Age and Sex Composition of Seals Killed by Polar Bears in the Eastern Beaufort Sea

Abstract

Background Polar bears ( Ursus maritimus ) of the Beaufort Sea enter hyperphagia in spring and gain fat reserves to survive periods of low prey availability. We collected information on seals killed by polar bears ( n = 650) and hunting attempts on ringed seal ( Pusa hispida ) lairs ( n = 1396) observed from a helicopter during polar bear mark-recapture studies in the eastern Beaufort Sea in spring in 1985–2011. We investigated how temporal shifts in ringed seal reproduction affect kill composition and the intraspecific vulnerabilities of ringed seals to polar bear predation. Principal Findings Polar bears primarily preyed on ringed seals (90.2%) while bearded seals ( Erignathus barbatus ) only comprised 9.8% of the kills, but 33% of the biomass. Adults comprised 43.6% (150/344) of the ringed seals killed, while their pups comprised 38.4% (132/344). Juvenile ringed seals were killed at the lowest proportion, comprising 18.0% (62/344) of the ringed seal kills. The proportion of ringed seal pups was highest between 2007–2011, in association with high ringed seal productivity. Half of the adult ringed seal kills were ≥21 years (60/121), and kill rates of adults increased following the peak of parturition. Determination of sex from DNA revealed that polar bears killed adult male and adult female ringed seals equally (0.50, n = 78). The number of hunting attempts at ringed seal subnivean lair sites was positively correlated with the number of pup kills ( r 2 = 0.30, P = 0.04), but was not correlated with the number of adult kills ( P = 0.37). Conclusions/Significance Results are consistent with decadal trends in ringed seal productivity, with low numbers of pups killed by polar bears in spring in years of low pup productivity, and conversely when pup productivity was high. Vulnerability of adult ringed seals to predation increased in relation to reproductive activities and age, but not gender.

Spring fasting behavior in a marine apex predator provides an index of ecosystem productivity

“…Spring is the most important, if not critical, foraging period for polar bears in which they gain mass lost over the previous winter (Rode, Regehr, Douglas, et al, 2014), particularly for females emerging from dens with newborn cubs (Pilfold, Derocher, Stirling, Richardson, & Andriashek, 2012;Stirling & McEwan, 1975;Stirling & Øritsland, 1995). Ringed and bearded seals haul-out to molt and pup in the spring, making them more accessible to polar bears than during any other time of the year.…”

“…Although nearly 40% of the ringed seals killed by polar bears in the Beaufort Sea are pups, older age classes of prey are also important contributors to total biomass consumed (Pilfold et al, 2012).…”

Polar bears: proceedings of the 18th working meeting of the IUCN/SSC Polar Bear Specialist Group, Anchorage, Alaska, 7-11 June 2016

“…Studies on polar bear predation have been limited by sample size and spatial extent, and hypotheses on the demographic composition of seal kills and the spatial distribution of polar bears and seals were incongruent. Pilfold et al (2012) used a longterm dataset of seals killed by polar bears (n = 650) and predation attempts at ringed seal subnivean lairs (n = 1396) in the Canadian Beaufort Sea to link the habitats polar bears use and the seals that polar bears kill during hyperphagia. Using DNA and field observations, it was determined that polar bears primarily killed ringed seals, but that bearded seals contributed a significant portion of kill biomass (Pilfold et al 2012).…”

“…Pilfold et al (2012) used a longterm dataset of seals killed by polar bears (n = 650) and predation attempts at ringed seal subnivean lairs (n = 1396) in the Canadian Beaufort Sea to link the habitats polar bears use and the seals that polar bears kill during hyperphagia. Using DNA and field observations, it was determined that polar bears primarily killed ringed seals, but that bearded seals contributed a significant portion of kill biomass (Pilfold et al 2012). An increase in seal kill frequency was observed as spring progressed, associated with the onset of ringed seal whelping.…”

Barents Sea polar bears (Ursus maritimus): population biology and anthropogenic threats

“…Nutritionally stressed polar bear females with COYs need a predictable food source when emerging from their maternity dens in spring so these ringed seal pupping areas are a vital resource. In such areas, the female bears hunt ringed seal pups and sometimes their mothers (Stirling & McEwan 1975;Pilfold et al 2012;C. Lydersen, pers.…”