Salmon packed with pollutants

Decaying fish dump PCBs in Alaska's lakes

Posted: 24 September 2003

Author: Michael Hopkin

Salmon travelling to Alaska's lakes to spawn are carrying large doses of industrial pollutants with them, according to a study¹ published in Nature magazine.

Environmentalists fear that the accumulation of these compounds, called polychlorinated biphenyls (PCBs), could have harmful consequences for the region's top carnivores: bears, eagles - and humans.

Each summer, millions of sockeye salmon (Oncorhynchus nerka) make the 1,000-km trip from the North Pacific back to the lakes where they were born. After spawning there, they die, and their carcasses decompose in the lakes' sediment.

The fish arrive loaded with PCBs from their oceanic feeding grounds, report Jules Blais of the University of Ottawa, Canada, and his colleagues. In the sediment of lakes with the most returning salmon, such as Frazer Lake on Kodiak Island in southern Alaska, PCB concentrations can be seven times those in lakes that receive no fish.

The results are akin to having a waste incinerator in Alaska's wilderness - pollution levels are as high as those in Lake Superior, close to the heavily populated northeastern United States. "This is a remote, pristine environment, but with PCB deposition comparable to an industrial site," says Blais.

Salmon cart chemicals - good and bad - upstream, agrees ecologist David Schindler of the University of Alberta, Canada. Dying fish, for example, furnish the lakes with vital nutrients. "If they can transport nutrients, they can also transport things that are not quite so beneficial," Schindler says.

Vicious circle

The problem is bioaccumulation - the build-up of contaminants in creatures at the top of the food chain. The North Pacific contains about 1 nanogram of PCBs per litre. By the time the average salmon has finished bulking up for its journey, its fat contains about 160 micrograms, Blais and co-workers report.

"The salmon are perfectly fine for eating," says Blais. But dead fish become fodder for insects at the bottom of the food chain, triggering a fresh round of bioaccumulation. "There's a snowball effect," Blais explains.

PCBs are released into the environment by the manufacture of materials such as flame-retardants and paints, and by burning waste. Their effects on human health are not clear, but are thought to include reproductive defects, memory impairment and reduced hand-eye coordination. PCBs break down very slowly, so they can spread widely and be difficult to track.

A case in point is Lake Laberge in Canada's Yukon Territory. In the early 1990s, PCB pollution - thought to have arrived by air from Eurasia - reached such a level that the inhabitants of this otherwise pristine area were warned not to eat trout from the lake. "PCBs have a way of producing surprises," says Blais.

The situation may improve in the future, Schindler predicts. Industrial PCB emissions have been falling for more than 20 years. "This could turn out to be one of the few environmental problems that we have dealt with in good time." References1. Krümmel, E. M. et al. Delivery of pollutants by spawning salmon. Nature, 425, 255 - 256, (2003). Article

Source: Nature magazine, 18th September 2003.

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