Polar bear counts for W Hudson Bay: ‘core’ area numbers are not comparable

Polar bear population estimates for Western Hudson Bay have recently become contentious because one specialist has been making statements that confuse the issue. As we all wait for the release of the report on the WHB aerial survey of 2016, it’s worth going over the recent history of these counts and what they have revealed.

Churchill_Polar_Bear_2004-11-15 Wikipedia

The official count for bears in WHB (used by the IUCN Polar Bear Specialist Group, the IUCN Red List, and Environment Canada) is 1030, based on an aerial survey of the entire region conducted in 2011 (Stapleton et al. 2014).

Since last year, Andrew Derocher (University of Alberta) has been telling any media pundit who will listen that WHB polar bears have declined from about 1200 bears in the 1980s to only 800 or so bears today (one example here) — a statement that is clearly not true.  In recent months, however, whether due to complaints from the public or from his colleagues, he’s qualified that statement by saying it’s the number of bears in the “core” area of WHB that has declined.

But is Derocher’s revised statement a clear scientific interpretation of the facts? Have a look at the details below and see if you come to the same decision I have: that it’s not possible to compare WHB ‘core’ area polar bear population estimates over time.

Lunn et al 2016 Fig 1

This study area map is Figure 1 from Lunn et al. 2016. The 2011 mark-recapture study (Lunn et al. 2013, 2014) did not include bears from Areas A or D, and only a few from Area B. The 2011 aerial survey (Stapleton et al. 2014) covered Areas A-D, and found many more bears in area D than expected in the Lunn et al. study. This is why the estimate of 1030 from the aerial survey is considered the most accurate assessment.

WH estimate in 1987 was 1,194 (1,020-1,368) based on standard capture-recapture data from Area C only (the “core” comprising Wapusk National Park thought by researchers to represent the entire region).

WH estimate in 2004 was 935 (794–1,076), again based on capture-recapture data from the core area only (Regehr et al. 2007).

WH estimate in 2011 was 806 (653–984), also based on capture-recapture data from the core area only (Lunn et al. 2016).

WH aerial survey in 2011 examined the entire region (A-D) and generated an estimate of 1,030 (754-1406). This is currently the official estimate for Western Hudson Bay.

Note that Area D (south of Wapusk National Park to the Ontario border) was found by the aerial survey to contain many more bears in 2011 than were assumed to exist in that region by the mark-recapture analysis done the same year (Stapleton et al. 2014) — and no one knows if that was true for previous mark-recapture surveys or not.

Wapusk Nat Park_Hudson Bay_Google maps_w Churchill

The 2011 aerial survey estimate of 1030 was considered consistent with the 2004 estimate (i.e., 935) because of conflicts with the methodology and because the error ranges overlap. The subpopulation in 2015 was considered by the PBSG to be STABLE (see screencap of the lower portion of their status table taken 24 Jan 2015 below):

2015 Jan 24 PBSG sea ice metrics example2 bottom snip

To claim that core population estimates based on capture-recapture surveys can be compared between 1987 and 2011, as Derocher continues to do (“Core of population used to be 1200 bears is now ca. 800“), is scientifically unsupported.

The core population estimate for 2011 cannot be compared to previous ones because we know it did not represent the entire population. It is possible that population numbers dropped a bit across the entire region between 2004 and 2011 (the data is inconclusive) but not defensible to imply that numbers in the core area declined by 33% due to recent (2012-2017) sea ice changes (see below).

We know three important things that counter Derocher’s comparison of “core” estimates:

  • the 2011 capture-recapture estimate of ~800 missed “a large number of bears” outside the core at the time of the fall count
  • the core estimate, by definition, was always meant to be a count of all bears in the entire WHB region, not just Area C (Churchill plus Wapusk Nat. Park) because researchers assumed all bears either came ashore here or migrated to this area before freeze-up in early fall
  • a population decline of 22% was documented between 1987 and 2004 in the core area, using similar methods, indicating that the only documented decline occurred before 2005

One recent study showed that at least some WHB bears (12%) changed the location where they came ashore in summer depending on prevailing ice conditions that year (Cherry et al. 2013), which may account for all or most of the bears found to the south of the core area in 2011, which was an early breakup year. According to data collected by Castro de la Guardia and colleagues (2017), copied below, breakup in 2011 occurred at June 17 which was the earliest breakup date for Western Hudson Bay ice since 2003.

As mentioned above, it is not know if a similar pattern of distribution outside the core area C occurred in previous early breakup years because no one checked — they just assumed all WHB bears enter core area C during the early fall every year, regardless of the pattern or timing of ice melt, and were therefore available to be counted.

castro-de-la-guardia-et-al-derocher-2017-fig-3-no-caption

Figure 3 from Castro de la Guardia et al. (2017) showing freeze-up and breakup dates and ice-free days 1979-2015 for Western Hudson Bay, where breakup for 2011 (middle panel) was the earliest since 2003, approximately Day 168 (June 17).

References

Castro de la Guardia, L., Myers, P.G., Derocher, A.E., Lunn, N.J., Terwisscha van Scheltinga, A.D. 2017. Sea ice cycle in western Hudson Bay, Canada, from a polar bear perspective. Marine Ecology Progress Series 564: 225–233. http://www.int-res.com/abstracts/meps/v564/p225-233/

Cherry, S.G., Derocher, A.E., Thiemann, G.W., Lunn, N.J. 2013. Migration phenology and seasonal fidelity of an Arctic marine predator in relation to sea ice dynamics. Journal of Animal Ecology 82:912-921. http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12050/abstract

Lunn, N.J., Servanty, S., Regehr, E.V., Converse, S.J., Richardson, E. and Stirling, I. 2016. Demography of an apex predator at the edge of its range – impacts of changing sea ice on polar bears in Hudson Bay. Ecological Applications, in press. DOI: 10.1890/15-1256

Regehr, E.V., Lunn, N.J., Amstrup, S.C., and Stirling, I. 2007. Effects of earlier sea ice breakup on survival and population size of polar bears in Western Hudson Bay. Journal of Wildlife Management71: 2673-2683. http://onlinelibrary.wiley.com/doi/10.2193/2006-180/abstract

Stapleton S., Atkinson, S., Hedman, D., and Garshelis, D. 2014. Revisiting Western Hudson Bay: using aerial surveys to update polar bear abundance in a sentinel population. Biological Conservation 170:38-47. http://www.sciencedirect.com/science/article/pii/S0006320713004618#

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