Polar bear (Ursus maritimus) COSEWIC assessment and status report: chapter 6

6. Limiting Factors and Threats

The main, proximate limiting factors (i.e., immediate factors that directly cause mortality or reduce reproduction) affecting polar bear distribution and abundance are starvation (access to and abundance of ice-dependent seals), human-caused mortality (almost exclusively from hunting), and intraspecific predation. Other potential proximate factors include contamination, especially that associated with offshore development of hydrocarbon reserves and increased sea traffic in the Arctic, and the accumulation of environmental contaminants (mainly organochlorines) in tissues of polar bears. Climate change will likely influence all of the proximate limiting factors to polar bears listed above and may therefore become the ultimate limiting factor for the species. Review papers (e.g., Stirling and Derocher 1993; Barber and Iacozza 2004; Derocher et al. 2004; Stirling and Parkinson 2006), documents of ATK (e.g., Dowsley 2005), and observational reports (e.g., Amstrup et al. 2006; Monnett and Gleason 2006) offer insight into the possible impacts of past and continued climate warming on polar bears. Empirical studies of correlations between variables of climate change and body condition (Stirling et al. 1999; Obbard et al. 2006), and, recently, demographic rates (Obbard et al. 2007; Regehr et al. 2006, 2007a,b; Stirling et al. 2007) are more useful for quantitative predictions of how climate warming might underlie proximate limiting factors and threats faced by bears (see also Section 4.2). Here we treat climate change as an integral part of any discussion of the limiting factors to polar bears.

6.1 Food Availability

Recently, researchers of the Canadian Wildlife Service and USGS Alaska Science Center established a relationship between earlier break-up of sea ice in Western Hudson Bay and decreased survival of non-prime adult age classes of polar bears (Regehr et al. 2007a; see also Section 4.2), providing the first quantitative evidence for effects of climate-related stressors on polar bear population dynamics. Coupling reduced survival for most age and sex classes in Western Hudson Bay with observations that body size of females coming off the sea ice has declined in association with earlier break-up in spring (Stirling et al. 1999), decline in the polar bear subpopulation of Western Hudson Bay (Section 7.10) is best explained by reduced access to food. Similar conclusions have been reached for polar bears of the Southern Hudson Bay (Regehr et al. 2007b; Rode et al. 2007).

In Western Hudson Bay, there is some evidence that ringed seal reproduction has been reduced by climate warming (Ferguson et al. 2005; Stirling 2005); hence, polar bears may be responding to reductions in numbers of seals. It is possible that, at least in the context of maintained levels of human-caused mortality: 1) density of polar bears will track seal numbers; or 2) if earlier dates of break-up of sea ice do not allow enough time for bears to accumulate fat reserves required to endure fasting, declines in abundance may occur before or in the absence of observable declines in seal numbers. It is also important to note that not all segments of a polar bear population are expected to immediately decline in response to a reduction in food carrying capacity.In Western Hudson Bay, Regehr et al. (2007a) suggested that prime-aged adults were most likely “in better body condition than other polar bears (E. Richardson, CWS, unpublished data), able to divert resources from reproduction to survival in times of nutritional stress, …better at catching seals, and more able to take seal kills away from subordinate polar bears.” This may explain why no association between survival of prime-aged adult females and males and earlier break-up of sea ice was detected by Regehr et al. (2007a). For all other age classes, a statistical relationship between earlier break-up and decreased survival was shown.

Derocher et al. (2004) provide a synopsis of possible scenarios of changes in food availability to polar bears in the context of climate change, including the potential for climate warming to benefit some subpopulations, at least over the shorter term. This might apply to polar bears at the extreme northern edge of the species’ range (e.g., Viscount Melville Sound, western Lancaster Sound, Norwegian Bay, Kane Basin, and the Arctic Basin), where low primary productivity and multi-year sea ice limits densities of and access to ringed seals (Kingsley et al. 1985). Even within areas of relatively close proximity, the impact of climate change might vary substantially. For example, during the period of decreased abundance of polar bears in Western Hudson Bay attributed to impacts of climate change (above, Section 4.2 and 7.10), there was no observable decline in numbers of bears in Southern Hudson Bay (Section 7.11), although concordant declines in body condition were evident (Obbard et al. 2007). Following a new analysis of mark-recapture data first presented inKolenosky et al. (1992), researchers of the Government of Ontario reported that abundance of polar bears in Southern Hudson Bay was 641 bears (95% CI 401–881) in 1986 and 681 (95% CI 401–961) in 2005 (Obbard et al. 2007).

