Rescuers make progress after more than 100 Atlantic white-sided dolphins stranded off Cape Cod, Massachusetts

(CNN) — After more than 100 dolphins were found stranded off Cape Cod, Massachusetts, the numbers had dwindled down to between 10 to 20 dolphins circling in the harbor by Saturday morning following a 12-hour rescue effort by the International Fund for Animal Welfare, a spokesperson said.

In what the IFAW described as possibly “the largest single mass stranding event” in the organization’s 26-year history, 125 Atlantic white-sided dolphins were located stranded early Friday in shallow mud flats of Wellfleet in an area called the “Gut,” or Great Island at the Herring River.

Stacey Hedman, the fund’s communications director, described the area as a “very difficult location with dangerous mud.”

The town of Wellfleet is a little over 100 miles southeast of Boston.

On Saturday, Hedman said other than the remaining 10 to 20 dolphins, the rest of the mammals were successfully herded out – “at least for a time.”

Hedman added: “Those spotted this morning are likely the same dolphins. They are still swimming but close to shore,” noting that low tide was set to begin at noon.

The non-profit, which rescues, rehabilitates and releases animals, described the Herring River Gut as “the epicenter of our mass strandings” in a Facebook post on Friday.

“We had 125 Atlantic white-sided dolphins strand and 10 died before we arrived,” Hedman said in an email to CNN on Friday.

The dolphins were herded in a coordinated response effort Friday to encourage their movement back to deeper waters, IFAW said in a news release Saturday.

Rescuers began on foot and switched to boating efforts as the water returned to high tide just before 5 p.m. Friday, which worked in their favor to refloat the animals, according to the fund. Rescuers continued until sunset.

The rescue operation, in which at least 25 fund workers and 100 volunteers assisted, was ongoing on Saturday.

The Whale and Dolphin Conservation, the Center for Coastal Studies, AmeriCorps of Cape Cod and the New England Aquarium are also supporting the operation.

“At first light Saturday morning, an IFAW team was on the water to find 10 of the dolphins swimming in a dangerous area, and efforts have been successful to herd them towards deeper water,” the fund stated in the release. “IFAW scouts found another group of 25 dolphins swimming but close to shore in Eastham.” Eastham is just south of Wellfleet.

“This rescue had many challenges due to the number of dolphins, the large size of many of the animals, how spread out they were over a large area, the difficult mud conditions, and the complicated locations from which we could reach them,” said Misty Niemeyer, stranding coordinator for IFAW, in a statement.

“It was a 12-hour exhausting response in the unrelenting sun, but the team was able to overcome the various challenges and give the dolphins their best chance at survival,” Niemeyer said.

Hedman noted there is no set explanation for why dolphins strand.

“Cape Cod is a global stranding hotspot due to the curvature of our shores and the fluctuation of our tides,” Hedman said.

More dolphins get stranded along Cape Cod’s 12-mile stretch of shoreline than any other place globally, according to the fund.

This story has been updated with additional information.

CNN’s Ashley R. Williams contributed to this report.

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