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Why do polar bears hunting at sea have higher pollution levels than those staying on land?

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Polar bears that hunt mainly in the sea have higher pollutant levels in their bodies than those staying on land during the sea ice free periods — but why? A new study in ACS’ Environmental Science & Technology reports the likely reasons.

As the climate changes, myriad animal populations are experiencing effects. In particular, Arctic sea-ice is in decline, causing polar bears in the Barents Sea region to alter their feeding and hunting habits.

Forced change

Barents Sea polar bears can roughly be divided into two groups which move along two differing routes during the year: Those living on the sea ice on the ocean surface, hunting their prey at sea, and those living on the coast, staying on land to fast or to hunt land-based food. In recent decades, the sea-ice bears have shifted northwards as southern sea-ice recedes.

The changes in sea ice availability have forced both types of bears to adjust how they find food. The bears staying close to the sea now have farther to migrate, while longer periods without sea-ice have forced coastal bears to feed on land-based prey or to rely on their fat reserves. Previous studies have shown that sea-based bears have higher levels of pollutants in their bodies, such as persistent organic pollutants (POPs), but little was known about why that difference exists.

To solve this mystery, scientists from the Norwegian Polar Institute, NILU – Norwegian Institute for Air Research and other collaborators collected an array of data that paints a clearer picture of how climate change affects polar bears.

Pollutants bioaccumulate in the food chain

In their study, the researchers gathered data on feeding habits, migration patterns, energy expenditure and geography to determine how the two polar bear types differed. They also measured pollutant levels in the prey that polar bears typically consume. The results indicated that several factors cause sea-based bears to accumulate more pollutants than those that stay on land.

Sea-based bears feed on a higher proportion of marine life, especially those that are higher up in the food chain – such as seals. This leads to multiple layers of polluted food, compared to those staying on land. In addition, they have higher energy requirements, which in turn causes them to consume more prey.

The bears hunting in the sea were also found to feed on prey located closer to pollutant sources and transport pathways. These combined factors highlight the unique threat that polar bears face in this region, and how increased sea hunting could enhance pollutant accumulation in polar bears as sea ice recedes.

“Although polar bears live far from people and pollution, their bodies contain some of the highest levels of environmental toxins that we know of”, says senior scientist Dorte Herzke from NILU. “They get it through their diet, and that is where our responsibility lies: Polar bears cannot choose from a menu, they must eat what they have always eaten. We humans are the ones who have to make sure that the world’s top predators can find food that does not contain environmental toxins.”

Read the article Pelagic vs Coastal—Key Drivers of Pollutant Levels in Barents Sea Polar Bears with Contrasted Space-Use Strategies in Environmental Science & Technology