Reply to response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay by Stirling et al. (2008)

We address the three main issues raised by Stirling et al. [Stirling, I., Derocher, A.E., Gough, W.A., Rode, K., in press. Response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay. Ecol. Complexity]: (1) evidence of the role of climate warming in affecting the western Hudson Bay polar bear population, (2) responses to suggested importance of human–polar bear interactions, and (3) limitations on polar bear adaptation to projected climate change. We assert that our original paper did not provide any “alternative explanations [that] are largely unsupported by the data” or misrepresent the original claims by Stirling et al. [Stirling, I., Lunn, N.J., Iacozza, I., 1999. Long-term trends in the population ecology of polar bears in western Hudson Bay in relation to climate change. Arctic 52, 294–306], Derocher et al. [Derocher, A.E., Lunn, N.J., Stirling, I., 2004. Polar bears in a warming climate. Integr. Comp. Biol. 44, 163–176], and other peer-approved papers authored by Stirling and colleagues. In sharp contrast, we show that the conclusion of Stirling et al. [Stirling, I., Derocher, A.E., Gough, W.A., Rode, K., in press. Response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay. Ecol. Complexity] – suggesting warming temperatures (and other related climatic changes) are the predominant determinant of polar bear population status, not only in western Hudson (WH) Bay but also for populations elsewhere in the Arctic – is unsupportable by the current scientific evidence.The commentary by Stirling et al. [Stirling, I., Derocher, A.E., Gough, W.A., Rode, K., in press. Response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay. Ecol. Complexity] is an example of uni-dimensional, or reductionist thinking, which is not useful when assessing effects of climate change on complex ecosystems. Polar bears of WH are exposed to a multitude of environmental perturbations including human interference and factors (e.g., unknown seal population size, possible competition with polar bears from other populations) such that isolation of any single variable as the certain root cause (i.e., climate change in the form of warming spring air temperatures), without recognizing confounding interactions, is imprudent, unjustified and of questionable scientific utility. Dyck et al. [Dyck, M.G., Soon, W., Baydack, R.K., Legates, D.R., Baliunas, S., Ball, T.F., Hancock, L.O., 2007. Polar bears of western Hudson Bay and climate change: Are warming spring air temperatures the “ultimate” survival control factor? Ecol. Complexity, 4, 73–84. doi:10.1016/j.ecocom.2007.03.002] agree that some polar bear populations may be negatively impacted by future environmental changes; but an oversimplification of the complex ecosystem interactions (of which humans are a part) may not be beneficial in studying external effects on polar bears. Science evolves through questioning and proposing hypotheses that can be critically tested, in the absence of which, as Krebs and Borteaux [Krebs, C.J., Berteaux, D., 2006. Problems and pitfalls in relating climate variability to population dynamics. Clim. Res. 32, 143–149] observe, “we will be little more than storytellers.”     

Polar bears of western Hudson Bay and climate change: Are warming spring air temperatures the “ultimate” survival control factor?

Long-term warming of late spring (April–June) air temperatures has been proposed by Stirling et al. [Stirling, I., Lunn, N.J., Iacozza, J., 1999. Long-term trends in the population ecology of polar bears in western Hudson Bay in relation to climatic change. Arctic 52, 294–306] as the “ultimate” factor causing earlier sea-ice break-up around western Hudson Bay (WH) that has, in turn, led to the poorer physical and reproductive characteristics of polar bears occupying this region. Derocher et al. [Derocher, A.E., Lunn, N.J., Stirling, I., 2004. Polar bears in a warming climate. Integr. Comp. Biol. 44, 163–176] expanded the discussion to the whole circumpolar Arctic and concluded that polar bears will unlikely survive as a species should the computer-predicted scenarios for total disappearance of sea-ice in the Arctic come true. We found that spring air temperatures around the Hudson Bay basin for the past 70 years (1932–2002) show no significant warming trend and are more likely identified with the large-amplitude, natural climatic variability that is characteristic of the Arctic. Any role of external forcing by anthropogenic greenhouse gases remains difficult to identify. We argue, therefore, that the extrapolation of polar bear disappearance is highly premature. Climate models are simply not skilful for the projection of regional sea-ice changes in Hudson Bay or the whole Arctic. Alternative factors, such as increased human–bear interaction, must be taken into account in a more realistic study and explanation of the population ecology of WH polar bears. Both scientific papers and public discussion that continue to fail to recognize the inherent complexity in the adaptive interaction of polar bears with both human and nature will not likely offer any useful, science-based, preservation and management strategies for the species.     

