| Polar
Bears of Churchill - Population Changes
Polar bears are coming ashore
earlier in western Hudson Bay. The ice breaks up around June 25th
now as opposed to July 15th.
However, polar bears actually
come ashore about three weeks after ‘break-up’. Break-up
occurs in early July while most bears do not come ashore until
the end of July, sometimes even August. Conversely, they go out
on the ice about one or two weeks earlier than what is considered
freeze-up (usually leaving around November 14 whereas freezeup
is considered November 21).
Realistically, this means
they are on land for about 15 weeks. During this time, they mostly
remain in a state of ‘walking hibernation’. Their
system slows down and consumes less energy. However, this does
not mean that they are fasting. They eat lyme grass, berries,
peat, kelp and whatever they can find. Researchers at the La Perouse
Bay Snow Goose Camp have watched young bears and mothers with
cubs routinely hunt snow geese, basically chasing them up against
willow bushes, stomping on a few of them and then sitting down
for a nice meal of goose guts.
Of course, none of these meals
compare to a good and fat seal pup and, therefore, an early break-up
remains critical to survival and to continued cub production.
Scientists believe that an early break-up of even one week will
translate to a 3-8% reduction in the annual survival rate, primarily
affecting old bears and lone subadults.
Researchers believe that warming
springs are primarily due to the effects of global climate change.
Massive hydroelectric projects
have change the runoff patterns and freshwater content of Hudson
Bay. While summer runoff has not significantly increased (6% more),
winter runoff into Hudson Bay has changed drastically. There is
now over 50% more runoff between November and April than prior
to hydroelectric development. That translates to about an extra
inch of freshwater covering the entire bay. To me, that seems
like a lot of water!
There has been some suggestion
that this could also be affecting break-up. However, little research
has been done in this area and its effects remain unclear.
While break-up is critical
and changing, freeze-up is much less so and not changing. Contrary
to several media reports, there has never been a study showing
a significant change in freeze-up of Hudson Bay. The biggest problem
with a late freeze-up is an increase in polar bear activity near
the community of Churchill. Its colder, bears are more active
and getting bored.
It is important to get back
on the ice but life is still tough out there. Hunting is sparse
through the winter months and polar bears still spend much of
their time in a state of walking hibernation or even hunkered
down, waiting out severe weather. Polar bears in western Hudson
Bay do not reach their lowest weights until March, so when you
think about it, the polar bears we see in ‘bear season’
are really still in modest condition.
Bears do not do the majority of their hunting until spring, pretty
much starting in April when the ringed seal pup birthing season
starts. Seal pups are about 50% fat and an easy meal for polar
bears. In fact, about 80% of a bear’s diet consists of ringed
seals under one year of age.
Despite a long-term decline
in adult body-condition, first year cub survival actually seems
to be increasing in the area. From a roughly 65% survival rate
25 years ago, it fell to 50% in the late 80s and climbed up to
75% in the late 90s. It is possible that this is a result of warming
spring temperatures. Young cubs do not have the insulative qualities
needed to survive harsh temperatures. Warm springs should help
them but, of course, only in the short-term.
This increase in cub survival
coincided with a change in cub production for the population as
a whole. Twenty five years ago, females used to wean their cubs
after 16 months in Hudson Bay, lately, they seem to have returned
to the 2.5 year cycle similar to other polar bear populations.
While the birth rate has decreased, it is still higher than most
other polar bear populations, however the recent population study
indicates that it is not keeping up with current mortality rates.
Only
time will tell whether this is another cycle in a larger pattern
or the first stages of a population collapse triggered by global
climate change and hydro-electric development.
-
prepared by Kelsey Eliasson
Click
here to view the prime source for this article: Population Ecology
of Polar Bears of Western Hudson Bay by Stirling, Lunn and Iacozza
- Appearing in the September, 1999 issue of the scientific journal,
Arctic
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