Post by grrraaahhh on Mar 26, 2010 19:25:32 GMT -9
Predation of Belugas and Narwhals by Polar Bears in Nearshore Areas of the Canadian High Arctic
On 18 August 1988 we found four narwhals and two dead belugas stranded on a low beach at Creswell Bay, Somerset Island. All of the narwhals and two of the belugas had been attacked and partially eaten by polar bears. At Cunningham Inlet, where belugas concentrate in large numbers, we have noted ten strandings over the period 1980-88, without bear predation on these occasions. One bear, hunting from an ice floe in deep water at Cunningham Inlet, killed two sub-adult belugas in July 1985. Belugas seem to exhibit curiosity towards swimming polar bears that might serve to drive bears out of the area and reduce the risk of predation. The potential large summer food resource for bears represented by odontocete whales in the High Arctic Archipelago seems to be underutilized. The timing and location of beluga concentrations are known and dates of probable strandings are somewhat predictable, which might allow us to assess the extent of bear predation on whales in the future.
During May 1970, while conducting field work at Grise Fiord in the Canadian Arctic Archipelago, a local hunter reported that a polar bear (Ursus maritimus) had successfully caught 3 beluga (Delphinapterus leucas) during March near King Edward VII Point (76°08'N., 81°08' W.), the extreme southeast cape of Ellesmere Island, Northwest Territories. As none of the fifteen local hunters had ever witnessed such an event, and only one had ever heard of it before, I assumed bear predation on whales to be very rare, and consequently recorded whatever information I could obtain at the time.
According to the hunter's narrative, movement of a partially grounded iceberg about 200 meters offshore had prevented freezing of a small area of water surrounding the berg. Winter trapment of whales is known to occur during unusual conditions of sea-ice formation1, and as the open sea was at least 30 kilometers distant from this locality in March it seems probable that a small number of beluga had endeavored to pass the winter in the open water alongside this berg. At some time in March a medium-sized female bear had caught and removed an adult female beluga together with another adult and a grey-colored sub-adult beluga both of unspecified sex; the adult female beluga was dragged about 7 meters from the edge of the water, the other two a shorter distance only.
Inspection of the carcass indicated loss of all skin and fat, and most of the meat from head and trunk; fracture of the occipital bones had occurred, but it is not known if this damage was suffered before or after death. An eyewitness account of a polar bear killing beluga in Novaya Zemblya however, relates how the bear lies with outstretched paws on the ice and delivers a blow to the head when the whale surfaces within range.
In this region of the Canadian Arctic Archipelago, beluga generally change color from grey to white at around 375 cm. in length. 3 Assuming the two white-colored beluga were around 400 cm. in length, their weight is calculated to be about 935 kilograms4; the grey-colored sub-adult measured 275 cm. and had a computed weight of 350 kilograms.
There appears no reason to doubt that the hunter reporting this event had, as he believed, discovered the beluga shortly after they were caught in March, nor that the tracks of the medium-sized female bear near the carcasses at that time were those of the predator. According to the description given, such a bear would weigh in the range of 130 to 180 kilograms, or about one-fifth the probable weight of each adult beluga it had successfully killed and removed from the water.
Source: Freeman, M. Polar Bear Predation on Beluga in the Canadian Arctic, ARCTIC, Vol 26, No 2 (1973), p.163-163 (PDF LINK)
"The high density of Belugas migrating in the spring along restricted openings in sea ice provides opportunities for predation by Polar Bears. Although there are not many documented sightings of Polar Bear predation on Belugas from Alaskan coastal areas (seven were reported by Lowry et al. (1987) since 1967, four of which occurred in April 1984 during a period of dense ice cover), the rate is high considering the low likelihood that anyone would come across a bear kill on the sea ice and report such a sighting to a scientist. Furthermore, moving ice and snow accumulation tend to cover evidence of kills. There have been several reports of Polar Bears eating Belugas in the Canadian High Arctic, including Freeman (1973), Heyland and Hay (1976), Mitchell and Reeves (1981), and Smith (1985), but the only documented observations of Polar Bears killing Belugas were made by Degerbol and Freuchen (1935), Kleinenberg et al. (1964), and Smith and Sjare (1990), along with an observation made by Harry Brower Sr.', a principal whaler in Barrow. However, the sighting reported by Lowry et al. (1987) and the original sightings of dead Belugas reported here included blood tracks on the ice, indicating relatively fresh kills. Because of their negative buoyancy. Belugas tend to sink when killed (Kemper 1980; Finley et al. 1982; Lowry 1985). This reduces opportunities to scavenge a dead Beluga until internal gasses expand, the carcass floats to the surface, and it becomes available for scavenging by Polar Bears. Since most sightings of dead Belugas on ice occur when sea ice is dense and whales are forced to use small openings in the ice, it appears that the whales are being killed by the bears and not scavenged after having succumbed due to other causes. Entrapped whales are probably vulnerable to predation, but pulling a whale onto the ice is an impressive feat because a Beluga may weigh five times as much as a Polar Bear (Freeman 1973)."
