Greenland sharks joined the list of top Arctic predators that suffer under heavy loads of accumulated pollution in their bodies.
Biologists already knew that polar bears, orcas (killer whales) and people build up dangerous levels of toxins from feeding at the top of the Arctic food chain. Now, marine scientists have found evidence that a certain population of Greenland sharks (Somniosus microcephalus) could be one of the most contaminated Arctic predators.
Greenland sharks living near the Svalbard Islands, a Norwegian territory far to the north of Scandanavia, contained high levels the pesticides DDT and chlordane and the industrial chemical PCB, according to a study published last year in Science of the Total Environment.
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These chemicals, collectively known as persistent organic pollutants (POPs), were banned or restricted in the United States and 90 other nations in a 2001 treaty. However, POPs don't break down quickly in the environment. Instead, they build up over time in the food web, a process known as bioaccumulation.
POP particles stick to vegetation and microorganisms that animals eat, or are ingested directly. Then the chemicals stay in the food chain and build up as one creature devours another.
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In the case of sharks near Svalbard, scientists suggested that the predators were more contaminated because they ate more seals than other Greenland shark populations. The seals contained higher levels of POPs than other prey, because the POPs in each fish the seals ate subsequently became trapped in the seals' fat. When the sharks ate the seals, the POPs continued to build up.
Previous research on Greenland sharks from near Iceland and the Davis Strait in Canada found lower levels of POPs, likely because these sharks ate fewer seals.
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Liver samples from the Svalbard seal-eating sharks revealed lower than normal levels of Vitamin A and higher levels of Vitamin E, which may have been a biological defense reaction to the POPs.
"We can conclude that the contaminants lead to reduced levels of vitamin A and increased levels of vitamin E in the sharks around Svalbard, but we don't know if this affects their health or reproduction," study co-author Bjørn Munro Jenssen of the Norwegian University of Science and Technology said in a press release. "We would have to study the species for many years.
"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. Lower levels of vitamin A in the body lead to a reduced immune defense and may affect reproduction negatively," Jenssen said.
Jenssen noted that sharks in other areas may be even more contaminated.
"Seals living along the coast in Trøndelag in mid-Norway have pollutant levels five times higher than those in seals around Svalbard," said Jenssen. "In the Oslo fjord the levels are ten times higher."
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Greenland sharks can grow to more than 21 feet long (6.4 meters) and weigh 2,200 pounds (1,000 kg), rivaling the size of the great white shark. The sluggish predators spend much of their time in the deep, but come near the surface to sneak up on sleeping seals. The sharks may live for up to a century, which gives them plenty of time to accumulate POP pollution.
Image: Greenland shark, Somniosus microcephalus. (NOAA Okeanos Explorer Program, Wikimedia Commons)