Without thick, solid ice expanses, seal babies drown or are crushed by broken-up chunks of ice.

For the harp seals, "good ice is about 30 to 70 centimeters [12 to 28 inches] thick and covers 60 to 90 percent of the water," said marine biologist Garry Stenson, who works for Canada's Federal Department of Fisheries and Oceans and helps to monitor and assess harp seal populations.

But ice cover in the sub-Arctic and North Atlantic Ocean has declined about 6 percent per decade since the 1970s. (Read "The Big Thaw" in National Geographic magazine.)

And as climate change continues to degrade the amount of good ice, the average pup survival rate is likely to