Spawning Salmon Carry Unwanted Guests to Alaska

Like many remote places on the globe, Alaska receives its share of gunk from more populated areas. A few years ago, researchers found traces of lindane, a pesticide, in the bark of trees near Denali National Park. The chemical, carried on winds and condensing once it reached the cold air of the north, came from tree farms thousands of miles away.

Now, researchers have found pollutants that swim into Alaska. Actually, salmon do the swimming, but the fish carry nasty baggage they picked up in the ocean--DDT, PCBs and other manmade chemicals. Researchers, led by Goran Ewald of Lund University in Sweden and including Nicky Szarzi, a fisheries biologist for the Alaska Department of Fish and Game in Homer, studied fish in two Alaska lakes and found evidence that salmon are passing on pollutants to other fish.

The researchers examined grayling in Round Tangle Lake and Lower Fish Lake, both located in the Alaska Range. Salmon migrate to and from Lower Fish Lake, which is connected by a network of rivers and streams to the Gulf of Alaska, but salmon do not return to Round Tangle Lake, which is linked to the Tanana River. Though Round Tangle Lake is theoretically connected to the ocean, its outlet to the Tanana River features a series of waterfalls and rapids that prevents salmon from returning, according to Fronte Parker, an Alaska Department of Fish and Game sport fish biologist in Delta Junction.

Beside the presence or absence of salmon, a striking difference between the lakes is that the grayling of Lower Fish Lake have more significant traces of DDT and PCBs in their fat than do the grayling in Round Tangle Lake, the lake that contains no migrating salmon.

Ewald and his colleagues saw that the grayling of Lower Fish Lake had similar levels of pollutants in their fat as did the salmon that shared the lake with them. The researchers theorize that the grayling gained the pollutants by feasting on the remains of spawned-out red salmon and salmon eggs.

Scientists don't know exactly where the salmon are picking up the elevated levels of DDT, a pesticide banned in the U.S. but used in other countries, and PCBs, a chemical byproduct of electrical component manufacture. The salmon had elevated levels of pollutants when they returned from the ocean, a fact the researchers discovered as they checked the fish throughout a 250-mile migration to the lake from the Gulf of Alaska by way of the Copper and Gulkana rivers and Fish Creek.

After hatching as fry and spending a year or two in Lower Fish Lake, salmon migrate to the ocean, where they spend a few years before returning to spawn. In those few years, the fish may encounter pollutants thousands of miles from the lake in which they were born. By recovering tagged fish, researchers have found pink and king salmon from northwest Alaska that crossed the Bering Sea to Russia's Kamchatka Peninsula. A king salmon tagged in the central Aleutian Islands was recovered a year later in the Salmon River, Idaho, after a journey of 3,500 miles.

In these global excursions, the red salmon Ewald studied somewhere picked up the manmade chemicals. The researchers found the returning salmon burned plenty of fat as they made their way upstream to Lower Fish Lake, but when the fat disappeared, the chemicals remained, becoming concentrated in what little fat wasn't used in the upstream journey. After they spawned in Lower Fish Lake, the salmon died and were gobbled by grayling, which showed levels of chemicals four times higher than the grayling at Round Tangle Lake, where there are no salmon.

While the levels of pollutants are too low to affect humans (good thing, too, because dipnetting excursions to the Copper River fill many Alaskans' freezers), the study shows once again that nowhere on this planet are we isolated from messes made by our own species.