WELLFLEET, Mass. -- Rescue crews freed six dolphins Thursday that were stranded in mud near Wellfleet, but weren't able to save two white-sided dolphins, WCVB-TV reported.

Whether the six dolphins will survive is unclear because they were out of water for many hours.

Workers have transported the six dolphins from where they were stuck in Drummer Cove and Lieutenant's Island to Herring Cove in Provincetown, where they were wheeled down on dollies and then lifted back into the deeper water on tarps. It was all part of a race against time for the 300-pound animals.

"It's not that they can't be out of the water. They're air breathers. That's not the stress," said Katie Moore, manager of marine mammal rescue and research for Yarmouth-based International Fund For Animal Welfare. "The stress is that they are not built to actually sustain their weight out of the water, so the pressure that they have on their skeletal system, on their internal organs, is such that they start to really go downhill quickly, physically."

About 16 dolphins were discovered around noon Thursday, stranded on the mud flats. By Friday morning, rescuers were able to reach the remaining dolphins still trapped in Drummer's Cove. Two survived, two died, and rescuers hoped that three that were missing were on their way out to sea.

"They (were) stranded in an area in Wellfleet that was very inaccessible. We had mud that was basically something that you sink hip-deep into when you get out there," said Moore.

Rescuers could only reach eight of the dolphins. Two perished.

Atlantic white-sided dolphins can measure up to 8 feet in length and are common in the waters off the Cape. They frequently become stranded around Cape Cod because of geography, rescuers said.

"This is one of the top three places in the world to have mass strandings, annually. Cape Cod is just a hot spot, globally ... we're a hook that sticks out in the water," Moore said.

Scientists aren't sure why animals become stranded. One theory is that they chase prey into shallower waters and then get stuck.