The former Beatle and his wife arrived Wednesday night in Charolottetown, Prince Edward Island - a fishing village on Canada's Atlantic coast - and landed a helicopter on the ice in the Gulf of St. Lawrence on Thursday.
The mission is to to put a white hot spotlight on the plight of the fluffy white pups, which are calved and weaned from their mothers on the frigid ice before being clubbed to death.
It's a new cause for both McCartneys, although hardly out of character. The pop star, who three years ago used his clout to argue for better treatment of chickens, is a longtime advocate of the vegetarian lifestyle (won over by his late wife, Linda, early in their marriage) and has spoken out on environmental issues including the hole in the ozone layer. And Heather is an activist on behalf of landmine victims and the global campaign to ban landmines.
The McCartneys, dressed in bright orange thermal jump suits, took helicopters with about a dozen journalists in tow to the ice floes just north west of Iles-de-la Madeleine, Quebec.
They rolled on the ice with one pup and expressed sadness that it would likely be killed in several weeks when the hunt officially gets underway.
"She's only three or four days old and they won't even get a chance to have a solid meal or even swim," said Heather Mills McCartney. "We've come out here to discuss the fate of these seals. In about three weeks time, these baby seals are due to be clubbed to death or shot. For many years people have tried to have this brutal practice stopped."
The United States has banned Canadian seal products since 1972 and the European Union banned the white pelts of baby seals in 1983.
The British government also is considering banning the import of seal goods. Groups such as Respect for Animals and the Humane Society of the United States, which are coordinating the McCartneys' visit, are encouraging people to boycott Canadian seafood as a show of solidarity.