Greenland sharks have high levels of toxic pollutants

The Greenland shark (Somniosus microcephalus) is one of the largest species in the world. It can grow to as much as seven metres long and weigh over a metric ton. It lives in deep water, at depths of 200 to 600 meters, and it lives farther north than any other shark. It is also long-lived, and can live to be 100 years old. Other names for it are the grey shark or gurry shark.

“We had this theory that the sharks would occasionally move south, but it turns out that they stay in the waters around Svalbard,” says Professor Bjørn Munro Jenssen, at the Norwegian University of Science and Technology (NTNU). He specializes in pollutants and arctic biology.

NTNU, the Norwegian Polar Institute and Windsor University in Canada have learned a great deal new about this species during their research in the ocean off Svalbard, Spitsbergen.

Eat a lot of seals

The researchers wanted to study behaviour, distribution, population size, concentrations of pollutants and the effect of pollutants on the species.

43 sharks were marked with a tracking device and a depth gauge. The researchers also took liver samples.

This species has not been extensively studied before, and for a long time, researchers thought that the Greenland shark is a scavenger. But as it turns out, the shark also eats live prey– both fish and seals.

The liver samples showed accumulation of alarmingly high concentrations of PCBs, brominated flame retardants (BFRs) and other pollutants.

“We think this is due to their diet, because Greenland sharks around Svalbard eat a lot of seals. Since the seals are high on the food chain, it leads to an accumulation of pollutants in the shark,” Jenssen says.

Since Greenland sharks live for a very long time, contaminants can accumulate in their bodies over decades. The species is also poor at excreting the pollutants.

Previous studies show lower levels

The researchers were surprised to find such high levels of contaminants in the sharks. Previous studies of Greenland sharks around Iceland, Greenland and in the waters off Canada have shown much lower contaminant levels.

This is due to differences in diet, the researchers believe.

“The reason we found lower concentrations [in these areas] is because the sharks eat less seal”, says Jenssen.

43 percent of the individuals around Svalbard had seal remains in their stomachs. In Canada, around Iceland and off Greenland, however, they found seal in only 14 percent of sharks.

“There the sharks prey lower down in the food chain, mostly on fish, which again results in less accumulation of pollutants,” Jenssen says.

Defence mechanism

The liver samples from the Greenland sharks around Svalbard also had low concentrations of vitamin A and high concentrations of vitamin E.

“We can conclude that the contaminants lead to reduced levels of vitamin A and increased levels of vitamin E in the sharks around Svalbard”, says Jenssen.

Vitamin E is an antioxidant, and the increased levels might be a defence mechanism.

“It seems that animals mobilize vitamin E stored in the liver and send it into the blood stream. Greenland sharks seem to be able to do this when needed” he says.

Lower levels of vitamin A in the body lead to a reduced immune defence and may affect reproduction negatively.

“But we don’t know if this affects their health or reproduction. We would have to study the species for many years,” he adds.

Polar bears

Researchers have studied contaminants, their accumulation and the effect they have on animals and plants extensively in Svalbard. The focus has particularly been on polar bears.

Studies have shown that polar bears have high concentrations of fat-soluble pollutants in their adipose tissue and blood. The pollutants affect the bear’s hormone levels and immune system.

The exposure starts when the bears are just cubs, because they get large doses of the contaminants from the mother bear’s fatty milk, which is all that they eat the first three or four months.

Transfer value

Bears and sharks are not the only creatures that are exposed to daily doses of contaminants – humans are also at the top of the food chain.

For that reason, the researchers believe that knowledge on Greenland sharks may help us understand how pollutants, especially hormone inhibiting substances, also affect humans.

“These finds are very provocative. The species highest up on the food chain are the most affected. We are among them,” Jenssen says.

External links

• Bjørn Munro Jenssen's profile

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