Long-term trends in the population ecology of polar bears in western Hudson Bay in relation to climatic change

From 1981 through 1998, the condition of adult male and female polar bears has declined significantly in western Hudson Bay, as have natality and the proportion of yearling cubs caught during the open water period that were i ndependent at the time of capture. Over this same period, the breakup of th e sea ice on western Hudson Bay has been occurring earlier. There was a sig nificant positive relationship between the rime of breakup and the conditio n of adult females (i.e., the earlier the breakup, the poorer the condition of the bears). The trend toward earlier breakup was also correlated with r ising spring air temperatures over the study area from 1950 to 1990. We sug gest that the proximate cause of the decline in physical and reproductive p arameters of polar bears in western Hudson Bay over the last 19 years has b een a trend toward earlier breakup, which has caused the bears to come asho re in progressively poorer condition. The ultimate factor responsible for t he earlier breakup in western Hudson Bay appears to be a long-term warming trend in April-June atmospheric temperatures.