A school of 10 whales that washed up on Monday in northern France may have done so voluntarily after the death of the dominant male, an expert said.

Seven of the whales found on the beach in Calais have not survived.

"The group was in the middle of a deep-sea migration toward the Faeroe Islands to reproduce and feed," said Jacky Karpouzopoulos, of the Centre for Marine Mammal Research at La Rochelle.

"It's possible that this was a voluntary family beaching, whereby the dominant male died at sea and the rest followed his body," she said.

Six of the black pilot whales were already dead when they were found. Among them was a 4.5-meter male and a large female.

An autopsy will be carried out to determine the cause of death.

A seventh, female whale died while local firefighters and an animal rescue team attempted to return her to sea.

The other three whales - a female and two calves - were successfully returned to sea when the tide came in.