Why Less Summer Ice Increases Bear Populations

Guest essay by Jim Steel, Director Emeritus, San Fransisco State University

“Annual primary production in the Arctic has increased yearly … Should these trends continue, additional loss of ice during Arctic spring could boost productivity >3-fold above 1998–2002 levels” ¹  -Dr. Kevin Arrigo, Stanford University

While the Inuit argue it is the time of the most polar bear, CO₂ advocates suggest they may soon go extinct, implying the loss of thick, multiyear ice in September is denying the polar bears the icy platform from which they hunt seals. In reality, less summer ice has a negligible effect on normal hunting, but a decisively positive effect on the bears’ main prey. Recent periods of more open water in the summer have undeniably benefitted the whole food chain.

The bears’ most important feeding period extends from March to June when bears binge on breeding ringed seals and their pups. This is the time when hunting on sea ice is most important, but unlike the highly publicized reductions in September ice, the reduction in springtime ice has been quite minor and no respectable models predict the disappearance of winter ice. Without the sun, winter air temperatures range from ‑15°F and ‑52°F and ample ice will always form, providing ringed seals with ample breeding habitat

The Arctic Ocean is relatively poor in nutrients, and benefits greatly from ocean cycles that import nutrients from the Bering Sea and the Atlantic. The intruding warm and nutrient-rich currents also cause less ice, which promotes more photosynthesis. Between 2003 and 2007, productivity in the Arctic Ocean increased by 23% relative to the 1998-2002 average. When phytoplankton increase, zooplankton flourish, treating whales, sea birds and young Arctic cod to a bountiful feast.² In addition to more food, the warmer surface temperatures stimulate the “cold-blooded” Arctic cod grow faster and bigger, and bigger fish are better able to survive the winter. Fishery scientists have concluded, “at least in the short term, the lengthening of the ice-free season presently observed in Arctic seas could result in improved recruitment and larger populations of Arctic cod.”³

More Arctic cod sustains more ringed seals, harp seals, harbor seals, beluga whales and several species of seabirds. Ringed seals feed intensively on cod in the open waters of summer in order to store the fat needed to survive the winter. Ringed seals suffer when sea ice is slow to break up. In 1992 when breakup of sea ice was delayed by 25 days, the body condition of all ringed seals declined.⁴ In contrast during the most recent decade with more open water, the number of ringed seal pups in the western Hudson Bay tripled relative to the 1990s.⁴ With more seal pups the polar bears’ body condition also improved. Polar bear experts observed that recent improvement in the bears’ body condition but never published it.⁵ Instead papers that try to portray the bears as starving, only report the cycle of decline up to 1999.¹⁴

Ringed Seal Biology

Because a larger body size conserves heat more efficiently, animals living in polar regions are typically the largest among related species. (i.e., polar bears and Emperor penguins) Paradoxically, the ringed seal is the smallest yet most abundant of all Arctic seals, and they remain in the Arctic all winter. Both males and females are featherweights (weighing in at about 110-150 pounds) compared to the male Pacific walrus (weighing in at approximately 3500 pounds). The secret to this tiny seal’s success is the relative warmth of the ocean’s water (+28°F or higher). Seals avoid deadly -20°F air temperatures by staying in the water. In fact, for most of the year, ringed seals spend more than 90% of their time swimming, inaccessible to polar bears.⁶ Even during the winter when seals are tethered to their breathing holes, they never spend more than 20% of the time out of the water. Although all polar bears (except those that are nursing newborn cubs) remain active during the winter hunting on thick winter ice, the bears continue to lose weight because the odds are slim that they will stumble upon a resting seal.

However, the bear’s odds improve mightily during seals’ breeding season. For about 6-8 weeks from late March through May, adult ringed seals spend about 50% of their time hauled out on the ice, giving birth and nursing their pups in lairs just beneath a layer of snow.⁶ Consequently female polar bears emerge from their maternity dens at just the right time to binge on fat, helpless ringed seal pups.⁷Researchers reported that one 17-year-old female with three cubs-of-the-year was handled in November 1983 when she weighed just 218 lbs. The following July, she was without cubs, probably pregnant, and weighed 903 lbs, a four-fold weight change in just eight months.⁸ However her gain may have been even greater, as she likely continued to lose weight from November until March or April when the first seal pups appeared.

