Everywhere one looks today, signs point to an Earth that seems to be shrinking.
Dashed by this growing realization is the illusion that anyplace, no matter how remote or far-flung, is immune from the modern-day goings on in this increasingly developed and industrialized world. Today, we know that pollutants carried by wind and water reach every corner of this tiny blue planet.
Wood River Valley residents witnessed the truth in this earlier this week when reports came out suggesting that Silver Creek—the famed fly-fishing destination near Picabo—is tainted with mercury, an element known to be especially harmful to pregnant women as well as to children.
While the reports don't precisely pinpoint the source or sources of the contaminant, preliminary investigation suggests the culprit could at least in part be industrial gold-mining operations in northern Nevada and a concrete plant across the state line in Durkee, Ore. Both are upwind from the Magic Valley region and are known sources of airborne mercury.
The Idaho chapter of The Nature Conservancy, which owns and manages an 883-acre preserve in the upper sections of Silver Creek, paid for the survey that identified the high mercury levels. The survey— carried out by the U.S. Geological Survey—tested contaminant levels in a total of 20 brown trout.
Staff of The Nature Conservancy commissioned the survey because of high mercury readings in other waterways in southern Idaho, Silver Creek Preserve Manager Dayna Gross said.
Gross said high levels of mercury were found in 2004 when the USGS sampled fish in Silver Creek. However, she said, those results weren't considered conclusive because the tissue samples sat for too long before being tested.
"We asked them to take more samples this year," Gross said.
She said that this year's samples will be used as a baseline for future testing.
"We don't know how much has been in the system historically," she said.
Gross said high mercury readings have even been found in places formerly considered pristine, such as high alpine lakes.
"I think it's going to be a bigger and bigger issue," she said.
The issue of fish consumption isn't an issue of concern in the upper stretches of Silver Creek through the preserve, which is a designated catch-and-release area. Only in select areas of the creek does the Idaho Department of Fish and Game allow fish to be kept.
Throughout the state, many bodies of water have been shown to contain fish with high levels of mercury in their flesh. In southern Idaho, those include Salmon Falls Creek Reservoir, where test results showed fish to have the highest known levels of mercury in the state.
While stagnant bodies of water such as lakes and reservoirs more commonly show high levels of mercury in fish, it wasn't too much of surprise to find high levels in Silver Creek, said Terry Maret, supervisory biologist with the USGS Idaho Water Science Center in Boise.
Maret said that the profusion of plant growth as well as microbial activity that exists in shallow bodies of water such as Silver Creek enhances the transformation of the elemental form of mercury to its organic form, methyl mercury, which is more dangerous to humans. He said those microorganisms exist in soil, sediment and water.
"Silver Creek is an ideal place for that to happen," he said.
In all, samples from 20 different brown trout were sent to the USGS national mercury laboratory in Middleton, Wisc. The samples were taken from fish caught by electrofishing at the Martin Bridge near Picabo and near the Silver Creek Preserve visitor center.
Maret said the fish ranged in size from 12 to 22 inches. Because older fish generally contain higher levels of mercury in their tissue, the larger ones showed higher levels of mercury, he said.
At the Picabo site, the 10 trout sampled showed an average concentration of .5 parts per million. Further upstream near the preserve visitor center, the 10 sampled trout showed a higher average concentration of .67 ppm. Maret said the highest concentration in a single trout was a reading of 1.91 ppm.
He said that compared with spring-fed streams such as Silver Creek, snowmelt rivers such as the Big Wood typically have lower concentrations of methyl mercury because they have less plant and microbial life to process the elemental mercury. Testing results from brown trout caught near Stanton Crossing on the Big Wood River in 2004 and 2006 showed mercury concentrations at much lower levels of .04 and .05.
The Idaho Department of Health and Welfare is responsible for setting safety standards for the amount of methyl mercury that can be safety ingested by humans. Standards state that women of childbearing age and children shouldn't consume more than .1 micrograms of methyl mercury per kilogram of bodyweight per day, department Program Manager Jim Vannoy said.
Determining how much fish is safe to eat requires a complex calculation that takes into account measurements like those obtained from Silver Creek fish, Vannoy said. He said that when surveys show samples of .2 or .3 ppm and higher, Health and Welfare considers issuing a fish advisory.
"We're going to be looking at these levels," Vannoy said.
If the results of the Silver Creek survey are deemed valid, he said, the agency will likely issue some sort of fish advisory for Silver Creek.
Vannoy said it takes much higher concentrations than those seen in Silver Creek for Health and Welfare to consider an all-out warning stating that fish shouldn't be consumed.
"This will be a limit-your-consumption advisory," he said.
Health and Welfare doesn't want people to feel like they should quit eating fish, which are healthy sources of protein and omega-3 fatty acids, Vannoy said.
"We'll determine how many meals you could eat," he said.
According to Health and Welfare, mercury contamination in sportfish can be the result of natural or industrial sources. The metal, which is familiar as the silver-colored liquid in thermometers, exists in the environment in rocks, soils, water and air. Maret said other natural sources of airborne mercury include forest fires.
Manmade sources include coal-fired power plants and mining. Mining releases mercury into the air during the smelting process.
"It's likely there are other sources besides local ones that are the sources of these high values," Maret said.
The only way to discover if a fish is contaminated with mercury is to test it using sophisticated methods like those The Nature Conservancy paid for. Mercury cannot be removed from fish using cleaning or cooking methods.
The nervous system, including the brain, is the main area of the human body affected by mercury exposure. The first noticeable effects of long-term, low-level exposure are trembling hands, numbness of the extremities and behavioral changes. As mercury builds up in the body, a person's ability to walk, talk, see and hear may be impaired.
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Two fly-fishermen cast their lines in the famous waters of Silver Creek, near Picabo. A recent survey of 20 Silver Creek brown trout caught by staff of the U.S. Geological Survey has revealed abnormally high levels of mercury in the fish. 
