The polar bear is a predator at the peak of the marine food chain in the Arctic. It mainly eats seals, such as the ringed seal, which it hunts on the ice. Polar bears are extremely dependent upon sea ice for hunting and living on, and will be affected by changes in the ice. As a top predator in the marine food web, it is exposed to high levels of contaminants, especially the persistent organic pollutants. These are slowly degradable contaminants which are stored in fat and increase in concentration up the food chain.
Like the ringed seal, the polar bear is exposed to both pollution and decreasing sea ice. Effects from persistent organic pollutants have been demonstrated on the hormone, vitamin, enzyme, and immune systems of polar bears. This is a stress factor which may pose a threat to the population in Svalbard. In addition, it has been observed that higher mortalities of cubs coincide with higher contaminant loads in Svalbard and Franz Josef Land compared with Russia, Alaska and Greenland. This indicates that the reproductive ability of the polar bear may also be weakened by contaminants. Ringed seals form an important food item for polar bears, and they in turn feed on such creatures as crustaceans and polar cod. The content of contaminants in ringed seals was investigated in 1996 and 2004, and PCB levels fell significantly in this period.
Effects from persistent organic pollutants have been demonstrated on the hormone, vitamin, enzyme, and immune systems of polar bears. Lower concentrations of pollutants may mean that these effects will decline in the years to come. Since the fitness of polar bears and their food supply can vary considerably in the course of a year, there will be periods when bears will be more prone to the effect of the fat-soluble pollutants.
In addition, it has been observed that higher mortalities of cubs coincide with higher contaminant loads in Svalbard and Franz Josef Land compared with Russia, Alaska and Greenland. This indicates that the reproductive ability of the polar bear may also be weakened by contaminants.
The sea ice is the most important polar bear hunting ground. The primary threat to the polar bear will therefore be global warming and melting sea ice. If these lead to food becoming less readily available, the concentration of contaminants may rise because the polar bear must turn to its body fat and burn that, but it is not clear how the contaminants will affect the bear in such periods. Changes in the type of prey due to altered habitat use may also result in changes in how the animals are exposed to different types of contaminants. Future studies will be able to reveal how levels and effects of different substances may be determined by changes in habitat use as a consequence of climate change.
Nivåene av PCB, DDT, bromerte flammehemmere og toksafen er betydeligere lavere i ringsel fra Svalbard enn i ringsel fra Østersjøen. På bakgrunn av dagens miljøgiftnivå hos ringsel på Svalbard er det ikke grunn til å tro at miljøgifter har effekter på dyrenes immun-, hormon eller reproduksjonssystem. Men siden ringselen kan ha perioder med lite mat og at den taper vekt under pelsskifte (moulting) siden den spiser lite, kan det ikke utelukkes at miljøgiftene i disse periodene kan påvirke helsen til ringseler på Svalbard.
Den primære trusselen mot ringsel er, i likhet med isbjørn, global oppvarming som smelter havisen. Ringsel er avhengig av is for fødsler, diing, hvile og for skifte av ham. Ringselen bruker kun is når den ikke er i havet, den legger seg aldri på land. I tillegg bruker den områder dekket av is til næringssøk.