Toxic mercury levels in seal meat linked to melting Arctic ice

Climate change in the Arctic may be boosting levels of highly toxic mercury in seals, one of the most commonly eaten traditional foods in the North.

A new study published in the journal Environmental Science and Technology has found a strong correlation between mercury in seal meat and shrinking sea ice, one of the most visible and well-publicized consequences of global warming.

"Sea ice patterns and contamination sources of ringed seals observed here suggest that the net outcome of longer ice-free seasons in the Arctic may lead to higher mercury levels in circumpolar seal populations," the report says.

Levels of mercury, a potent nerve poison, have long been rising in the North as emissions from sources such as coal-burning power plants in southern latitudes drift into the Arctic and remain. That increase has been reflected in animals such as beluga whales and Arctic seabirds.

Seals haven't shown a long-term increase, but their mercury levels fluctuate from year to year.

In an attempt to understand those changes, scientists at the Department of Fisheries and Oceans in Winnipeg compared levels of mercury in ringed seals from Prince Albert Sound off Victoria Island in the High Arctic to annual number of ice-free days. They found mercury levels increase after any significant difference from the average.

"You get this amazing correlation, where it seems to suggest that concentrations [of mercury] that you measure in the ringed seal are associated with sea ice extent," said Gary Stern, one of the co-authors of the paper.

The link between the two, suggests Stern, is the Arctic cod.

Cod thrive in low ice years, providing a banquet for hungry seals. But the fish concentrate the minute amounts of mercury found in everything they eat and pass that concentration along to their hunters.

Seals prefer cod to just about anything else, so Stern says that when they eat more of them, they wind up with more mercury in their bodies.

In years when the ice cover is heavy, only the older cod survive. That means that the only cod available to the seals are the ones with relatively high mercury levels, says Stern.

"If you have more ice you have higher concentrations and if you have less ice you have higher concentrations."

More ice than usual, however, is getting to be a less and less common event in the Arctic. Sea ice is in long-term decline.

Last February, scientists said the total extent of Arctic waters covered by ice was about 287,000 square kilometres less than average.

Stern says the next step is to return to Victoria Island and see if seals actually do eat more cod in low-ice years.

He emphasizes that while seals are still considered a safe and healthy part of northern diets, the link between sea ice and mercury means health officials will have to keep a close eye on the animals.

"It's important for us to continue to monitor, because if the levels are going up we want to make sure."

Still, the unexpected link between sea ice and mercury levels offers an important lesson in how climate change can produce surprising effects that directly effect how people live in their environment, says Stern.

"As we lose more ice, as the climate seems to warm, especially in the North, we really do need to keep monitoring this."