Fish in US Rivers Contaminated With Mercury

Many of the larger fish in rivers of the western United States have mercury levels that exceed federal guidelines for safe human food.

The fish most affected are those that eat other fish, as living at the peak of the food web makes them vulnerable to accumulating persistent toxins like methylmercury.

Mercury also tends to concentrate more in bigger fish.

Fish-eating fish, or piscivores, average triple the mercury concentrations of fish feeding at a lower trophic level on other organisms. Three-quarters of the pikeminnow assessed between 2000 and 2004 had mercury concentrations above the threshold for safe consumption by children and pregnant women. Other carnivorous species with unhealthy mercury accumulations include walleye, bass and northern pike. Although piscivorous fish are relatively uncommon in the region, 57 percent of the stream reaches they were found in had fish tainted above safety levels.

The threshold for avoiding mercury toxicity is much lower among fish-eating mammals, such as mink and otter. Nearly all streams with predatory fish longer than 12 cm had fish considered harmful to wild animals. As well, smaller fish of all kinds from 18 percent of fish-bearing stream lengths had mercury levels above what these mammals can tolerate.

The more popular river species among anglers in the 12 western states surveyed, such as rainbow and cutthroat trout, are the least contaminated of all. Only 2.3 percent of the stream lengths with members of the Salmonidae family contained fish too toxic for people to eat safely.

The results from assessing fish in 207,000 km of streams and rivers suggest a widespread and fairly uniform loading of mercury in waterways lying anywhere from South Dakota west to Washington and south to Arizona and California. The degree of human disturbance in a watershed has no bearing how much mercury its fish contain.

These patterns lead the study's authors to conclude that mercury reaches rivers in western North America from the atmosphere. They suspect the source is pollution blown across the Pacific Ocean from Asia.