After the surviving seal pups are weaned, adult ringed seals seek out ice edges and floes where they can lay in the sun and molt their skin during two weeks of peak sunlight in June. Although not as vulnerable as baby seals, molting seals spend 60% of their time on the ice. Once their molt is complete, ringed seals are swimming in distant open waters from July through October, far from the jaws of most hungry bears. But with the passing of September’s equinox, the sun begins to fade and adult seals return to the coast to stake out their winter territories. And savvy polar bears line the coast in anticipation.

The seals must arrive before the new ice thickens in order to develop a series of breathing holes. When the ice first forms, the seals use their heads to punch open holes in the thin ice. Then as the fast-ice thickens, they must constantly chew and claw at the ice to maintain their breathing holes throughout the winter.

Because seals require thinner ice to create their breathing holes,⁹ areas dominated by thick multiyear ice always sustain far fewer seals and far fewer bears.^14,15 In regions like the northern Canadian Archipelago, winds pile ice against the shoreline. The winds crumple the ice and heave layers of thin ice into piles of thick rubble. The ice rubble resists melting and sets the stage for thicker multiyear ice to increase in the following years. Climate scientists have detected various cycles that alternately drive thick ice out of the Arctic or confine and compress the ice.¹⁰ These cycles range from 6 to 20 years and are associated with the North Atlantic Oscillation/Arctic Oscillation.

Since the mid 1990’s, Arctic sea ice has been behaving more like Antarctic sea ice and that has been good news for plankton, cod, seals, and bears. When the Arctic Oscillation swung to a positive phase, thicker multiyear ice was blown out from the Arctic into the north Atlantic.¹⁰ As a result the thinner replacement ice now melts more rapidly each summer, and biologically that is highly beneficial. (In the Antarctic, ice is not constrained by continents, and thick multiyear ice is relatively scarce, Although Antarctic sea ice expands much more than Arctic ice, it also melts more rapidly each summer. Still the Antarctic winter sea ice has expanded to its greatest limits during the most recent decades.).

The Deception

In 2012, polar bear experts Ian Stirling and Andrew Derocher (who predicts by the middle of this century, two-thirds of the polar bears will be gone due to rising CO2) published “Effects of climate warming on polar bears: a review of the evidence.”¹⁶ To illustrate the importance of ringed seal pups they wrote, “In the mid-1970s and again in the mid-1980s, ringed seal pup productivity plummeted by 80% or more for 2–3 years…. A comparison of the age-specific weights of both male and female polar bears from 1971 to 1973 (productive seal years), to those from 1974 to 1975 (years of seal reproductive failure), demonstrated a significant decline in the latter period.” ¹⁶

Without argument, bears always benefit from more seal pups, but Derocher’s retelling of the seals’ decline in a section titled, “Why progressively earlier breakup of the sea ice negatively affects persistence of polar bear subpopulations” was (to be kind) highly deceptive! The seals’ productivity had plummeted because the Arctic had cycled to years of heavy ice, not due to “a progressively earlier break-up. Somehow that critical point escaped peer review.

Instead of directly mentioning the heavy ice connection, they simply referenced Stirling’s 2002 paper. In that paper Stirling contradicted the “review”, “Heavy ice conditions in the mid-1970s and mid-1980s caused significant declines in productivity of ringed seals, each of which lasted about 3 years and caused similar declines in the natality of polar bears and survival of subadults, after which reproductive success and survival of both species increased again.” ⁷ In 2012, Stirling coauthored another paper with a seal researcher and concluded all declines were caused by heavy ice years. Their paper proposed that “the decline of ringed seal reproductive parameters and pup survival in the 1990s could have been triggered by unusually cold winters and heavy ice conditions that prevailed in Hudson Bay in the early 1990s, through nutritional stress”.⁷

Located south of the Arctic Circle, the Hudson Bay and Foxe Basin are naturally ice free by the end of every summer, yet these regions host robust bear populations. The lack of ice provides two benefits: it insures ample thin autumn ice required by breeding ringed seals, and it permits the summer immigration of Beluga whales, harp seals, and harbor seals into the bay. Any bear that failed to get its fill of ringed seal pups in the spring, can supplement its diet with these open-water immigrants.

Scientists can estimate a bear’s diet by taking samples of fat from the rump of a (heavily sedated) polar bear. Each prey species has a highly specific combination of essential fats. By analyzing those unique fats, they can tell what the bears have eaten. Using this method scientists have determined that ringed seals provide about 70% of the bear’s diet in the Hudson Bay. The remainder of the diet consists of resident Bearded seals, Harbor seals that typically avoid ice, and Harp seals and Beluga whales that immigrate into the bay only during the open-water season.

