Science News - September 29, 2004

Fish transport toxins hundreds of miles

Pushed hundreds of miles across the globe, pollutants often settle far from their origin because of wind, rain, or fire. Add to that salmon. A study published on ES&T’s ASAP website (es048744q) has found that organohalogens in salmon migrating upstream to spawn in a lake can end up in the lake’s resident fish. How this transfer occurs is still a mystery.

“These fish spend about three years out at sea before their migration,” says Göran Ewald, a professor of environmental science at the Technical University of Denmark and the corresponding author for the new study. Once they begin migrating, the salmon stop eating and deplete their fat stores by almost 95%, for energy use. Previous studies by Ewald and a report published as an ASAP article in September (es049607w) found that fat-soluble pollutants, such as PCBs and dibenzo-p-dioxins, become magnified in salmon during this time of migration. For example, PCB levels in the fat increase up to 9.7 times, depending on the migration run.

Salmon migrate hundreds of miles from the ocean to spawn in their natal rivers and lakes, but they also transport any pollutants that have built up in their bodies during years spent at sea.

In the newest research, Ewald and his colleagues followed the salmon one step further along their migration route by tracking the fish 400 kilometers (km) upstream into a pristine Alaskan lake. Once they reached their lake, the salmon spawned and died. Ewald’s group then analyzed the levels of chlorinated fatty acids in Arctic grayling, which live in the lakes. In the lakes where the salmon spawned, the grayling had 5 times the levels of chlorinated fatty acids of graylings from a lake 2 km away where salmon are not found. Ewald is not certain whether the graylings are acquiring the chlorinated fatty acids by consuming the salmon roe or from feeding on the dead fish themselves.

“It has to be a direct transfer of fatty acids,” he says. He adds that the levels of chlorine in the grayling were about 1 microgram per gram of fish, which is not a toxic level. However, the modified fatty acids could have other consequences. “We have research that now shows that the fish can’t utilize these fatty acids because the chlorine blocks enzyme digestion,” reports Ewald.

Ewald has not done follow-up studies to see whether salmon migrations are causing chlorinated compounds to build up in the lake, but Frank Wania, an environmental analytical chemist at the University of Toronto (Canada), says the finding is already important. “Studying the transport process is itself very interesting, whether it leads to an enrichment of the pollutants or not,” he says.

While biotransport is probably not significant to the movement of large amounts of pollutants on a global scale, it may have important local effects on pollution levels. And biotransportant may be more prevalent in the environment than suspected. Ongoing research in Norway has found that seabird droppings can contaminate lakes underneath a rookery with organochlorines, and an earlier paper (Environ. Sci. Technol. 1993, 27, 2198–2206) reported that eels transport large quantities of the pesticide Mirex out of the Great Lakes and up the Saint Lawrence River, which adjoins both Canada and the United States. —PAUL D. THACKER