When it comes to hunting prey, seal populations are healthy, stable, and readily available for polar bears to hunt–the accessibility is the problem. In fact, conditions and reproduction of ringed seals and barbed seals, which comprise almost 80% of polar bear diets, have either increased or remained stable since the 1970s (Rode et al., 2014). This helps demonstrate that seals are plentiful and not the problem, emphasizing the problem is sea ice.

The actual act of hunting is most prominent in the late spring, when fat, recently weaned and naive ringed seal pups are abundant. Polar bears reach their lightest weights of the year in late March, just before the birth of this cohort of seal pups. This suggests that it is the success of their hunting in spring and early summer that enables them to maximize the body reserves necessary for survival, reproduction, and nursing of cubs through the rest of the year (Stirling, Lunn, & Iacozza, 1999). Normally, summer months bring slight warming to the Arctic, causing the phenomenon known as “breakup”, which refers to the loss of all or almost all sea ice.

 During this time, all bears must depend on their stored fat resources while fasting on land until the freeze-up again in the fall (Rode 2011). Bears depend on the ice to refreeze each winter, allowing them access to the ice and therefore their hunting platforms. Studies show that the melting of sea ice is 2.5 weeks sooner than it was 30 years ago and polar bears are losing hunting time (Derocher, Lunn, & Stirling, 2004). Limited time to hunt means storing enough fat for the summer months becomes less and less achievable. It would be very much like going to your pantry everyday, only to find that one day you return and your shelving has raised itself higher and higher. Sooner or later, that shelving is completely out of reach, and you’ll find yourself with nothing but crumbs and the old bag of peanuts at the bottom.

https://blogs.umass.edu/natsci397a-eross/does-climate-change-pose-a-significant-threat-to-polar-bears/#more-6229