The effects of declining Arctic sea ice on local ecosystem productivity are not well understood but have been shown to vary inter-specifically, spatially, and temporally. Because of their upper trophic-level position in relatively simplified Arctic food webs, marine mammals may be useful indicators of variation in ecosystem productivity. Polar bears (Ursus maritimus) are an apex predator that primarily consume ice-associated seals that feed on benthic and pelagic organisms. As such, their productivity integrates changes in sea ice conditions and the ecosystem supporting them. Declining sea ice availability has been linked to negative population effects for polar bears but does not fully explain observed population changes. Here we examined relationships between spring feeding behavior of polar bears and sea ice conditions, prey productivity, and general patterns of ecosystem productivity in the Beaufort and Chukchi Seas. Fasting status (≥ 7 days) was estimated using serum urea and creatinine levels of 1,448 samples collected from 1,177 adult and subadult bears across three subpopulations.
Results/Conclusions
Temporal and spatial patterns in fasting were consistent with observed variation in ecosystem productivity across the region. Fasting probability increased for Beaufort Sea bears between 1983-1999 and 2000-2016 and was related to an index of ringed seal body condition. These changes were concurrent to declines in polar bear body condition as well as other species within the food chain dependent on Arctic cod (Boreogadus saida) in the Beaufort Sea. Because polar bears require greater than one seal kill per 5 days in the spring to meet nutritional needs, increased fasting over periods of 7 days or more in the southern Beaufort Sea may have contributed to recent population decline. In contrast, fasting probabilities of Chukchi Sea polar bears declined between periods and were lower than Beaufort Sea bears consistent with studies demonstrating that polar bears, ringed seals, and bearded seals have maintained or improved body condition despite recent sea ice loss in this region. Consistency between regional and temporal variation in spring polar bear fasting and food web productivity suggest that polar bears may be useful indicators of variation in these systems. Further, our results suggest that ecosystem changes have the potential to impact upper trophic species during seasons with minimal sea ice loss.