"We simply haven't done enough studies to know all of the impacts, especially on fish-eating animals," Hughes added.
The study, based on data collected from EPA's Environmental Monitoring and Assessment Program, encompassed nearly 188,000 miles of perennial streams and rivers in the western United States. The selection of the more than 600 creek and river sample sites was done randomly to address regional questions about the prevalence of mercury contamination among fish in western rivers; not to do a stream-by-stream analysis of the impact.
The EPA's "tissue-based water quality criterion" is set at 0.3 micrograms of mercury per gram of fish tissue. In the survey, only 2.3 percent of the stream network that contained large (5 inches or longer) salmonids had mercury levels at or exceeding that 0.3 mg level. Salmonids were the most prevalent group of species, found in 41 percent of the network sampled.
On the other hand, large bass, walleye and other piscivores were found in only 10 percent of the streams and rivers, yet 57 percent of them met or exceeded the 0.3 microgram level.
"The difference in mercury levels between salmonids and piscivores is likely a function of their diet," Hughes said.
"Most of the salmonids we sampled were insect-eaters. Older, very large salmon may respond more like piscivores, but we didn't sample salmonids of that size."
In looking at mercury levels in fish across the West, the most important factors "were where the fish were in the food web, and their ages," Herlihy said. "The older a fish is, the larger it is, and the more fish that it eats, the more likely it is to bioaccumulate mercury."
Mercury levels of 0.1 mi
Source:Oregon State University