WELLFLEET, Mass. -- The ninth and final stranded dolphin was rescued on Cape Cod on Friday after crews worked for more than a day to save 16 wayward dolphins who stranded on mud flats in Wellfleet.

In all, nine of the 16 stranded dolphins were rescued and released into deeper waters. Unfortunately, five of the animals had died, including one that had to be euthanized, and two were unaccounted for.

Workers had successfully transported six of the rescued dolphins from where they were stuck in Drummer's 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. The stress is that they are not built to actually sustain their weight out of the water and 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," said Katie Moore, manager of marine mammal rescue and research for Yarmouth-based International Fund For Animal Welfare.

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.

After spending the night stuck in the mud defenseless against scavenger predators like seagulls that are attracted to their eyes, one surviving dolphin was rescued from the mud flats on Friday morning.

"They 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.

The eight that were rescued were driven 18 miles up the coast to the deeper waters off Provincetown and successfully released.

On Friday afternoon, the New England Aquarium said they received radio transmissions from tags placed on two of the animals, indicating that they were in deeper waters about five miles south of where they were released.

The Aquarium said the transmissions were a positive sign because they showed that the two tagged dolphins had survived the initial trauma of the stranding.

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

"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. Wellfleet in particular is a hot spot. It's a hook within a hook, so it's a kind of a double trap for these animals. I think sometimes people think these animals throw themselves on shore. The reality is that they come into these little estuaries and the water just runs out from underneath them, 'cause they're not used to that," Moore said.

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