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Davis Strait Polar Bears: Too Many Bears or Not Enough Sea Ice?

Reference
Peacock, E., Taylor, M.K., Laake, J. and Stirling, I. 2013. Population ecology of polar bears in Davis Strait, Canada and Greenland. Journal of Wildlife Management 77: 463-476.
Davis Strait polar bears are the most southerly subpopulation: while a few bears in the southernmost portion of the Southern Hudson Bay region (James Bay) spend their summers at about 52°N and those in the southernmost portion of Western Hudson Bay region range to about 55°N, bears in the Davis Strait polar bear subpopulation occur regularly in spring to at least 50°N with some moving as far south as 47°N. It is a large region that varies north to south in terms of harp seal (Pagophilus groenlandicus) abundance (i.e. prey) and risk of being hunted.

A comprehensive assessment of the population has not been conducted since the 1970s, although some mark/recapture work was done in the 1990s (Stirling and Parkinson 2006). Stirling and Parkinson had this to say about Davis Strait sea ice (where 'breakup' is defined as the date in the year when ice cover reached 40% or less): "The maximum percentage ice coverage in Davis Strait [in spring] varies considerably more between years than that in Western Hudson Bay, Foxe Basin, or Baffin Bay: it was below 50% in 1981, 1986, and 2004, but above 85% in 1983, 1984, and 1993 (Fig. 7a). Correspondingly large interannual fluctuations are apparent in the timing of ice breakup (Fig. 7b), making a linear trend less meaningful. The long-term slope of the trend line for Davis Strait, -0.64 ± 0.69 days/year (p = 0.35), is not statistically significant; in contrast, the short-term trend, from 1991-2004, is decidedly negative (Fig. 7b)." The graph provided by Stirling and Parkinson shows that the earliest breakup date occurred in 1981 while the latest was only three years later (1984); the breakup date for 2004 was virtually identical to 1986.

But have these recent changes in sea ice cover negatively affected the survival or abundance of polar bears in Davis Strait? Lily Peacock and colleagues proposed to find out. The authors summarize their research this way: "we conducted a new capture study of polar bears in Davis Strait from 2005 to 2007. We pooled polar bear mark-recapture and harvest recovery data from 1974 through 2009, and estimated current rates of reproduction and survival, and population abundance and growth rate. To examine survival rates in an ecological context, we considered the effects of geography, harp seal abundance, and ice conditions on annual survival, and how the rates have changed over time. We further discuss the impacts of harvest rate and population density."

Bears were captured in the spring during 1974-1979, in spring and fall during 1991-1994 and 1997-1999, and in the fall only during 2005-2007. A total of 866 females and 990 males were captured and 145 harvested bears were accounted for. Abundance estimates and population growth rate estimates were generated from models that incorporated data on age and reproductive status collected from the mark-recapture work, and took into account harvest information. Survival for various age classes of polar bears was calculated by models that related survival over time to summer ice concentration ("mean bi-weekly total annual ice concentration 15 May - 15 October") and harp seal abundance.

And what did they find? Peacock et al. (2013) state that "the overall amount of sea ice declined and breakup has become progressively earlier" since the 1970s. Despite this, they "estimated the abundance of the Davis Strait polar bear subpopulation to 2,158, which results in a relatively high population density of polar bears of approximately 5.1 bears/1,000 km2 of sea ice habitat (Taylor and Lee 1995). This density is greater than polar bear densities in other seasonal-ice subpopulations, which are approximately 3.5 bears/1,000 km2." This estimate of 2,158 bears is a substantial increase over the 1,400 bears estimated in 1993 by Derocher et al. (1998). So despite declining sea ice since the 1970s, by 2007 the density of bears in this region had reached a higher level than other subpopulations with similar ice conditions.

The authors also concluded that "survival and reproduction of bears in southern Davis Strait was greater than in the north and tied to a concurrent dramatic increase in breeding harp seals (Pagophilus groenlandicus) in Labrador." They suggest that the Davis Strait polar bear subpopulation is characterized by "low recruitment rates, average adult survival rates, and high population density." The high density of bears in this region may be affecting recruitment (i.e. reproduction): the authors state that "low reproductive rates may reflect negative effects of greater densities or worsening ice conditions." It seems that polar bear populations with a high density of animals may show changes to life history parameters ["density-dependent responses"] that are similar to those expected in populations affected by declines in sea ice. This may seem a surprising outcome but it is not unprecedented. Similar results were found for Barents Sea polar bears studied between 1988 and 2002, prompting researcher Andrew Derocher (2005) to state: "given that the population may be showing density-dependent responses, it is not possible to differentiate the climatic effects from population effects."

Effects of high density and less time spent feeding due to reduced sea ice appear to be viable explanations for the observed conditions in Davis Strait and these factors are not necessarily mutually exclusive. Nevertheless, polar bears in Davis Strait appear to be increasing, not declining as reported by the IUCN/SSC Polar Bear Specialist Group (Obbard et al., 2010) and others, despite the declines in sea ice.

Additional References
Derocher 2005. Population ecology of polar bears at Svalbard, Norway. Population Ecology 47: 267-275.

Derocher, A., Garner, G.W., Lunn, N.J. and Wiig, Ø. (eds.) 1998. Polar Bears: Proceedings of the 12th meeting of the Polar Bear Specialists Group IUCN/SSC, 3-7 February, 1997, Oslo, Norway. Gland, Switzerland and Cambridge UK, IUCN.

Obbard, M.E., Theimann, G.W., Peacock, E. and DeBryn, T.D. (eds.) 2010. Polar Bears: Proceedings of the 15th meeting of the Polar Bear Specialist Group IUCN/SSC, 29 June-3 July, 2009, Copenhagen, Denmark. Gland, Switzerland and Cambridge UK, IUCN.

Stirling, I. and Parkinson, C.L. 2006. Possible effects of climate warming on selected populations of polar bears (Ursus maritimus) in the Canadian Arctic. Arctic: 59: 261-275.

Taylor, M. and Lee, J. 1995. Distribution and abundance of Canadian polar bear populations: a management perspective. Arctic 48: 147-154.

Archived 28 August 2013