Polar Bears at the Top of POPs

Wild prey provide predators with plenty of protein and energy, but they also pass along persistent organic pollutants (POPs), such as polychlorinated biphenyls (PCBs) and chlorinated pesticides, that bioaccumulate in their fatty tissue and degrade very slowly. According to scientists, these concentrations multiply five to ten fold with every step in the food chain, putting top feeders at the highest risk of health effects.

The polar bear's almost exclusive diet of fatty ringed seals makes it one of the most highly contaminated of all Arctic mammals. Environment Canada scientists and their colleagues in other polar nations have been monitoring bear tissues for more than two decades to determine geographical trends in the type and concentration of POPs. This information is useful in determining the sources of these pollutants, the effectiveness of global controls, and the potential for human exposure through the ingestion of wild foods.

PCBs and other toxins can mimic, block or disrupt the function of naturally occurring hormones in both humans and wildlife, and have been implicated as a causative factor in cancer, embryonic malformation, sterility, growth retardation, immunologic dysfunction, and reproductive abnormalities. Relatively high levels of PCBs and compounds related to the pesticide chlordane have been found in polar bears—with concentrations in the order of 10 parts per million (ppm) in the Canadian Arctic and up to 100 ppm in the European Arctic, likely due to pollution from sources in Europe, Asia and North America.

There is increasing evidence that PCBs and other organochlorines may affect the immune systems of wildlife, even at very low concentrations. Studies of harbour seals that were fed PCB-contaminated fish showed that immune suppression occurred at a much lower level than other effects—and at levels similar to those found in polar bears in some areas of the Arctic. In 1998, a team of Norwegian scientists that immunized 35 bears and measured contaminant concentrations in their blood found that bears with higher levels of PCBs had lower levels of a group of proteins that function as antibodies. The study was repeated in 1999 with Canadian polar bears, and the data are currently being analyzed.

Organochlorine levels have not changed much since the beginning of accurate polar bear population assessments in the early 1970s, so reproductive effects are difficult to determine. PCBs, DDT or their methylsulfone metabolites are suspected of causing reproductive failure in grey and ringed seals in the Baltic Sea, but at much higher concentrations than in the Arctic. The fact that POPs relocate to target organs when fat reserves are used makes it likely that female bears, fetuses and cubs are at the highest risk from toxic effects because the females fast for up to seven months of the year. Although this theory remains to be proven, a recent study showed that females who had lost cubs sometime during the first year after birth had higher levels of contaminants than those that had kept them.

During the first two years of their lives, cubs have PCB concentrations about twice that of their mothers, largely because contaminants accumulate in the milk they feed on. The breast milk of Inuit women from northern Quebec who eat marine mammals has also been found to contain PCB concentrations 2 to10 arial higher than that of women in the southern part of the province.

Last year, Environment Canada and researchers at the University of British Columbia carried out tests to determine whether testosterone metabolism in male bears is affected by increased concentrations of enzymes in the animals' livers induced by high levels of PCBs. The Department is also working with Carleton University in Ottawa to test the hypotheses that hydroxy PCB metabolites found in polar bear blood may be interfering with the transport of Vitamin A and a thyroid hormone in blood.

By improving our understanding of the effects of persistent toxic pollutants on human and wildlife health—and determining geographic and temporal trends in contamination levels—the results of these and other studies will help policy makers identify priorities and effective strategies for controlling airborne toxics.

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