Although it remains uncertain as to how every subpopulation will respond to climate warming, it follows that there is a minimum period of at least annual sea ice (likely modified by factors such as prey availablity) conducive to the presence of polar bears. Only rarely have polar bears been observed to kill seals while swimming in open water (Furnell and Oolooyuk 1980), and killing of seals and walrus when hauled out on land will likely never replace the advantage of killing seals from sea ice (Derocher et al. 2004). Polar bears do not live where there is not at least annual sea ice, but many species of seals do. If climate warming were to prevent the formation of winter sea ice, or to substantially increase the duration of the open-water season in areas with seasonal ice currently used by polar bears, there is no hope that their numbers will remain viable in the affected subpopulation.

6.2 >Human-Caused Mortality

The most important proximate, limiting factor to the polar bear is presently human-caused mortality. In Canada this largely occurs through regulated hunting. For most age and sex groups, much of the annual mortality can be attributed to the known human-caused mortality (Section 5.1; Table 6). Over-harvest is of major concern for some demographic units, particularly where there is outdated information on abundance or a lack of enforceable quotas (Section 9.3). In the past, some Canadian polar bear subpopulations have been over-hunted (because of overestimation of population abundance and therefore quotas), with unfortunate examples coming from M’Clintock Channel (Taylor et al. 2006a) and the Viscount Melville Sound (Taylor et al. 2002). Today, substantially reduced mean rates of annual kill (34.0 bears [1979–1999] reduced to 1.8 bears [2002–2007] for M’Clintock Channel; 19.6 bears [1985–1990] reduced to 4.8 bears [2002–2007] for Viscount Melville Sound) are projected by simulations to have reversed trends in these subpopulations (Taylor et al. 2002, 2006a), although numbers remain severely reduced (Table 6, Sections 7.4 and section7.7). The same method has been used to project ongoing and severe instances of over harvest in polar bears of Baffin Bay and Kane Basin (Sections 7.12 and section7.13).

The most important problems with over-hunting are for the subpopulations of Kane Basin and Baffin Bay, and Western Hudson Bay in the context of lowered natural survival rates and climate change (Section 7.10). The governments of both Nunavut and Greenland have substantially increased their kill in Baffin Bay and Kane Basin in recent years (the 2006–2007 harvest of polar bears in Baffin Bay was 99 for Nunavut [increased from a quota of 54 in 2004] and 75 for Greenland; PBTC 2008). However, the Baffin Bay subpopulation was projected by Taylor et al. (2005) to be able to sustain an annual harvest of approximately 90 bears in 1997. Although Greenland has, as of January, 2006, instituted its first quota for polar bear hunting, which should reduce the kill of polar bears in Kane Basin and Baffin Bay (West Greenland harvest is not to exceed 100 bears/year; PBTC 2006), reversal of what we expect to be severe population decline (Table 6) is not likely to have occurred. The current combined Nunavut/Greenland regulated harvest of polar bears in Kane Basin and Baffin Bay continues to be excessive (Table 6, Sections 7.12 and section7.13).

Table 6. Status of subpopulations of polar bears within or shared by Canada. See footnotes next page and Sections 7.1–7.14 for details.
Subpopulatio
(primary data source)
Prvious Abundance Estimate N1
(year of estimate)
Previous Abundance Estimate 95% CI of N1 Current Abundance Estimate N2 (year of estimate) Current Abundance Estimate 95% CI of N2 Human-caused Mortality
Permitted harvest (bears/year)
Human-caused Motality
2002–2007 mean kill (bears/year)
Results of Population SimulationsTable notea
l± 1 SETable noteb
at currentkill
Results of Population SimulationsTable notea
PVATable notec
Results of Population SimulationsTable notea
l± 1 SE

under harvest moratoriumTable noted
2008 Status
Based on weight of evidenceTable notee
Baffin Bay
(Taylor et al. 2005,