Effects of Earlier Sea Ice Breakup on Survival and Population Size of Polar Bears in Western Hudson Bay

ABSTRACT Some of the most pronounced ecological responses to climatic warming are expected to occur in polar marine regions, where temperature increases have been the greatest and sea ice provides a sensitive mechanism by which climatic conditions affect sympagic (i.e., with ice) species. Population-level effects of climatic change, however, remain difficult to quantify. We used a flexible extension of Cormack-Jolly-Seber capture-recapture models to estimate population size and survival for polar bears (Ursus maritimus), one of the most ice-dependent of Arctic marine mammals. We analyzed data for polar bears captured from 1984 to 2004 along the western coast of Hudson Bay and in the community of Churchill, Manitoba, Canada. The Western Hudson Bay polar bear population declined from 1,194 (95% CI = 1,020-1,368) in 1987 to 935 (95% CI = 794-1,076) in 2004. Total apparent survival of prime-adult polar bears (5–19 yr) was stable for females (0.93; 95% CI = 0.91-0.94) and males (0.90; 95% CI = 0.88-0.91). Survival of juvenile, subadult, and senescent-adult polar bears was correlated with spring sea ice breakup date, which was variable among years and occurred approximately 3 weeks earlier in 2004 than in 1984. We propose that this correlation provides evidence for a causal association between earlier sea ice breakup (due to climatic warming) and decreased polar bear survival. It may also explain why Churchill, like other communities along the western coast of Hudson Bay, has experienced an increase in human-polar bear interactions in recent years. Earlier sea ice breakup may have resulted in a larger number of nutritionally stressed polar bears, which are encroaching on human habitations in search of supplemental food. Because western Hudson Bay is near the southern limit of the species' range, our findings may foreshadow the demographic responses and management challenges that more northerly polar bear populations will experience if climatic warming in the Arctic continues as projected.     

What to eat now? Shifts in polar bear diet during the ice-free season in western Hudson Bay

Under current climate trends, spring ice breakup in Hudson Bay is advancing rapidly, leaving polar bears (Ursus maritimus) less time to hunt seals during the spring when they accumulate the majority of their annual fat reserves. For this reason, foods that polar bears consume during the ice‐free season may become increasingly important in alleviating nutritional stress from lost seal hunting opportunities. Defining how the terrestrial diet might have changed since the onset of rapid climate change is an important step in understanding how polar bears may be reacting to climate change. We characterized the current terrestrial diet of polar bears in western Hudson Bay by evaluating the contents of passively sampled scat and comparing it to a similar study conducted 40 years ago. While the two terrestrial diets broadly overlap, polar bears currently appear to be exploiting increasingly abundant resources such as caribou (Rangifer tarandus) and snow geese (Chen caerulescens caerulescens) and newly available resources such as eggs. This opportunistic shift is similar to the diet mixing strategy common among other Arctic predators and bear species. We discuss whether the observed diet shift is solely a response to a nutritional stress or is an expression of plastic foraging behavior.     

Habitat use by harbour seals (Phoca vitulina) in a seasonally ice-covered region, the western Hudson Bay

Over the last few decades, the period of ice cover in Hudson Bay has decreased, owing to climate warming, with breakup occurring approximately 3 weeks earlier than it did 30 years ago. The trend towards lengthening of the open water season has led to speculation that ringed seal numbers would decline, but then harbour seals might become numerous enough to replace ringed seals in the diet of polar bears. The movement patterns of 18 harbour seals equipped with satellite-linked transmitters in the Churchill River estuary (western Hudson Bay) were examined, as well as the dive behaviour of 11 of these seals. During the ice-free period, seals followed a general central place-foraging strategy, making repeated trips between their haul-out site in the Churchill River estuary and nearshore areas (<20 km) near the river mouth and estuary. Seal behaviour changed significantly as ice started to form along the coast of western Hudson Bay: animals remained significantly farther from the Churchill River haul-out site and from the coast and performed longer and deeper dives. However, throughout the entire tracking period, whether ice was present or not, all animals restricted their movements to a narrow band of shallow coastal waters (<50 m depth) along a 600-km stretch of the western Hudson Bay coastline, centred on the Churchill River estuary haul-out site. This natural self-limitation to nearshore shallow waters could restrict the potential for the population to increase in size and replace ringed seals as a primary energy resource for polar bears.     

