Sea Bright, N.J. ? A group of bottlenose dolphins have been confounding humans since they took up residence in two rivers near the Jersey shore six months ago. Now that it’s winter, some people are worried they’ll never make it out.

Three dolphins have died out of the original group of about 15 that spent the summer and fall in the Shrewsbury and Navesink rivers, waterways just north of Asbury Park.

Federal wildlife experts say the remaining dolphins are healthy, and should be able to make it through the winter if they choose to stay. They cite the cases of dolphins that successfully spent winters in Massachusetts, Virginia and even northern Scotland.

But some animal advocates worry the dolphins will meet the same fate as four that drowned in the Shrewsbury River in 1993 when ice closed in on them, or the 26 dolphins killed by a sudden freeze in 1990 in Texas’ Matagorda Bay.

“It would seem to me that the natural habitat for dolphins in the winter when it gets cold is much farther south in warmer waters,” said U.S. Rep. Frank Pallone Jr., D-N.J. “Isn’t it stressful for them to be in this colder environment? Since they are mammals, what happens if the ice freezes over and they can’t breathe?”

The National Oceanic and Atmospheric Administration, which has jurisdiction over the dolphins, doesn’t plan to intervene unless the dolphins appear to be in imminent danger.

“This is a normal sort of way of life for these critters,” said David Gouveia, marine mammal program coordinator for the agency’s fisheries service. “We’re optimistic that things are going to work out just fine.”

Others are not as confident.

“We have seen the disastrous consequences when dolphins remain in the area during the winter months,” said state Sen. Sean Kean, R-Wall. “Now it’s up to NOAA to determine a course of action to try to get the dolphins back to the ocean before it’s too late.”

Looming over this debate is the dismal history of dolphins that have wound up in the Shrewsbury in previous years. In at least two instances, dolphins lingered too long in the river and died when rescue attempts went awry.