Rugh, D.J., Shelden, K.E.W., 1993. Polar bears, Ursus maritimus, feeding on Beluga Whales, Delphinapterus leucas. Canadian Field-Naturalist 107, 235–237.
Great article there. There are plenty of articles on google. Unfortunately I am unable to copy and paste anthying on pdf.
Here is one with other related literature:
The authors found 4 narwhals Monodon monoceros and 2 belugas Delphinapterus leucas stranded on a low beach at Creswell Bay, Somerset Island. All of the narwhals and 2 of the belugas had been attacked and partially eaten by polar bears Ursus maritimus. Belugas seem to exhibit curiosity towards swimming polar bears that might normally drive bears out of the area and reduce the risk of predation. The potential large summer food resource for bears represented by odontocete whales in the High Arctic Archipelago seems to be underutilized.
Surveys of belugas and narwhals in the Canadian High Arctic in 1996
Stuart Innes, M P Heide-Jørgensen, J L Laake, K L Laidre, P Richard, P Richard in NAAMCO Scientific Publications (2002)
Distribution and Abundance of Belugas, Delphinapterus leucas , and Narwhals, Monodon monoceros , in the Canadian High Arctic
T G Smith, M O Hammsll, D J Burrage, G A Sleno in Noûs (1985)
Interactions between polar bears and overwintering walruses in the central Canadian High Arctic
W Calvert, I Stirling in International Conference on Bear Research and Management (1990)
Summer and autumn movements and habitat use by belugas in the Canadian high Arctic and adjacent areas
P R Richard, M P Heide–Jørgensen, J R Orr, R Dietz, T G Smith in Arctic (2001)
Denning ecology of polar bears in the Canadian Arctic archipelago
F Messier, M K Taylor, M A Ramsay in Journal of Mammalogy (1994)
By Candace Savage with photography by Malcolm Ramsay
Two polar bears belly up to feast at a breathing hole kept open by whales trapped under sea ice for about 60 days last spring in Lancaster Sound. By the time the ice broke up, the bears at this hole had killed at least eight belugas.
ABOUT TWENTY YEARS AGO, as a promising young biologist, Malcolm Ramsay deliberately chose to focus his attention on one of the simplest predator-prey systems on Earth. The High Arctic seemed to offer nature in its most elemental, stripped-down mode: one large predator and one main species of prey — the polar bear and the ringed seal. But when he thinks back on that decision now, Ramsay can’t help shaking his head. For that apparently "simple" system has turned out to be full of quirky surprises.
He was reminded of this fact again last May, on what was supposed to have been a routine research trip over Lancaster Sound, off the north coast of Baffin Island. The plan was straight-forward. With the help of a helicopter pilot and colleague Susan Polischuk, Ramsay would fly out over the sea ice, as he had done hundreds of times before, to look for polar bear tracks. Since the animals regularly come this way to hunt for seals, this would not be as difficult as one might suppose: the prints stand out, crisp and clear, in the polished snow. By locating one of these dotted lines and following it to its source, the researchers had an excellent chance of tracking down a bear, which could then be tranquilized with a dart shot from the air. Once the animal had been immobilized, the team would land and collect a set of measurements — weight, age, sex, reproductive status, and more — to add to their store of baseline data.
Lancaster Sound, Baffin Island (Nunavut, Canada).
At first, everything went as expected. After a couple hours of searching, they came across a braided skein of tracks and, half an hour later, the cry went up that a female and two yearlings had come into sight. But then something odd happened. As Ramsay was busy preparing the tranquilizers, Polischuk shouted out that she had seen another bear. "And another. And another. And look, over there!"
As her excited voice tallied more and more sightings, Ramsay listened in disbelief. Apart from mating season (which was over for the year), adult polar bears do not hunt or travel together on the sea ice. So why had all these individuals — more than 20 — suddenly chosen to gather here, on a stretch of frozen ocean?