Spatial and temporal patterns of problem polar bears in Churchill,Manitoba

Human–bear interactions near the town of Churchill, Manitoba occur annually because the Western Hudson Bay polar bear population spends 4–5 months on-land each year when the sea ice melts completely. Significant changes have occurred in the Hudson Bay ecosystem and in the bear population as a result of climate warming; however, how these changes may have influenced human–bear interactions near Churchill is unclear. This study examined the temporal and spatial patterns of 1,487 problem bears captured in the Churchill area from 1970 to 2004. We also examined the relationship between problem bears and environmental variables as well as the Nunavut harvest. The number of individual problem bears caught near Churchill varied from 10 to 90 individuals per year and increased over time. Subadult males comprised 39%, subadult females 23%, adult males 18%, females with young 14%, and solitary females 6% of captures. Bears that became problem individuals were in closer proximity to the Churchill area. Nutritional stress and a northward shift in the distribution of the bears that spend the summer on-land in northeastern Manitoba may account for the increase in problem bear numbers. The date of sea ice freeze-up, which is getting progressively later, was the best predictor explaining the annual variation in the occurrence of problem bears. These results provide an understanding of how a warming climate may directly impact polar bear behaviour. This information may allow wildlife managers to predict relative levels of human–bear interactions and thereby implement effective management strategies to improve human safety and the conservation of polar bears.     

Trophic matches and mismatches: can polar bears reduce the abundance of nesting snow geese in western Hudson Bay?

Climate change driven advances in the date of sea ice breakup will increasingly lead to a loss of spring polar bear foraging opportunities on ringed seal pups creating a phenological trophic ‘mismatch’. However, the same shift will lead to a new ‘match’ between polar bears and ground nesting birds. This new match will be especially prevalent along the Cape Churchill Peninsula of western Hudson Bay where both polar bears and nesting snow geese are abundant. Easily foraged goose eggs will provide at least some of the earlier arriving polar bears with compensation for the energy deficit accrued through lost seal hunting opportunities. We examine the potential impact of changes in the extent and pattern of polar bear egg predation on snow goose abundance using projection models that account not only for increases in the temporal overlap of the two species but also for autocorrelation and stochasticity in the processes underlying polar bear onshore arrival and snow goose incubation. Egg predation will reduce reproductive output of the nesting lesser snow geese and, under all but trivial rates, will lead to a reduction in the size of their nesting population on the Cape Churchill Peninsula. Stochasticity associated with the asymmetrical advances in polar bear onshore arrival and the snow goose incubation period will lead to periodic mismatches in their overlap. These, in turn, will allow snow goose abundance to increase periodically. Climate driven changes in trophic matches and mismatches may reduce snow goose numbers but will not eliminate this over‐abundant species that poses a threat to Arctic landscapes.     

Demographic and traditional knowledge perspectives on the current status of Canadian polar bear subpopulations

