Polar bear cannibalism and sea ice, the spring of 1976

Remember Ian Stirling’s claim that late freeze up in Western Hudson Bay in 2009 was forcing polar bears to resort to cannibalism (here and here), with gut-wrenching images and video provided for the media? Or Steve Amstrup’s claim for a similar phenomenon in the Southern Beaufort in 2004?

I pointed out that Stirling’s claim was way overblown and that Amstrup’s incidents were almost certainly the result of heavy ice in the spring (not low ice in summer), similar to the heavy ice conditions and polar bear starvation documented in the same region back in 1974-1976.

It turns out that the heavy ice conditions documented in the Eastern Beaufort in the mid-1970s had much broader effects on polar bears and ringed seals than has been appreciated.

In the course of looking for something else (as has happened so many times before!), I found a report of cannibalism in 1976 associated with heavy spring ice – in the Chukchi Sea, near Barrow, Alaska. This region is well west of the Eastern Beaufort region where starving polar bears were reported from 1974 to 1976 (see the map below).

Chukchi Beaufort locations_PolarBearScience_sm

The report on this incident was made by biologist Jack Lentfer in his summary of polar bear research in Alaska for the IUCN Polar Bear Specialist Group’s 6th meeting (1976, pg. 187 and 192, “Polar bear management and research in Alaska 1974-76”). See the map above for locations of communities mentioned (click to enlarge):

“Both 1975 and 1976 were “heavy” ice years. Bears traveled south to the southern Chukchi Sea and northern Bering Sea with a movement of heavy ice south early in the winter of these years. Unusually high kills [of polar bears] occurred on St. Lawrence Island and near the villages of Wales and Shishmaref in areas where bears are seldom encountered.

Sea ice was heavier than usual and heavy ice extended further south than usual in 1975 and 1976. This may have caused a movement of ringed seals, the principal food of bears, to the west and south (Burns et al. 1976) and a similar movement of some bears. In both years significantly fewer seals were killed by bears in the Barrow, Alaska area than in years when ice had not been so heavy. Also, perhaps related to reduced availability of seals, the first instance of predation by an adult bear, a male, on cubs was noted.

Both cubs in a litter were nearly completely consumed on 13 April 1976.” [my bold and links]

Of course, “perhaps related to reduced availability of seals” is an important qualifier – it wasn’t necessarily the case. Maybe this male was hungry enough to eat the cubs (and not simply kill them) or maybe he was one of those rare rogue bears who will kill and eat anything they can catch. It’s impossible to say for sure without further details. But it is pertinent that the incident occurred in the spring (April 13), when prey for polar bears is usually plentiful.

The most important point of this story, in my opinion, is not the cannibalism but the ice conditions: the fact that the spring ice in eastern Alaska (eastern portion of the “Southern Beaufort”) were so heavy in 1975 and 1976 that many polar bears and ringed seals were noticed to have moved west and south (into the Chukchi and Bering Seas).

[Even then, apparently, the ice was still heavy enough around Barrow for the incident of cannibalism to be blamed on lack of seals.]

However, it sounds like conditions were better in the portion of the Chukchi to the south of Barrow in 1975/76, where there would have been only first year ice.

Seal biologist John Burns described the unusual nature of the movements of seals and bears in his quarterly report on research he was doing on the distribution and abundance of seals (Burns and Fay 1975). I’ve copied the first page below, in which he says (in part):

Survey results substantiated that a major, short-term shift in density had indeed occurred. Density of seals in the Beaufort Sea was down 10 fold with a corresponding increase in the Chukchi Sea of between 10 and 15 fold.

 …In my opinion, the overriding factor affecting ringed seal distribution is the distribution of favorable sea ice conditions. From past experience it was obvious the prevailing sea ice conditions in the Beaufort Sea were, by and large, unfavorable for breeding ringed seals whereas they were excellent in the Chukchi Sea.

 …The distribution of ringed seals directly affected the distribution of their most significant predator, the polar bear.” [my bold, full text below]

Burns 1975 report on Chukchi ice

As Burns points out, the ice conditions of 1974-1976 in Alaska and western Canada were unusual. In their 2008 paper on Eastern Beaufort polar bears, Ian Stirling and colleagues said that prior to 2003-2006 (when there was heavy spring ice and some very thin bears), they had not seen similar development of shorefast pressure ridges since 1974 — even though they had worked there from 1971 to 1979 and from 1985 to 1987.

This suggests that conditions in the eastern Beaufort were worse in the spring of 1974 (with almost as bad conditions the following two years, see previous post here), but that in the Chukchi Sea, the worst effects were evident in 1975 and 1976.

Stirling says ice conditions were as bad in 2003-2006 as 1974, but were they really? Was there a 10-15 fold increase in Chukchi Sea ringed seals and mass movements of polar bears in the mid-2000s? If so, I’d like to see the reports.

Is it possible that we’ll see deadly spring conditions (as severe as ’74-’76) within the next 20 years in Alaska? I’d say the odds are much better for a bout of heavy ice springs than for polar bears being extirpated from Western Hudson Bay by 2050, as Steve Amstrup and colleagues have prophesied.

References
Burns, J. J. and Fay, F. 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]

Lentfer 1976. Polar bear management and research in Alaska 1974-76. Pg. 187-197 in [Anonymous]. Polar Bears: Proceedings of the 6th meeting of the Polar Bear Specialists Group IUCN/SSC, 7 December, 1976, Morges, Switzerland. Gland, Switzerland and Cambridge UK, IUCN. http://pbsg.npolar.no/en/meetings/

Stirling, I., Richardson, E., Thiemann, G.W. and Derocher, A.E. 2008. Unusual predation attempts of polar bears on ringed seals in the southern Beaufort Sea: possible significance of changing spring ice conditions. Arctic 61:14-22. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/3/3

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