Elsewhere the Lancaster Sound population dines on Beluga whales nearly as much as they eat ringed seals. In the summer Belugas also herd cod into shallow embayments but get helplessly stranded when the tide goes out. Belugas are also frequently trapped by rapidly advancing winter ice. In the South Beaufort Sea, ringed seals account for 15% to 70% of the bears’ diet, while Bowhead Whales contribute from 2% to 52%, and Beluga Whales from 1% to 33%, with percentages varying widely amongst individuals.¹¹

In the Davis Strait off the coast of Labrador, polar bears will binge on baby Harp seals that breed on the pack-ice. Harp seals are another conservation success story. Since sustainable hunting regulations were imposed, they increased from less than 2 million in the 1970s to over 5.5 million in the 1990s and the bear population grew accordingly. Contrary to popular global warming theory, Harp Seals are now spreading south. In the 1980s only 5 seals were reported on Sable Island off the coast of Nova Scotia. By 1994 the Harp Seal population had ballooned to over 1100.¹²

In Foxe Basin just north of the Hudson Bay, ringed seals make up about 50% of the diet. In addition to harbor seals, harp seals and bearded seals, walruses contribute 7% of the bears’ diet. Wherever walruses are abundant, they are preyed upon by bears. Along the Laptev Sea polar bears have been observed making pits behind piles of driftwood, in which they hide and wait for walruses to come ashore. On Wrangle Island the bears wait on ice-free shores, anticipating the traditional walrus haul-outs to feast on the weariest walrus that lumber ashore. Impress by their resilient hunting behavior, researchers have remarked that such varied hunting behavior explains how polar bears have thrived during the past 10,000 years when summer sea ice was much less prevalent than today.¹³

Literature Cited

1. Arrigo, K. and van Dijken, G. (2004) Geophysical Research Letters, vol. 31, L08304, doi:10.1029/2003GL018978
2. Michaud, J., t al. (1996) Feeding success and survivorship of Arctic cod larvae, Boreogadus saida, in the Northeast Water polynya (Greenland Sea). Fisheries Oceanography, vol. 5, p. 120-135.
3. Fortier, et al. (2006) Survival of Arctic cod larvae (Boreogadus saida) in relation to sea ice and temperature in the Northeast Water Polynya (Greenland Sea). Canadian Journal of Fisheries and Aquatic Science, vol. 63, p. 1608–1616
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10. Rigor, I.G., J.M. Wallace, and R.L. Colony (2002), Response of Sea Ice to the Arctic Oscillation, J. Climate, v. 15, no. 18, pp. 2648 – 2668.
11. Thiemann,G. et al. (2011) Individual patterns of prey selection and dietary specialization in an Arctic marine carnivore. Oikos, doi: 10.1111/j.1600-0706.2011.19277.x
12. Lucas, Z., and Daoust, P. (2002) Large increases of harp seals (Phoca groenlandica) and hooded seals (Cystophora cristata) on Sable Island, Nova Scotia, since 1995. Polar Biology, vol 2, p. 562–568.
13. Ovsyanikov N.G., and Menyushina I.E. (2008) Specifics of Polar Bears Surviving an Ice Free Season on Wrangel Island in 2007. Marine Mammals of the Holarctic. Odessa, pp. 407-412
14. Stirling, I. et al. (1999) Long-term Trends in the Population Ecology of Polar Bears in Western Hudson Bay in Relation to Climatic Change. Arctic vol . 52, p. 294-306.
15. Stirling, I. and Derocher, A. (1990) Factors Affecting the Evolution and Behavioral Ecology of the Modern. Bears: Their Biology and Management, Vol. 8, A Selection of Papers from the Eighth International Conference on Bear Research and Management, Victoria, British Columbia, Canada, February 1989 (1990), pp. 189-204.
16. Stirling, I and Derocher, A. (2012) Effects of climate warming on polar bears: a review of the evidence. Global Change Biology (2012) 18, 2694–2706, doi: 10.1111/j.1365-2486.2012.02753.x

Adapted from the chapter The Resilient Polar Bear and 10,000 years of Climate Change in in Landscapes & Cycles: An Environmentalist’s Journey to Climate Skepticism