Salmon carry PCBs to Alaskan lakes

Salmon travelling to Alaska's lakes to spawn are carrying large doses of industrial pollutants with them, a study has shown. The accumulation of these compounds 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.

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

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 north-eastern United States. "This is a remote, pristine environment, but with PCB deposition comparable to an industrial site," says Blais.

The problem is bioaccumulation - the build-up of contaminants in creatures at the top of the food chain. The North Pacific contains about one 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's team reports. "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."

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