IUCN/SSC Specialist Group 2006)
2074 (1998) 1544–2604 1546 (2004) 690–2402 105 +
Greenland
232.4 0.861 ± 0.075 0.998 1.054 ± 0.027 Declining
Davis Strait
(Peacock et al. 2006, PBTC 2007)
900 (1980) n/a 2251 (2006) n/a 52 +
Greenland, Quebec
60.0 n/a n/a n/a UnknownTable notef
Foxe Basin
(Taylor et al. 2006b, IUCN/SSC Specialist Group 2006)
2119 (1996) 1421–2817 2300Table noteg(2004) 1780–2820Table noteh 106 +
Quebec
98.6 n/a n/a n/a Unknown
Gulf of Boothia
(Taylor et al. 2008c)
n/a n/a 1528 (2000) 953–2093 74 56.4 1.025 ± 0.032 0.067 1.065 ±0.019 Increasing
Kane Basin
(Taylor et al. 2008a)
n/a n/a 164 (1998) 94–234 5 +
Greenland
12.8 0.935 ± 0.027 0.999 1.010 ± 0.010 Declining
Lancaster Sound
(Taylor et al. 2008b)
1031Table notei (1979) 795–1267 2541 (1998) 1759–3323 85 82.4 1.001 ± 0.014 0.260 1.023 ± 0.012 Stable
M'Clintock Channel
(Taylor et al. 2006a)
700 (1978) n/a 284 (2000) 166–402 3 1.8 1.022 ± 0.015 <0.001 1.031 ± 0.038 Increasing
Northern Beaufort Sea
(Stirling et al. 2007)
867 (1986) 726–1008 1200 (2006) 825–1135 65 34.4 0.994 ± 0.023Table notej 0.419Table notej 1.031 ± 0.021Table notej StableTablenotej
Norwegian Bay
(Taylor et al. 2008c)
n/a n/a 190 (1998) 102–278 4 3.0 0.997 ± 0.026 0.439 1.006 ± 0.016 Stable?
Southern Beaufort Sea
(Regehr et al. 2006, 2007b)
1800 (1983) 1300–2500Table noteh 1526 (2006) 1211–1841 81 53.4 0.938 ± 0.030 0.945 0.980 ± 0.029 Declining
Southern Hudson Bay
(Obbard et al. 2007a,b)
641 (1986) 401–881 681Table notek (2005) 401–961 25 +
Ontario,
Quebec
36.2 0.969 ± 0.055 Tablenotel 0.670 Tablelnotel 1.028 ± 0.076Table notel StableTablenotel
Viscount Melville Sound
(Taylor et al. 2002)
161 (1993) 121–201 215 (1996) 99– 331 7 4.8 1.037 ± 0.033 0.072 1.059 ± 0.063 Increasing
W. Hudson BayTable notem
(Regehr et al. 2007a)
1194 (1987) 1020–1368 935 (2005) 794–1076 46 +
Manitoba
46.8 0.940 ± 0.013
0.903 ± 0.014
0.999
0.999
0.999 ± 0.011
0.964 ± 0.011
Declining
Total Current estimate 15000
(11000–19000)Table noten
- - - >643 734.6Tableonoteo n/a n/a n/a DecliningTable notep

How human-caused mortality interacts with climate warming and impending changes to abundances of or access to seals is of considerable importance to the conservation of polar bears. Perhaps the most important impact of climate change on the manner in which polar bears are killed in Canada will come in the form of an anticipated increase in bear-human conflicts (Derocher et al. 2004; Stirling and Parkinson 2006). As alluded to in Section 6, reductions in food availabilty may result in increases in nutrionally stressed bears spending longer periods of time onshore, where humans live.Increases in problem bear activity in areas most affected by climate warming have been reported in recent years, including the Southern Beaufort Sea (Schliebe et al. 2006) and Western Hudson Bay (McDonald et al. 1997; Stirling et al. 1999; Stirling and Parkinson 2006), and a positive interaction between climate warming and human-caused mortality may pose a serious problem. Stirling and Parkinson (2006) clearly show that for Western Hudson Bay, the earlier the ice breaks up the more problem bears there are in a year, and vice versa (see Figure 4 of Stirling and Parkinson [2006]). The perception of increased numbers of problem bears has also been voiced repeatedly by Inuit participants at meetings of the Federal/Provincial/ Territorial Polar Bear Committee, and through collection of ATK (see Section 7). In some cases, higher numbers of problem bears may indicate higher abundance, an argument given for increasing hunting quotas in almost every subpopulation in Nunavut in 2005 (IUCN/SSC Polar Bear Specialist Group 2006); however, the best available scientific data support the notion that perceived higher numbers of bears is the result of bears coming off the ice in poorer condition and becoming problem animals in greater numbers due to earlier break-up of sea ice (see Stirling and Parkinson 2006).

Of particular importance, simulation models (Section 7.1; Table 6) suggest that for the subpopulations of Western Hudson Bay and Southern Beaufort Sea, both impacted by effects of climate change, even with a harvest moratorium declines would be inevitable. That is, the populations are no longer viable at their present abundances. Currently, this is not likely to be the case for most subpopulations of polar bears in Canada (Table 6), but vital statistics may be changed in the future by changes in ice availability.

