Fatty acid‐based diet estimates suggest ringed seal remain the main prey of southern Beaufort Sea polar bears despite recent use of onshore food resources

Abstract

Polar bears (Ursus maritimus) from the southern Beaufort Sea (SB) subpopulation have traditionally fed predominantly upon ice‐seals; however, as the proportion of the subpopulation using onshore habitat has recently increased, foraging on land‐based resources, including remains of subsistence‐harvested bowhead whales (Balaena mysticetus) and colonial nesting seabirds has been observed. Adipose tissue samples were collected from this subpopulation during the springs of 2013–2016 and analyzed for fatty acid signatures. Diet estimates were generated for the proportional consumption of ringed seal (Pusa hispida), bearded seal (Erignathus barbatus), and beluga whale (Delphinapterus leucas), relative to onshore foods, including bowhead whale remains and seabird, as represented by black guillemot (Cepphus grylle mandtii) nestlings and eggs. Quantitative fatty acid signature analysis (QFASA) estimated that the ice‐obligate prey, ringed seal, remained the predominant prey species of SB polar bears (46.4 ± 1.8%), with much lower consumption of bearded seal (19.6 ± 2.0%), seabird (17.0 ± 1.2%), bowhead whale (15.0 ± 1.4%), and hardly any beluga whale (2.0 ± 0.5%). Adult and subadult females appeared to depend more on the traditional ringed seal prey than adult and subadult males. Diet estimates of SB polar bears showed significant interannual variability for all prey (F12, 456 = 3.17, p < .001). Longer‐term estimates suggested that both types of onshore prey, bowhead whale remains and seabird, have represented a moderate proportion of the food resources used by SB polar bears since at least the start of the 21st Century.

Dietary ecology of Alaskan polar bears (Ursus maritimus) through time and in response to Arctic climate change

“…to increase their onshore resource use in recent years, compensating for seal loss with more flexible strategies (Bourque et al, 2020;Gormezano & Rockwell, 2013). Although we observe some polar bears to have increased opportunistic foraging of harder foods in recent decades, as sea ice conditions deteriorate and polar bears experience longer summer fasts, this may not be a physiologically sustainable sustenance strategy.…”

“…The higher hard-food DMTA signal likely indicates increased carcass utilization of seals, scavenging at bowhead whale bone piles, browsing or hunting on the terrestrial landscape, and/or utilizing human refuse (Bourque et al, 2020;Gormezano & Rockwell, 2013;McKinney et al, 2017). Additionally, the standard deviation of 21stcentury polar bear Asfc values is elevated relative to other polar bear specimens (Table 1; Figure 2a), suggesting increased variability of feeding strategy among individuals of this group, perhaps in response to irregular availability of prey in recent years (Hamilton et al, 2014).…”

“…Specialist limitations notwithstanding, polar bears which ordinarily hunt on ice and exploit marine resources of ringed and bearded (Erignathus barbatus) seals may be increasing labile land resource use at the subpopulation level with Arctic warming. In particular, the bears may be feeding more frequently on human-killed bowhead whale (Balaena mysticetus) carcasses and other alternative terrestrial diets to compensate for the loss of lipid-dense, high-trophic prey (Bourque et al, 2020;Dyck & Kebreab, 2009;Gormezano & Rockwell, 2013;McKinney et al, 2017). Polar bears might also be expected to utilize their kills more fully when prey are scarce, consuming suboptimal tissues such as bone.…”

Diet-driven mercury contamination is associated with polar bear gut microbiota

“…The polar bear ( Ursus maritimus ) is a long-lived, wide-ranging apex predator, which feeds on a variety of high trophic position seal and cetacean species 13 – 15 . As such, polar bears through their diet are exposed to among the highest levels of biomagnifying contaminants 16 , including methylmercury (MeHg), of all the Arctic species.…”

“…2000 have led to shifts in prey availability and recruitment 24 , 25 . Polar bears primarily consume a diet of seals, namely ringed seals ( Pusa hispida ) and bearded seals ( Erignathus barbatus ) 14 , 15 , which occupy high trophic positions 26 and can accumulate high levels of contaminants 27 – 29 . Such diet-driven exposure to contaminants, including mercury 30 , may in turn may have a knock-on effect on gut microbiota diversity and composition.…”