Spawning Salmon Haul Toxins to Alaska Lakes, Experts Find
By ANAHAD OCONNOR
Published: September 23, 2003
Pacific salmon contaminated by industrial pollutants in the ocean are carrying the chemicals to Alaska's lakes, where they may affect people and wildlife, according to a new study published in the Sept. 18 issue of Nature.
After spending most of their lives in the ocean, where they absorb widespread industrial chemicals like PCB's, sockeye salmon flock to Alaska's interior lakes in huge numbers to spawn and then die.
Each salmon accumulates just a small quantity of PCB's. But when the fish die together in the thousands, their decaying carcasses produce a sevenfold increase in the PCB concentrations of the spawning lakes, the study found.
"They die in such huge numbers that it almost looks like you can walk across the lakes," an author of the study, Dr. Jules Blais, said. "When they die, they release everything they've built up over the course of their lives. There's the possibility that the PCB's they give off are having an effect on the freshwater environment."
Dr. Blais, an assistant professor of biology at the University of Ottawa, and his team were drawn to Alaska after learning of studies that found killer whales in the Northwest to be among the most contaminated marine mammals in the world.
Other scientists had shown that salmon, a huge part of the killer whale's diet, and grayling fish could carry PCB's in their fat stores from the seas to the lakes. So Dr. Blais and his team, made up mainly of scientists from Canadian universities, decided to find out where the PCB's ended up.
Analyzing sediment and muscle tissue from sockeye salmon they collected in eight lakes throughout Alaska, the scientists found that the lakes with the highest numbers of spawning salmon had the highest concentrations of PCB's.
Crustaceans and insects that graze on the contaminated salmon carcasses become food for small fish, which are in turn eaten by salmon. Over time, PCB levels in the lakes grow.
The salmon act as biological pumps, Dr. Blais said, carrying pollutants upstream where they can reach bears, eagles and people whose diets consist mainly of fish, seals and whales.
"This speaks to scientists about how pollutants are being transported — by fish, not by air," said Dr. Derek Muir, an environmental chemist who has studied PCB's in the Arctic for 20 years but was not involved with this research.
Dr. Deborah Swackhamer, an environmental scientist at the University of Minnesota, has seen populations of eagles, otters and minks nearly wiped out after eating contaminated fish from the Great Lakes.
"In the Great Lakes," Dr. Swackhamer said, "the difference was that we knew where the contaminants were coming from, atmospheric and industrial pollution. But in Alaska you have no other source. So these fish are serving as vectors. There's just no end to the surprises of how contaminants can reach pristine places in our environments."
Dr. Blais said that the phenomenon was detrimental to the freshwater environments where the salmon spawned, but that it should not stop most people from eating fish.
"In this case, you'd have to eat about seven meals a week to reach threshold levels," he said. "It wouldn't be fair to imply that the salmon are unsafe to eat."
Industrial chemicals can find their way into the Arctic because air pollution tends to migrate toward colder areas. The airborne pollutants cool and condense, often descending in high concentrations on cold ecosystems, where they enter the food chain.
PCB's, or polychlorinated biphenyls, can cause cancer, neurological problems and reproductive defects in humans.
Industrial pollutants have been known to seep into food chains in some of the most remote parts of the world, said Dr. Muir, who works with Environment Canada.
In the mid-1990's, scientists discovered fish in a lake on Bear Island, off the coast of Norway, that had high levels of PCB's and other pollutants. A study found that contaminated droppings from seabirds flying over the lake were the source.
Polar bears in the Arctic may also be eating food tainted by PCB's, in their case, seals.
"Studies in Canada and Norway show that the immune systems of polar bears are affected by PCB's," Dr. Ross Norstrom, who studied environmental pollution with the Canadian Wildlife Service, wrote in an e-mail message. "It's not known whether this translates into a higher disease incidence or any effect at the population level. But there's a suggestion from one Canadian study that polar bear cub survival might be compromised by high PCB exposure in their mother's milk."
PCB's were among 12 highly toxic chemicals known as persistent organic pollutants that were banned by 122 countries in 2000. Dioxins and the pesticide DDT, among others, were included in the ban.