Subpopulation growth rates and the probability of decline at current harvest levels were determined for 13 subpopulations of polar bears (Ursus maritimus) that are within or shared with Canada based on mark–recapture estimates of population numbers and vital rates, and harvest statistics using population viability analyses (PVA). Aboriginal traditional ecological knowledge (TEK) on subpopulation trend agreed with the seven stable/increasing results and one of the declining results, but disagreed with PVA status of five other declining subpopulations. The decline in the Baffin Bay subpopulation appeared to be due to over‐reporting of harvested numbers from outside Canada. The remaining four disputed subpopulations (Southern Beaufort Sea, Northern Beaufort Sea, Southern Hudson Bay, and Western Hudson Bay) were all incompletely mark–recapture (M‐R) sampled, which may have biased their survival and subpopulation estimates. Three of the four incompletely sampled subpopulations were PVA identified as nonviable (i.e., declining even with zero harvest mortality). TEK disagreement was nonrandom with respect to M‐R sampling protocols. Cluster analysis also grouped subpopulations with ambiguous demographic and harvest rate estimates separately from those with apparently reliable demographic estimates based on PVA probability of decline and unharvested subpopulation growth rate criteria. We suggest that the correspondence between TEK and scientific results can be used to improve the reliability of information on natural systems and thus improve resource management. Considering both TEK and scientific information, we suggest that the current status of Canadian polar bear subpopulations in 2013 was 12 stable/increasing and one declining (Kane Basin). We do not find support for the perspective that polar bears within or shared with Canada are currently in any sort of climate crisis. We suggest that monitoring the impacts of climate change (including sea ice decline) on polar bear subpopulations should be continued and enhanced and that adaptive management practices are warranted.     

Temporal change in the morphometry–body mass relationship of polar bears

Accurate information on animal body mass is often an essential component of wildlife research and management. However, for many large-bodied species, obtaining direct scale weights from individuals may be difficult. In these cases, morphometric equations (e.g., based on girth or length) may provide accurate and precise estimates of body mass. We developed predictive equations to estimate the body mass of free-ranging polar bears (Ursus maritimus) in western Hudson Bay, Canada. Using multiple linear and non-linear regression, we identified a strong relationship between polar bear body weight and linear measures of straight line length and axillary girth. The mass–morphometry relationship appeared to change over time and we developed separate equations for polar bears measured during 2 time periods, 1980–1996 and 2007–2009. Non-linear models were more accurate and provided body mass estimates within 5.8% (R² = 0.98) and 6.1% (R² = 0.98) of scale weight in the earlier and later time periods, respectively. Earlier equations developed for polar bears in this subpopulation performed poorly when applied to recently sampled individuals. In contrast, some contemporary equations from other regions performed reasonably well, suggesting that temporal changes within a subpopulation may be more pronounced than regional differences and can render earlier predictive equations obsolete. Our results have important implications for current and future studies of polar bear body condition and the effects of ongoing climate warming. © 2011 The Wildlife Society.     

The Energetic Value of Land-Based Foods in Western Hudson Bay and Their Potential to Alleviate Energy Deficits of Starving Adult Male Polar Bears

Climate change is predicted to expand the ice-free season in western Hudson Bay and when it grows to 180 days, 28–48% of adult male polar bears are projected to starve unless nutritional deficits can be offset by foods consumed on land. We updated a dynamic energy budget model developed by Molnar et al. to allow influx of additional energy from novel terrestrial foods (lesser snow geese, eggs, caribou) that polar bears currently consume as part of a mixed diet while on land. We calculated the units of each prey, alone and in combination, needed to alleviate these lethal energy deficits under conditions of resting or limited movement (2 km d^-1) prior to starvation. We further considered the total energy available from each sex and age class of each animal prey over the period they would overlap land-bound polar bears and calculated the maximum number of starving adult males that could be sustained on each food during the ice-free season. Our results suggest that the net energy from land-based food, after subtracting costs of limited movement to obtain it, could eliminate all projected nutritional deficits of starving adult male polar bears and likely other demographic groups as well. The hunting tactics employed, success rates as well as behavior and abundance of each prey will determine the realized energetic values for individual polar bears. Although climate change may cause a phenological mismatch between polar bears and their historical ice-based prey, it may simultaneously yield a new match with certain land-based foods. If polar bears can transition their foraging behavior to effectively exploit these resources, predictions for starvation-related mortality may be overestimated for western Hudson Bay. We also discuss potential complications with stable-carbon isotope studies to evaluate utilization of land-based foods by polar bears including metabolic effects of capture-related stress and consuming a mixed diet.     

