Indicator Assessment Reports

Arctic Borderlands Ecological Knowledge Co-op

Background

Is it possible to use fat levels of polar bears as an indicator of body condition that might be sensitive to climate change? Stirling and Derocher (1993) have argued that the impacts of climate warming may appear first in polar bear populations near their southern limit in Hudson Bay. Factors that affect the timing or extent of ice cover may have important impacts of polar bears and many of these impacts may translate into a change in body condition of polar bears, especially females. Stirling and Lunn (in press) recently examined the possible links between food supply (ringed seals), female reproduction and body condition.

In the Hudson Bay area, Stirling and Lunn found that an index of body condition (based on weight and length of individuals) differed between males and females. Declines in female body condition and cub survival seemed to be associated while male body condition during the same period did not decrease. There are two potential difficulties with collecting information on polar bear body condition in the Beaufort Sea area:

• Only a fraction of the annual harvest of polar bears are females and information on the body condition of males may not be that useful based on results from Hudson Bay.
• The measurements to determine body condition may be difficult to carry out in a consistent manner if there are many different observers. This would weaken the possible conclusions from such an indicator.

New Suggestion: Ringed Seal Productivity

Stirling and Lunn (in press) reported that the availability of ringed seal pups (from April to July) was a critical factor influencing polar bear reproductive success. They examined the fraction of young ringed seal pups that were taken by hunters from Sachs Harbour and Holman during the open water harvest and were able to interpret these changes with corresponding changes in female polar bear reproduction. They suggested that changes in ringed seal productivity (i.e. the fraction of the harvest made up of pups) was tracked by changes in polar bear reproduction.

There is existing data available on ringed seal productivity from harvests taken at Sachs Harbour (1974-1979, 1985-1989 and 1992-1994), Holman (1971-1972, 1975-1981 and 1992-1994), Minto Inlet (1992-1994) and Paulatuk (1992-1994). Supporting this type of work may be a useful way of tracking changes in the food chain that have important links to top predators like polar bears.

References

Stirling, I. and Derocher, A.E. 1993. Possible impacts of climate warming on polar bears. Arctic 46:240-245.

Stirling, I. and Lunn, N.J. (in press). Environmental fluctuations in Arctic marine ecosystems as reflected by variability in reproduction of polar bears and ringed seals. In: Woodin, S.J. and Marquiss, M. (eds.) Ecology of Arctic Environments, Blackwell Science Ltd., p. 167-181.

Written by S. Gilbert, March 1997.