6.3 Other Limiting Factors

As noted in Section 5.2, polar bears, like all ursids, kill and eat members of their own species, and intraspecific predation is a potential regulating factor (i.e., density-dependent limiting factor) for polar, brown, and black bears (Taylor 1994). As such, intraspecific predation would be expected to occur at a higher frequency when polar bear subpopulations are at relatively high density. It is unlikely that polar bear subpopulations as they exist today are approaching abundances that, at least in the past, would have been associated with population carrying capacity. However, intraspecific predation continues to be observed and recent, potential spikes in instances of intraspecific predation (e.g., Amstrup et al. 2006) suggest that, in areas most affected by climate change, carrying capacity of the environment has been lowered for polar bears. This hypothesis presents a plausible explanation for the purported link between climate warming and increased conflicts within the polar bear population, and may also explain recent instances of increased conflicts between humans and polar bears (Section 6).

Since the mid-1960s, exploration for energy and mineral reserves has led to an increased amount of industrial activity in the Arctic. Concerns about disturbance of bears at denning areas due to noise or construction have been voiced in recent interviews of ATK (Atatahak and Banci 2001; Keith et al. 2005), and new activity in the Arctic has the potential to increase killing of bears in defence of life or property. However, the primary threat to polar bears from industrial development may come from the potential for environmental contamination, especially large-scale oil spills. Oil is extremely toxic and potentially lethal to bears in even small amounts (Øritsland et al. 1981; Stirling 1990; Derocher and Stirling 1991). Although recent oil-spill simulations (Durner et al. 2001) suggest that relatively few bears in Canada (Southern Beaufort Sea) would encounter oil if a major spill occurred from existing operations, as climate change increases access to the polar basin we might anticipate increased risks to bears with development in the Canadian Arctic Archipelago. Extensive discovered and recoverable oil and gas reserves exist in Nunavut, including the 3.3 × 106 barrel (oil) and 17.4 × 108 ft³ (gas) reserves of the Sverdrup sedimentary basin (Drummond 2006), which overlap the subpopulations of Norwegian Bay, Lancaster Sound, Viscount Melville Sound, and Northern Beaufort Sea. Reserves of an additional 2.3 × 108 ft³ of natural gas occur in Baffin Bay (Drummond 2006). Continued development of the 1.0 × 107 barrel (oil) and 9.7 × 108 ft³ (natural gas) petroleum reserves of the Beaufort Sea/Mackenzie Delta in the Northwest Territories (Drummond 2006) may put additional pressure on the Southern Beaufort Sea subpopulation of polar bears.

In recent years, significant levels of various contaminants (organochlorines and other persistent organic pollutants) have been documented in polar bear tissues or tissues of their prey, particularly adipose tissue (e.g., Born et al. 1991; Norstrom et al. 1988, 1998; Norstrom and Muir 1994; Bernhoft et al. 1996; Letcher et al. 1995; Henriksen et al. 2001; Kucklick et al. 2002; Oskam et al. 2004; Wolkers et al. 2004; Smithwick et al. 2005; Muir et al. 2006). Effects of various compounds in the tissues of polar bears or of the seals they feed on remains largely unknown. Although contaminant levels in some subpopulations correlate with impaired endocrine function (Skaare et al. 2001; Oskam et al. 2004), immune function (e.g., Bernhoft et al. 2000; Skaare et al. 2002; Lie et al. 2004, 2005), and potentially bone mineral composition (Sonne et al. 2004), there has been little demonstration of demographic effects from contaminants on polar bears (Amstrup 2003).

Inuit interviewed for ATK have recently expressed concerns that studies for scientific research, whereby bears are immobilized using drugs and helicopters and snowmobiles are used to capture bears, may cause displacement of bears or result in long-term, adverse physiological effects (McDonald et al. 1997; Atatahak and Banci 2001; Dowsley and Taylor 2006a; Dowsley 2005). However, scientific research is clearly not a limiting factor on polar bears. Messier (2000), after analyzing 3,237 research handlings of polar bears for the period 1989–1997, concluded that long-term effects on polar bears of tagging and radio-collaring are largely negligible from the perspective of population dynamics. Nonetheless, polar bears are sometimes killed by accident during the course of scientific research. Messier (2000) reported that mortalities occurred at an average rate of 1 per 1,000 bears handled for management and population studies. Risk of mortality was higher for more complex handling protocols associated with studies of physiology (28 bears per 1,000 bears handled).

In all likelihood and within our lifetimes, due to climate warming, the Northwest Passage (recently renamed the Canadian Internal Waters by the Government of Canada) will remain open for increasing periods of time, making it attractive as a major shipping route. Routes from Europe to the Far East are reduced by as much as 4,000 km by travel through the waterway, as compared to the route through the Panama Canal. Polar bears in the vicinity of this new shipping route may be exposed to traffic and levels of pollution that no subpopulation of polar bear has yet experienced. How they will respond to these cumulative effects is unknown.

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