocean.
The agency had considering using a U-shaped flotilla of boats to try to herd the dolphins out of the river and back out to sea, or using recorded "socialization sounds" of other dolphins to try to lure them from the river into Sandy Hook Bay and ultimately the ocean.
Part of the proposal to herd the dolphins out of the rivers would involve playing loud or unpleasant noises underwater and creating a "visual barrier" such as a curtain of air bubbles in the water to try to guide the dolphins in a particular direction.
But Katie Touhey, who manages stranding responses for the Cape Cod Stranding Network in Massachusetts, said neither of those is likely to work.
"Based on our experiences, herding is unlikely to be effective here given the small number of animals and the distance to the ocean," she said. It is about eight miles from where the dolphins are now to the ocean.
The agency is hesitant to try moving the dolphins, noting that no one has ever tried to move this many dolphins that far.
It also said luring with sounds has never been tried with bottlenose dolphins, and herding has been tried before with other dolphin species with mixed results.
"In most successful cases these animals were in poor habitats or otherwise distressed, and nearer to safer waters," the agency said in its press release. "Herding raises the stress level of animals considerably, and in some cases causes dolphins to strand, which is often lethal."
Larry Hansen, an agency researcher who studies the northern coastal bottlenose dolphins, said it is possible they can survive the winter in the river.
"They can spend the winter in colder water and even endure icy conditions, as long as there is enough to eat," he said.
