Harp seals give birth on unstable pack ice, which imposes a hard deadline. The nursing period lasts around twelve days. In that window the mother delivers milk with a fat content in the region of 40 to 50 per cent, and the pup grows from roughly 10 kilograms at birth to something in the region of 30 kilograms.
The mother then stops nursing and leaves. The pup, still in its white lanugo coat, remains on the ice for a further period during which it neither feeds nor swims well, living off the blubber laid down during nursing. It sheds the white coat and enters the water only once that fasting period ends.
The strategy is efficient but brittle. Pup survival in a given year depends heavily on how long the ice holds, and poor ice years in the Gulf of St Lawrence have produced heavy pup mortality. Long-term consequences of declining seasonal ice cover for the population as a whole are not yet clear.