State officials Thursday formally added Applegate Lake to the list of reservoirs where people should limit consumption of bass and other warm-water fish because of elevated levels of mercury found in their tissues.
The Oregon Health Authority advised specific limits of monthly meals that pregnant women, children and others should have of bass, crappie, bluegill and yellow perch after federal testing last year showed unhealthy levels of mercury.
The smallmouth bass tested at Applegate Lake showed mercury levels three times higher than the threshold for such an advisory, said David Farrar, a public-health toxicologist for the OHA.
Bluegill and black crappie tests at the reservoir showed levels at just above the advisory levels, Farrar said.
"You can safely eat a lot more bluegill and crappie from the lake than bass," Farrar said.
As predatory fish-eaters that live their entire lives in one water body, bass accumulate far more toxins such as mercury than rainbow trout, which are stocked in the lake and feed primarily on insects, Farrar said. That's why trout consumption is not part of the advisory, he said.
Also, salmon and steelhead in rivers such as the Rogue River have shown very low levels of mercury in their tissues, Farrar said.
"We encourage people to keep eating fish and to choose the right fish," Farrar said.
While yellow perch in Applegate Lake were not tested, their life cycles were similar enough to bass to have them added to the advisory, Farrar said.
The OHA reported those findings in January, but waited until now to issue its advisories after calculating the numbers of monthly meals considered safe for these particular fish, Farrar said.
The advisories for Applegate Lake, which is on the Rogue River-Siskiyou National Forest about 30 miles southwest of Medford, are similar to advisories that have been in effect for two decades at Emigrant Lake east of Ashland.
Elevated mercury levels also have been found throughout the Rogue River Basin, primarily in pikeminnow samples from the Rogue River. Mercury levels in Emigrant Lake were traced to natural sources, but the source of mercury in Applegate Lake and elsewhere has not been determined.
Under the advisory, pregnant women, nursing mothers and children should eat no more than two meals per month of largemouth and smallmouth bass and yellow perch, and no more than four meals per month of bluegill and crappie.
Men and women beyond childbearing age should eat no more than five meals per month of bass and yellow perch and no more than 13 meals per month of bluegill and crappie.If the maximum amount of fish covered by an advisory is eaten in a month, people should not consume any more of any of the listed fish during that month, according to the advisory.
A meal is about the size and thickness of an adult hand.
Mercury is a toxin most dangerous to fetuses because it can cross the placenta. It is known to cause permanent problems with developing brains and is linked to such developmental problems as autism and lower IQs in children.
Mercury is a worldwide health issue and so prevalent in the aquatic food chain that it's present in virtually all fish, including those caught in the ocean.
Because the element builds up over time in a fish's flesh, older and larger fish tend to have higher concentrations of mercury. Since mercury chemically bonds with muscle tissue, it cannot be removed or significantly reduced by fish-cleaning methods, cooking, brining or smoking.
The OHA expects soon to issue a statewide advisory on bass consumption because bass sampled in lakes and reservoirs across Oregon show elevated levels of mercury, Farrar said. The only two exceptions are Henry Hagg Lake near Hillsboro and Paulina Lake near Bend, he said.
Reach Mail Tribune reporter Mark Freeman at 541-776-4470, or email at email@example.com.