In Mario Cyr’s 40 years as a marine photographer and expedition leader in Canada’s north, he has never seen harp seal pups clustered on shore. Every December, a population of harp seals arrive in the Gulf of St. Lawrence, traveling south from the Canadian Arctic and Greenland to give birth on the sea ice around the Îles de la Madeleine in late February and early March. These harp seal nurseries attract hundreds of people each year, eager to see the fuzzy white pups bellying around the ice and getting fat on their mothers’ milk.

But in recent days, hundreds of pups instead have appeared on the beach just outside the small community of Blanc-Sablon, Québec, some 350 miles northeast