NEWARK, N.J. - A striped dolphin found stranded on a Raritan Bay beach early Tuesday has died.

Bob Schoelkopf of the Marine Mammal Stranding Center in Brigantine said the dolphin was believed to be one of three found stranded at Sandy Hook on Monday. One of those dolphins died, and rescuers had helped the other two back to sea.

Schoelkopf said the dead dolphins would be taken to a veterinary clinic at the University of Pennsylvania to try to determine what caused the deaths and why the animals might have become stranded.

Rescuers erected a tent around the stranded dolphin Tuesday morning to shelter it from the bitter temperatures, but it died on the beach, Schoelkopf said. TV footage showed the dolphin shaking.

The three found at Sandy Hook are not part of a group of bottlenose dolphins that have been in the Shrewsbury and Navesink Rivers since June.

Those dolphins are at the center of a struggle between federal wildlife officials - who plan to leave them alone unless they appear to be in imminent danger - and animal rescuers, who want them either removed or coaxed out of the river and back out to sea. Some fear that the dolphins will be trapped in ice or die from the cold if they remain in the rivers.

Two of the original group of 15 died earlier this year.

The striped dolphin that stranded Tuesday is of an offshore species that often travels in pods of 100 or more.

The National Oceanic and Atmospheric Administration has recorded six strandings of striped dolphins this year, including the three in New Jersey.

NOAA scientists are studying whether changing ocean surface temperatures have any correlation to the strandings, which also are caused when animals become sick or injured, or follow prey too far inland.

"It's not so much a cause for alarm, but it underscores how important the stranding program's research is," said Trevor Spradlin, a marine mammal biologist with the NOAA. "Marine mammals are basically sentinels of the ocean's health, so we can look at trends over time and how they are changing their patterns."

The Institute of Marine and Coastal Sciences at Rutgers University also is researching why animals become stranded as part of a wider look at ocean health.

Josh Kohut, an assistant professor at the institute, said an ocean observation network has been set up off the New Jersey coast. It monitors ocean life through radar networks, space satellites, and underwater vehicles that record ocean-floor data.

"We're looking at how we can take the measures of temperature and salinity of the ocean and link it to the distribution of animals," he said. "Is the ocean environment changing, or is it that the ocean is changing their food supply - which is changing their habits? We're starting to be able to answer those questions."