The stress of Arctic warming on polar bears

Arctic ecosystems are changing rapidly in response to climate warming. While Arctic mammals are highly evolved to these extreme environments, particularly with respect to their stress axis, some species may have limited capacity to adapt to this change. We examined changes in key components of the stress axis (cortisol and its carrier protein—corticosteroid binding globulin [CBG]) in polar bears (Ursus maritimus) from western Hudson Bay (N = 300) over a 33 year period (1983–2015) during which time the ice‐free period was increasing. Changing sea ice phenology limits spring hunting opportunities and extends the period of onshore fasting. We assessed the response of polar bears to a standardized stressor (helicopter pursuit, darting, and immobilization) during their onshore fasting period (late summer–autumn) and quantified the serum levels of the maximum corticosteroid binding capacity (MCBC) of CBG, the serum protein that binds cortisol strongly, and free cortisol (FC). We quantified bear condition (age, sex, female with cubs or not, fat condition), sea ice (breakup in spring–summer, 1 year lagged freeze‐up in autumn), and duration of fasting until sample collection as well as cumulative impacts of the latter environmental traits from the previous year. Data were separated into “good” years (1983–1990) when conditions were thought to be optimal and “poor” years (1991–2015) when sea ice conditions deteriorated and fasting on land was extended. MCBC explained 39.4% of the variation in the good years, but only 28.1% in the poor ones, using both biological and environmental variables. MCBC levels decreased with age. Changes in FC were complex, but more poorly explained. Counterintuitively, MCBC levels increased with increased time onshore, 1 year lag effects, and in poor ice years. We conclude that MCBC is a biomarker of stress in polar bears and that the changes we document are a consequence of climate warming.     

Can whisker spot patterns be used to identify individual polar bears?

Studies of population dynamics, movement patterns and animal behavior usually require identification of individuals. We evaluated the reliability of using whisker spot patterns to noninvasively identify individual polar bears Ursus maritimus . We obtained the locations of polar bear whisker spots from photographs taken in western Hudson Bay, tested the independence of spot locations, estimated the complexity of each spot pattern in terms of information and determined whether each whisker spot pattern was reliable from its information content. Of the 50 whisker spot patterns analyzed, 98% contained enough information to be reliable, and this result varied little among observers. Photographs taken <50 m from polar bears were most useful. Our results suggest that individual identification of polar bears in the field based on whisker spot pattern variations is reliable. Researchers studying polar bear behavior or estimating population parameters can benefit from this method if proximity to the bears is feasible.     

Future sea ice conditions in Western Hudson Bay and consequences for polar bears in the 21st century

The primary habitat of polar bears is sea ice, but in Western Hudson Bay (WH), the seasonal ice cycle forces polar bears ashore each summer. Survival of bears on land in WH is correlated with breakup and the ice‐free season length, and studies suggest that exceeding thresholds in these variables will lead to large declines in the WH population. To estimate when anthropogenic warming may have progressed sufficiently to threaten the persistence of polar bears in WH, we predict changes in the ice cycle and the sea ice concentration (SIC) in spring (the primary feeding period of polar bears) with a high‐resolution sea ice‐ocean model and warming forced with 21st century IPCC greenhouse gas (GHG) emission scenarios: B1 (low), A1B (medium), and A2 (high). We define critical years for polar bears based on proposed thresholds in breakup and ice‐free season and we assess when ice‐cycle conditions cross these thresholds. In the three scenarios, critical years occur more commonly after 2050. From 2001 to 2050, 2 critical years occur under B1 and A2, and 4 under A1B; from 2051 to 2100, 8 critical years occur under B1, 35 under A1B and 41 under A2. Spring SIC in WH is high (>90%) in all three scenarios between 2001 and 2050, but declines rapidly after 2050 in A1B and A2. From 2090 to 2100, the mean spring SIC is 84 (±7)% in B1, 56 (±26)% in A1B and 20 (±13)% in A2. Our predictions suggest that the habitat of polar bears in WH will deteriorate in the 21st century. Ice predictions in A1B and A2 suggest that the polar bear population may struggle to persist after ca. 2050. Predictions under B1 suggest that reducing GHG emissions could allow polar bears to persist in WH throughout the 21st century.     

