I know I was not the only one that thought the last polar bear count conducted by the US Geological Survey (USGS) in the Southern Beaufort generated an untenable result but it appears that some of those challengers are in a position to demand a recount.
CBC News ran a story late last week announcing a plan is in the works to do an aerial survey of Southern Beaufort Sea polar bears across their entire range over two years starting in 2017 (11 March 2016, “Joint U.S.-Canada Beaufort Sea polar bear survey planned“).
With luck, this period will not include another bout of the same thick spring ice conditions that decimated the population in the mid-2000s (which many reporters concluded was due to reduced summer sea ice because that’s what leading biologists implied). Even if it does, lucky timing will not negate the fact that this population is routinely subjected to devastating population declines caused by natural changes to spring sea ice conditions from which they have recovered on numerous occasions.
Figure 1. This is Fig. 1 from Bromaghin et al. 2015, showing the study area and implying equal representation of CA and US bears in their count. Note about half of the Southern Beaufort Sea subpopulation is in the USA and the other half is in Canada. Canada has recently moved the eastern boundary to ~ Tuktoyaktuk.
Relevant bit from the CBC piece:
“Counts done annually on the U.S. side indicate the population has dropped by 40 per cent over the last 10 years, but those surveys are done on only a small part of the bears’ range and their results conflict with what hunters in both Alaska and the Northwest Territories have been seeing.
“Numbers they’re coming out with through their modelling don’t match what the hunters are observing when they’re out there on the ice, so we’d like to resolve that difference of opinion,” said Steven Baryluk with the Inuvialuit Game Council.
“We felt that undertaking a survey that’s co-ordinated across the range of the southern Beaufort bears would be a good way to do that.”
Consultations on the survey have been held in two Inuvialuit communities. Baryluk said they will be held in the remaining four over the next few weeks.
The last time a co-ordinated U.S.- Canada survey was done on Beaufort Sea polar bears was in 2004-2005.” [my bold]
But it’s not only hunters that are seeing more bears. As I’ve pointed out before, although the USGS population assessment ended in 2010, preliminary results from an aerial survey done by US Fish & Wildlife Service in the fall of 2012 (Polar Bear News 2013-2014) said numbers were the highest they’d been since 2002:
“The number of polar bears observed in 2012 was high relative to similar surveys conducted over the past decade. Body condition appeared relatively normal for this time of year with most bears reported to be in average body condition.”
Newsletter distributed by the USFWS, (pdf Polar Bear News 2013-2014 in which the preliminary report on the 2012 fall aerial survey appeared (pg. 17, lower right).
Recall that counting polar bears in the western Arctic is complicated by the fact that the bears routinely cross both the Canadian/US boundary in the east as well as the US/Russian boundary in the west – and that this movement is not only well known (Amstrup et al. 2005, see below) but it’s known to be more pronounced when thick spring ice conditions prevail (Burns et al. 1975).
This is Fig. 3 from Amstrup et al. (2005), showing the overlap of collared western North American polar bears – the Southern Beaufort, Chukchi Sea and Northern Beaufort subpopulations. When heavy spring ice conditions periodically develop in the Eastern Beaufort, many bears in that area probably move west into the Chukchi Sea, as ringed seals and bears did during the winters of 1974 and 1975 (Burns 1975).
The last SBS polar bear count
Here are my notes regarding the Bromighan et al. 2015 study (the 2001-2010 population estimate referred to in the above CBC report, which updated the previous 2004-2006 count conducted by Regher et al. 2006), which I covered in this post: S Beaufort polar bears largely recovered from known 2004-2006 decline, says new study (November 18, 2014).
Period of the study: 2001-2010 (2001-2005 used previously in 2008 ESA assessment).
Season of capture: Approximately late March to early May each year (April/May in Canada). Apparently, the condition of bears in summer and fall was not considered.
Method of capture: Bears were chased with helicopters, sedated, tattooed, ear tags attached; some adult females fitted with satellite radio collars in all years except 2010.
Sea ice relationships: They used only “summer-habitat” (area of optimal polar bear habitat available for July through October) and “Melt-season” (the time between melt and refreeze in the Beaufort each summer).
Areas of capture: US half (called “USGS”), 2001-2010 (late March-May); Canadian half (called “USCA”), 2003-2006 (April and May) and 2007-2010 but not to the eastern boundary (with subadults and females preferentially targeted).
Therefore, the sampling effort was uneven: there were no captures in the Canadian half of SBS from 2001 and 2002 and captures were biased towards females and subadults in 2007 through 2010. But it appears to be the Canadian (“USCA”) population estimate for 2010 that is being accepted as representative. Go figure.
Known movement of bears between SBS, Chuckchi Sea and NBS: This is called “heterogeneity in recapture probabilities”; the authors claim they accounted for this phenomenon (which the Polar Bear Specialist Group identified as a serious problematic issue for this region, and which was not taken into account as it should have been in the last population estimate, discussed previously here) in their abundance estimates. They said:
“In summary, although we are aware of the potential influence of temporary emigration and non-random movement, we believe any bias from these sources is likely to be small compared to the magnitude of temporal variation and trends in survival and abundance estimates.”
[See also Guest post: How ‘science’ counts bears July 3, 2013]
References
Amstrup, S.C., Durner, G. M., Stirling, I. and McDonald, T. L. 2005. Allocating harvests among polar bear stocks in the Beaufort Sea. Arctic 58:247-259. http://arctic.journalhosting.ucalgary.ca/arctic/index.php/arctic/article/view/426 Pdf here.
Bromaghin, J.F., McDonald, T.L., Stirling, I., Derocher, A.E., Richardson, E.S., Rehehr, E.V., Douglas, D.C., Durner, G.M., Atwood, T. and Amstrup, S.C. 2015. Polar bear population dynamics in the southern Beaufort Sea during a period of sea ice decline. Ecological Applications http://onlinelibrary.wiley.com/doi/10.1890/14-1129.1/abstract [paywalled]
Burns, J. J., Fay, F. H., and Shapiro, L.H. 1975. The relationships of marine mammal distributions, densities, and activities to sea ice conditions (Quarterly report for quarter ending September 30, 1975, projects #248 and 249), pp. 77-78 in Environmental Assessment of the Alaskan Continental Shelf, Principal Investiagors’ Reports. July-September 1975, Volume 1. NOAA, Environmental Research Laboratories, Boulder Colorado. [available online] pdf here.
Polar Bear News 2013-14. 2013. Polar bear newsletter of the US Fish & Wildlife Service, Anchorage, Alaska. Pdf here.
Regehr, E.V., Amstrup, S.C., and Stirling, I. 2006. Polar bear population status in the Southern Beaufort Sea. US Geological Survey Open-File Report 2006-1337. Pdf here.
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