As most people know, salmon spend their adult lives out to sea, but return to their native river to spawn and then die. As a result their entire body mass, which has been gained in the ocean, is delivered far inland throughout North America and left there for wildlife to feed on. Ecologically speaking, salmon act collectively as an enormous pump from ocean to land for vital nutrients such as nitrogen and phosphorus. But they are also increasingly pumping pollutants too, such as mercury.

Researchers from the University of Connecticut have completed the mammoth task of compiling an inventory of important nutrients and pollutants thus conveyed.

The researchers found that over the roughly 40-year period, the biomass of returning salmon has increased by 32%—roughly 75,000 tons. And the amount of nutrients they bring in has increased by 30% (kinda what you’d expect). But pollutants coming upstream have grown by 20%.

Each year salmon deliver about 7 kilograms of mercury, spread over streams from Alaska to California. That doesn’t sound much on a continental scale, but mercury is a serious pollutant, even in low concentrations. Moreover it gets concentrated in animal tissues as you go up the food chain. Too low to affect people all that much, but bad news for apex predators such as bald eagles.