The bears seemed to be converging on a chain of dark blotches, or ponds, that were dotted across the surface of Lancaster Sound. Within several of these patches, Ramsay could make out a tumult of vague, greyish forms thrashing and jostling about. Although he had never seen anything like it, Ramsay knew what he was looking at. He and his team had happened upon a "sassat" (pronounced sa-SAT).
Sassat is a Greenlandic word for a group of animals caught in the ice. In this case, the victims were whales — more than 40 belugas and at least one bowhead — which presumably had swum west from Baffin Bay during a warm spell in early spring and become entrapped when conditions changed. Now, they were keeping their breathing holes open as they surged to the surface for air. Until the ice broke up, they would be prisoners in this spot, unable to stray from their oxygen source.
Meanwhile, the trapped whales attracted the bears, which may have detected them through their super-keen sense of smell. The bears backed away when the helicopter set down near one of the ponds, but signs of their depredations were everywhere. Many of the whales in the water had been bitten and clawed, and eight dead belugas were sprawled along the margins of the ponds where, apparently, they had been dragged by the bears. (How does a quarter-tonne bear lift a tonne of dead weight? "Polar bears are very strong" is the best that Ramsay can say.[/b]) After stripping the blubber from the carcasses, the bears had ceded the rest to the thousands of glaucous and ivory gulls wheeling overhead.
Several months later, back in his office at the University of Saskatchewan in Saskatoon, Ramsay is still astounded by what he saw. "If we hadn’t been tracking those bears at that moment, we would have missed it," he says. The whale ponds were small (about five metres across) and it would have been the easiest thing in the world to have swept on past. "I know of few biologists who have ever seen a sassat."
Yet Ramsay believes that sassats and other similarly random events may be of critical significance to polar bears and other Arctic marine mammals. Sea ice is terra infirma, as restless as wind and waves, a realm of slow, relentless, capricious change. In this fluid medium, the geography of life and death can be radically redrawn within days or weeks. (The sassat, for example, was gone without a trace when the site was overflown again about a month later.) "If scientists aren’t there to witness these key events," Ramsay says, "we are missing what’s important."
As evidence, he points to his data from the sassat. The four bears he weighed and measured at the site were rippling with fat. At weights of about 300 kilograms for females and almost 500 for males, these animals were each about 150 kilograms above the average for late May. In fact, they were so gloriously obese that Ramsay predicts they may not have to eat again for another year.
Researcher Susan Polischuk is dwarfed by the intimidating bulk of a male polar bear, tranquilized after feeding at the sassat. He weighed in at 471 kilograms.
Impossible? For a mere human, yes. But, as Ramsay’s research has shown, polar bears are superbly adapted to life in the High Arctic, where food supplies are as vagrant as the sea ice itself. When resources are abundant, their bodies balloon with fat; when prey is scarce, they draw calories from this bank. Unlike most other mammals, which need a constant supply of protein, polar bears can go for months on virtually no food at all.
Ramsay and his colleagues have recently discovered that they do this by switching from a "normal" metabolism into a special "fasting" state, the same mode that other bears enter when they hibernate. In this condition, a polar bear often remains active, yet can go without food for four months, eight months, perhaps even a year, provided it has sufficient fat reserves. A bear that has gorged at a sassat will not have to prey on seals, which in turn will survive to eat more fish, and the perturbations will echo on through the food chain.
Viewed from this perspective, a sassat becomes much more than a spectacular, short-term event. "I believe that we have been privileged to catch a glimpse of something truly significant," Ramsay says, perhaps even one of the keys to understanding the unexpected intricacies of life in the High Arctic.
Imprisoned by the ice and gouged by bears, a pod of belugas (ABOVE) shares its breathing hole with a bowhead whale (BELLOW). Inuit hunters reported seeing 13 bears at one of the sassats.
Candace Savage is a Saskatoon-based writer and author of 18 books on wildlife, environmental issues and other subjects.
It has been shown only recently that belugas are subject to a seasonal skin moult. Belugas have a very thick skin that is at least 10 times thicker than that of dolphins and 100 times thicker than that of terrestrial mammals.
A beluga’s skin is 100 times thicker than that of land mammals, and is composed of a significant layer of fat that can reach up to 10 cm in thickness. Counting for over 40% of the whale’s body weight, the blubber functions as both an energy store and a natural insulator from the cold.