Polar bears obtain the majority of their annual nutrient reserves during the late winter. They hunt seals on the surface of sea ice. During this time, the sea ice is also at its highest density. Scientists have been measuring the amount of arctic sea ice for decades, and their results can be seen below.

They will have to find alternate food sources.

Explanation:

From the data gone in the graph the polar bear obtains their majority of the food during the later winter months by hunting seals on the surface as they have adapted to consumes the food that has extreme fat.They spends most of there time hinting 40% in summer and 25% in spring and dependent on the seals and whales and fish and arctic birds. In the winter the sea ice is more denser than in the summers thus they have to find more of the alternative source of food.