Scientists have for the first time found evidence that polar bears
are drowning because climate change is melting the Arctic ice shelf.
The researchers were startled to find bears having to swim up to 60 miles across
open sea to find food. They are being forced into the long voyages because the
ice floes from which they feed are melting, becoming smaller and drifting farther
Although polar bears are strong swimmers, they are adapted for swimming close
to the shore. Their sea journeys leave them them vulnerable to exhaustion, hypothermia
or being swamped by waves.
According to the new research, four bear carcases were found floating in one
month in a single patch of sea off the north coast of Alaska, where average
summer temperatures have increased by 2-3C degrees since 1950s.
The scientists believe such drownings are becoming widespread across the Arctic,
an inevitable consequence of the doubling in the past 20 years of the proportion
of polar bears having to swim in open seas.
“Mortalities due to offshore swimming may be a relatively important and
unaccounted source of natural mortality given the energetic demands placed on
individual bears engaged in long-distance swimming,” says the research
led by Dr Charles Monnett, marine ecologist at the American government’s
Minerals Management Service. “Drowning-related deaths of polar bears may
increase in the future if the observed trend of regression of pack ice continues.”
The research, presented to a conference on marine mammals in San Diego, California,
last week, comes amid evidence of a decline in numbers of the 22,000 polar bears
that live in about 20 sites across the Arctic circle.
In Hudson Bay, Canada, the site of the most southerly polar bears, a study
by the US Geological Survey (USGS) and the Canadian Wildlife Service to be published
next year will show the population fell 22% from 1,194 in 1987 to 935 last year.
New evidence from field researchers working for the World Wildlife Fund in
Yakutia, on the northeast coast of Russia, has also shown the region’s
first evidence of cannibalism among bears competing for food supplies.
Polar bears live on ice all year round and use it as a platform from which
to hunt food and rear their young. They hunt near the edge, where the ice is
thinnest, catching seals when they make holes in the ice to breath. They typically
eat one seal every four or five days and a single bear can consume 100lb of
blubber at one sitting.
As the ice pack retreats north in the summer between June and October, the
bears must travel between ice floes to continue hunting in areas such as the
shallow water of the continental shelf off the Alaskan coast — one of
the most food-rich areas in the Arctic.
However, last summer the ice cap receded about 200 miles further north than
the average of two decades ago, forcing the bears to undertake far longer voyages
“We know short swims up to 15 miles are no problem, and we know that
one or two may have swum up to 100 miles. But that is the extent of their ability,
and if they are trying to make such a long swim and they encounter rough seas
they could get into trouble,” said Steven Amstrup, a research wildlife
biologist with the USGS.
The new study, carried out in part of the Beaufort Sea, shows that between
1986 and 2005 just 4% of the bears spotted off the north coast of Alaska were
swimming in open waters. Not a single drowning had been documented in the area.
However, last September, when the ice cap had retreated a record 160 miles
north of Alaska, 51 bears were spotted, of which 20% were seen in the open sea,
swimming as far as 60 miles off shore.
The researchers returned to the vicinity a few days later after a fierce storm
and found four dead bears floating in the water. “We estimate that of
the order of 40 bears may have been swimming and that many of those probably
drowned as a result of rough seas caused by high winds,” said the report.
In their search for food, polar bears are also having to roam further south,
rummaging in the dustbins of Canadian homes. Sir Ranulph Fiennes, the explorer
who has been to the North Pole seven times, said he had noticed a deterioration
in the bears’ ice habitat since his first expedition in 1975.
“Each year there was more water than the time before,” he said.
“We used amphibious sledges for the first time in 1986.”
His last expedition was in 2002, when he fell through the ice and lost some
of his fingers to frostbite.