Seasonal and annual movement patterns of polar bears
on the sea ice of Hudson Bay
E.K. Parks, A.E. Derocher and N.J. Lunn
(rewiew written by: Attila Bolyki)
This article written by Parks, Derocher and Lunn discusses the anual and seasonal movements of polar bears over the ice of Hudson Bay. Usually most animals’ life is influenced by the resources mandatory for their survival. These resources are food and breeding habitat. Polar bears (Ursus maritimus Phipps, 1774) belong to a non territorial group of animals that feed on ringed seals and bearded seals. The researchers used sattelite and GPS positioning systems to be able to follow the animals and observe their movements and also to find out if there’s a certain pattern in their yearly movements. The usage of advanced technology allows us to study the distribution of polar bears even in remote areas and gives us a cleaner picture of the structure of society in which these animals live.
Circumpolar bears have been divided into 19 different groups based on the sattelite data. Animals in these groups had different movement patterns. Variations within the groups were due to the ice conditions of the area in which they lived and also to the fact that every animal have different energy demand. According to Parks, Derocher and Lunn: „females have greater engergetic demand when they are accompanied by dependent offspring then when they are not, and therefore may adopt different space-usage strategies” (Parks et. al. 2006). Seal distribution and sea ice also play an important part in the variaton between the groups and among the animals. Ice thickness and concentration have been declining since 1970 which is due to the climate change that forces the ecosystem of the Arctic to change dramatically and rapidly. This of course on the big scale affects the air temperature and ocean circulation which results in massive shifts of the ecosystem. These enermous changes that happen over the years affect the arctic wildlife such as geese in a way that their reproductive success declines anually.
According to Stirling and Derocher „western Hudson Bay polar bear population live close to the southern limit of polar bear distribution and therefore expected to be affected by a warming climate sooner than other populations” (Stirling and Derocher 1993.; Derocher et al. 2004.). Every summer, the ice over the Hudson Bay area melts and the polar bears are forced to move to land. Here they spend months without food until October – November when ice returns and covers the bay again. At this time most bear return to ice to hunt with the exception of pregnant females that give birth to the cubs between mid-November and mid-December. Afterwards they return to the ice too becasue between April and July the most critical hunting period follows in the polar bears’ life. They must acquire great amounts of fat from the seals to be able to survive the ice-free period when food is absolutely limited. A 2005 study by Gagon and Gouth states that over the past 30 years ice break up has been occuring earlier which may cause a decline in the condition of polar bears. These radical changes result in prey distribution change, ice structure change which at the end changes the patterns of polar bear movement.
Researchers in this study wanted to quantify the movement of certain animals. These animals had a sattelite collar and were observed according to their reproductive status, season and changes in ice conditions. They hypothesized that reproductive status, season and ice coverage would affect these animals’ movement patterns and their home ranges. As a test for the hypothesis, the researchers compared and contrasted movement trends among certain reproductive classes annually and seasonally.
For this research study the polar bear population of south
of Churchill, Manitoba, Nunavut and nothwestern Ontario was observed, captured
and used. Hudson Bay has a mean deapth
of
Between 1991 and
1998, using helicopter capture method 41 female polar bears were captured from
the western Hudson Bay area. These
animals received a collar (Argos®) that was later used locating the
animals. Maximum of 10 bears were
collared each year with the exception of 1994 and 1995 when only 3 bears were
captured and „marked”. Bears were
catergorized into 5 groups: females with cubs of the year, females with
yearlings, solitary females, adult males and subadult males or females. Males were not collared since their neck was
too large in diameter and the collares could not have been secured. The accuracy of these collars ranged from
less than
Between 1991 and 1998, 6083 locations were registered. 16% of these locations were eliminated since the measurement wasn’t precise enough. The frequency of locations varied but the median was approximatelly 117 hours (~ 5 days). Since a lot of collars got damaged over the years commonly less than six locations were recorded per day. The life span of the collars deployed in September 2004 varied between 68 and 309 days.
It turned out that the home range size of the western Hudson Bay polar bear is apporoximatelly an enermous 106614 km2. Females with cubs had the smallest and largest home range sizes. Home range size decreased between 1992 and 1998 by 55% anually which was in positive correlation with ice break up of the bay. Home range size significantly increased in 1992 due to the eruption of Mount Pinatubo. Home range sizes varied with season (smallest in summer) but was almost the same for the different reproductive classes. The most significant differences could be observed during freeze up. Females with yearlings had a larger home range size than females with cubs but solitary females had almost the same too.
Another interesting data that
observed by the researchers was net displacement. Females with cubs and yearlings had similar
results while solitary females had lower displacements. The displacement for all the classes was
influenced by season. Summer had the
lowest displacement while freeze up had even higher displacement than
winter. During freeze up the net
displacement was about
When it comes to directional patterns the researchers concluded that the only significant movement among bears was when they were heading back on to the ice. Bears were moving in the northeast direction mainly. During the winter this best could be observed in case of females with cobs while other groups of animals showed no sign of directionality.
The last characeristic that was considered by the researchers is the total distances traveled by the bears. They state that the annual distance was 2080 km. The smallest and larges values were 574 km, traveled by a solitary female, and 4935 km, traveled by a female that had a cub that year. In the 1991 to 1998 period the total distance coverd by the animals was constantly declining. The researchers concluded that season had a great affect on the distances covered by the bears. They have traveled the most during the winter while the least traveling was done during the summer.
This study, made by Parks, Derocher and Lunn was primarily conducted to examine the movements of polar bears in the Hudson Bay area and observe these animal’s life and activity during the different seasons. The researchers observed that the animals used the available space rather differently, depending mainly on season. It turned out that bears are the least active while on dry land which is during the summer. During freeze up bear movement greatly incresed as they were moving back on to the ice. Also a rather interesting result is that bears with cubs were moving more than bears without company. I think these are understandable since these animals have access to their food on ice which explains their low movement rate on land and also those with cubs have to catch prey not just for themselves but also for their cubs which resusults in higher movement rates by „mohter” bears. Not too suprising that the home range size of these animals was influenced by the availability of prey, which are bearded and ringed seals mainly. It is important to feed sufficiently during the hunting period since when they move to dry land due to the absence to seals animals may not eat for up to eight months. I think this and many more intersting facts make the polar bear one of the greatest preditors of all times.
Citation:
Parks, Derocher, Lunn: Seasonal and annual movement patterns of polar bears on the sea ice of Hudson Bay, Canadian Journal of Zoology 84(9), 1281-1294, 2006.