The effects of forest fires on polar bear maternity denning habitat in western Hudson Bay

The effects of forest fires on polar bear (Ursus maritimus) maternity denning habitat in western Hudson Bay were determined by comparing the physical characteristics of 48 burned and 101 unburned maternity den sites from September 2001 to October 2003. Fire significantly altered vegetation composition and increased the depth of the active layer, resulting in a decrease in the stability of den sites, the collapse of dens, and degradation of the surrounding habitat. Although bears investigated burned areas, analysis of mark and recapture data, satellite telemetry, radio-telemetry, and field observations all demonstrated that bears did not use burned areas for denning. While peat denning habitat is likely not limiting at this time, the re-use and occupancy of peat den sites during the summer may be an important means of energy conservation for pregnant female bears in western Hudson Bay. Increased energy expenditures in association with increased search times for suitable den sites and the excavation of new dens can potentially affect reproductive success. Predicted increases in forest fire activity as a result of climate change, along with the long-term recovery of denning habitat may reduce the amount of suitable denning habitat in the future. Resource managers need to be aware of the possibility of a shift in the distribution of denning bears and further loss of maternity denning habitat in western Hudson Bay.     

Assessing stress in Western Hudson Bay polar bears using hair cortisol concentration as a biomarker

The development of novel biomarkers to help assess whether polar bear (Ursus maritimus) health is impacted by long-term physiological stress associated with climate change represents an emerging area of research. Reductions in sea ice cover and food availability are potentially stressful, and chronic stress can have deleterious effects that may impair individual and population level health. Cortisol is the principal effector hormone of the stress response and has previously been linked to aspects of polar bear life history (e.g., reproduction and growth) known to be negatively influenced by environmental change. Understanding stress is important for polar bears at the southern limit of their range, such as those in Western Hudson Bay (WH), where rapidly changing sea ice phenology threatens population viability. We examined the relationships between age, reproductive status, and body condition (fatness) and hair cortisol concentration (HCC) in 729 polar bear hair samples collected in WH from 2004–2013. Overall, there was a negative relationship between fatness and HCC, suggesting that bears in poorer body condition experienced higher levels of stress. However, when reproductive status was included in our analysis, this relationship only held for male and lone female bears. Females with dependent offspring had consistently low fatness and elevated HCC, likely because of the high cost of maternal care. We also found a positive correlation between HCC and age for: (1) bears in poor body condition, possibly due to nutritional stress compounding effects of aging; and (2) male bears, potentially due to stress and injury associated with intrasexual mate competition. These findings support the use of HCC as a biomarker for polar bear health. Furthermore, we have established a HCC benchmark against which future population-level effects of climate change in WH polar bears can be compared.     

Reproductive rates of ringed seals and survival of pups in Northwestern Hudson Bay, Canada, 1991–2000

Reproductive parameters were determined from a sample of ringed seals collected by Inuit hunters during their annual open water harvest in autumn at Arviat, Nunavut, on the western coast of Hudson Bay, Canada, in 1991–1992 and 1998–2000. Ovulation rates of adult females were high and similar to rates recorded from studies of ringed seals in other geographic areas. However, pregnancy rates averaged only 55% and were significantly lower than in other studies, and the proportions of young-of-the-year were only 4.8, 4.2, 7.5, 4.1, and 23% for the mentioned years, respectively, instead of being >30% as expected. These results appear to indicate that reproductive parameters of ringed seals and survival of their young are exhibiting long-term shifts rather than short-term fluctuations, and that the trend is downward. Furthermore, these downward trends in reproduction, in conjunction with changes in the proportions of different seal species in the diet of polar bears, climatic warming in western Hudson Bay, and progressively earlier breakup of sea ice over the last 25 years, suggest that major changes are occurring in the marine ecosystem of Hudson Bay. The pathways involved are poorly understood and merit further study.     

