The Marine Mammal Stranding Center said two of the dolphins died right away.
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The other six were assisted by a veterinarian, the center said, and their conditions were "rapidly deteriorating."
"The decision was made to humanely euthanize the dolphins to prevent further suffering, as returning them to the ocean would have only prolonged their inevitable death," the MMSC said.
All eight dolphins have been taken to the NJ state lab for necropsies.
"We share in the public's sorrow for these beautiful animals, and hope that the necropsies will help us understand the reason for their stranding," the MMSC said.
The dolphins were found along the beach between 50th and 52nd streets sometime before 11 a.m.
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Raw video: Rescuers work to save beached dolphins in Sea Isle City
Tim Ramsey of Sea Isle City says it all started as he and his wife were on the porch enjoying the sight of dolphins swimming near the shoreline.
"There was probably 50 dolphins just going back and forth all morning, and the next thing you know we saw some coming straight in," Ramsey said.
Neighbors called 911, and among the first on the scene was Sea Isle City police chief Anthony Garreffi. He said it became quickly apparent that not all the mammals survived.
"Two of them were obviously deceased, and then there were six of them that were still alive," Garreffi said.
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First responders, with the help of concerned neighbors, kept the dolphins as comfortable as possible with damp sheets and buckets of ocean water until rescuers from the Marine Mammal Stranding Center arrived.
We spoke with people who live nearby who say they don't know for sure what's going on, but they know it's not good.
"In all my 30 years down here I've only seen one big, huge dolphin washed up and he was dead when he washed up," said Alan Nesensohn.
"Your guess is as good as mine, but my first instinct is something is terribly wrong. I've never seen six dolphins come up on the beach ever," said Joann Fratantoro.
Some residents question whether this is connected to recent whale deaths and wind turbine pre-construction activities offshore.
"Find out what it is then. If it's not that, then what is it?" asked Eileen Gooch of Cape May Courthouse.
So far, there has been no proven connection.
The New Jersey League of Conservation Voters, a nonpartisan group that focuses on electing people who protect the environment, is advocating for the wind turbines -- stating they are safe.
"We will look at the results from professionals and scientists regarding today's dolphin stranding in Sea Isle City, and we will continue to advocate for actual solutions based on those results," the group said.