|
Salmon
dump pollutants on lake bed
Salmon
travelling to Alaska's lakes to spawn are carrying large doses of
industrial pollutants with them, a study has shown.
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.
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.»
|