Effects of climate warming on polar bears: a review of the evidence

Climate warming is causing unidirectional changes to annual patterns of sea ice distribution, structure, and freeze‐up. We summarize evidence that documents how loss of sea ice, the primary habitat of polar bears (Ursus maritimus), negatively affects their long‐term survival. To maintain viable subpopulations, polar bears depend on sea ice as a platform from which to hunt seals for long enough each year to accumulate sufficient energy (fat) to survive periods when seals are unavailable. Less time to access to prey, because of progressively earlier breakup in spring, when newly weaned ringed seal (Pusa hispida) young are available, results in longer periods of fasting, lower body condition, decreased access to denning areas, fewer and smaller cubs, lower survival of cubs as well as bears of other age classes and, finally, subpopulation decline toward eventual extirpation. The chronology of climate‐driven changes will vary between subpopulations, with quantifiable negative effects being documented first in the more southerly subpopulations, such as those in Hudson Bay or the southern Beaufort Sea. As the bears' body condition declines, more seek alternate food resources so the frequency of conflicts between bears and humans increases. In the most northerly areas, thick multiyear ice, through which little light penetrates to stimulate biological growth on the underside, will be replaced by annual ice, which facilitates greater productivity and may create habitat more favorable to polar bears over continental shelf areas in the short term. If the climate continues to warm and eliminate sea ice as predicted, polar bears will largely disappear from the southern portions of their range by mid‐century. They may persist in the northern Canadian Arctic Islands and northern Greenland for the foreseeable future, but their long‐term viability, with a much reduced global population size in a remnant of their former range, is uncertain.     

Home range distribution of polar bears in western Hudson Bay

Polar Biology - Sea ice in Hudson Bay is melting earlier and freezing later as the climate warms, resulting in declines in the condition, survival, and population size of polar bears (Ursus...     

Assessing polar bear (Ursus maritimus) population structure in the Hudson Bay region using SNPs

Defining subpopulations using genetics has traditionally used data from microsatellite markers to investigate population structure; however, single‐nucleotide polymorphisms (SNPs) have emerged as a tool for detection of fine‐scale structure. In Hudson Bay, Canada, three polar bear (Ursus maritimus) subpopulations (Foxe Basin (FB), Southern Hudson Bay (SH), and Western Hudson Bay (WH)) have been delineated based on mark–recapture studies, radiotelemetry and satellite telemetry, return of marked animals in the subsistence harvest, and population genetics using microsatellites. We used SNPs to detect fine‐scale population structure in polar bears from the Hudson Bay region and compared our results to the current designations using 414 individuals genotyped at 2,603 SNPs. Analyses based on discriminant analysis of principal components (DAPC) and STRUCTURE support the presence of four genetic clusters: (i) Western—including individuals sampled in WH, SH (excluding Akimiski Island in James Bay), and southern FB (south of Southampton Island); (ii) Northern—individuals sampled in northern FB (Baffin Island) and Davis Strait (DS) (Labrador coast); (iii) Southeast—individuals from SH (Akimiski Island in James Bay); and (iv) Northeast—individuals from DS (Baffin Island). Population structure differed from microsatellite studies and current management designations demonstrating the value of using SNPs for fine‐scale population delineation in polar bears.     

Reproductive biology and ecology of female polar bears (Ursus maritimus)

Data on age-specific natality rates, litter size, interbirth interval, age of first reproduction, reproductive senescence, age of weaning and cub survival were determined for a free-ranging population of polar bears inhabiting Hudson Bay, Canada, near the southern limit of the species range. Serum progesterone levels were also determined for females at different stages of their reproductive cycle to provide corroborative support for the reproductive parameters described. Animals were live captured using immobilizing drugs and each animal uniquely marked for future identification. First parturition occurred at four or five years of age and the age-specific natality rate increased with age until approximately 20 years, after which it dropped markedly. At least 40% of adult females displayed two-year interbirth intervals and 55% of cubs in their second year were independent of their mother. Mean size of cub litters in spring was 1.9 and 13% of litters had three or more cubs. The natality rate for 5–20-year-old females was estimated as 0.9, higher than that reported for any more northerly polar bear populations where two-year interbirth intervals are rare, fewer than 5% of yearling cubs are weaned and triplet litters occur with less than 1% frequency. Cub mortality was initially high and declined with age. Although cubs in western Hudson Bay were weaned at a younger age and a lighter weight than their counterparts in more northern populations, cub mortality rates were similar. The reason for the marked differences in reproductive parameters in the western Hudson Bay population is not known. We speculate that sea-ice conditions may be sufficiently different to allow weaned bears at a lighter body weight to hunt seals more successfully there than further north.