Mercury in seals linked to vanishing sea ice

Erika Engelhaupt in Environmental Science & Technology: Every summer, seal hunters in the village of Ulukhaktok in Canada’s Northwest Territories carve out small pieces of muscle from ringed seals during their annual subsistence hunt. As part of their collaboration with Arctic researchers, they carefully bag the tissue samples, draw blood, and measure each seal.

After more than 30 years of monitoring the seals, the researchers have noticed a disturbing pattern: levels of the dangerous metal mercury in the seals are connected to the state of sea ice on the ocean. The new research, published in ES&T, reveals that more mercury may be spiraling up the food chain as sea ice disappears.

“The trend now in the loss of sea ice suggests that mercury in ringed seals will increase over time,” says Gary Stern, a study coauthor from the University of Manitoba (Canada). His team, along with colleagues from Fisheries and Oceans Canada, measured mercury levels in ringed seals collected from 1973 to 2007 and related the levels to the length of the summer ice-free season in the seals’ habitat.

The Arctic environment already contains more mercury than animals there can take up, Stern explains. However, a shifting climate could result in ecosystems converting the mercury into highly toxic forms that biomagnify up the food chain.

The researchers found that mercury concentrations in seals were higher in years when the ice-free season was either very long or very short. The researchers think this is related to the seals’ food supplies. Cod are the most-mercury-contaminated food that ringed seals eat, so the seals’ summer mercury levels depend largely on how much cod the seals ate during their previous fall and winter hunting season. Older cod are the most contaminated, because the metal accumulates over time.

The researchers think that a longer ice-free season creates milder conditions that favor cod growth. So in years with less sea ice, seals have a longer ice-free period in which to catch cod, plus there may be more cod to eat. Alternatively, when sea ice stays frozen for a long time, young cod may have a harder time surviving the winter, leaving a greater proportion of older and more contaminated cod as a food supply….

Lee Cooper shot this portrait of a ringed seal