BOSTON (AP) – Another rare North Atlantic right whale has been found dead, this time off the coast of Cape Cod.

The discovery Thursday marks the sixth known death within the struggling species since November. There are only 300 to 350 North Atlantic right whales in existence.

The young female whale was discovered by researchers on South Monomoy Island, off Chatham, said Tony LaCasse, a spokesman for the New England Aquarium.

Scientists aren’t yet sure what caused the whale’s death. The whale was not caught in fishing gear and there were no obvious signs of a ship strike. The whale did have fractured vertebrae, but scientists don’t yet know if the fractures occurred after death in the rough surf around the island.

Of the six recently discovered dead whales, five have been females. That loss is particularly troubling because the recovery on the species depends on healthy females, LaCasse said.

“We can get by with fewer males, but we need those females,” he said.

The whales have migrated north from their winter calving grounds near Florida and there’s an unusually heavy concentration near the Cape because their food is unusually plentiful now. They whales are swimming near heavily traveled shipping lanes, and LaCasse said researchers are hoping there won’t be more deaths.

“We’re holding our breath,” he said.

AP-ES-04-30-05 1506EDT