Dolphin mass stranding, 'largest in US history'

Rescuers who helped free more than a 100 dolphins from the Cape Cod shoreline say they have confirmed that the mass stranding is the largest involving dolphins in US history.

There were two other events on record in Hawaii and the Florida Keys where dolphin species were observed circling in shallow water.

However the Cape Cod event, which began June 28, marks the highest number of dolphins beached in a single stranding event, according to the International Fund for Animal Welfare, which helped lead the rescue.

A final review of data and aerial imagery this week revealed that a total of 146 dolphins were involved in the stranding, according to the fund's communications director Stacey Hedman.

The group estimated that 102 dolphins survived the multi-day event.

There were 37 natural deaths and seven dolphins had to be euthanised.

Response efforts have continued on a smaller scale including the rescue, relocation, and release of nine of the same Atlantic white-sided dolphins on July 2.

Satellite tags tracked several of these animals safely offshore.

There's no set reason for why the dolphins became stranded.

Rescuers faced many challenges as they attempted to guide the dolphins back to open water, including difficult mud conditions and the dolphins being spread out over a large area.

"This stranding response was a tremendous effort for our staff, volunteers, and partners over multiple days," Hedman said, adding that with about a 70 per cent survival rate, the group consider the response a success.

The organisation has also received reports from whale watching vessels that have seen some of the dolphins — identified with temporary markings — now swimming among other groups of hundreds of other dolphins that had not been part of the stranding.