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NMSU Scientist Studies Mercury Problem in N.M. Reservoirs

LAS CRUCES - Elevated levels of mercury make some fish like wall-eye and bass caught in New Mexico's reservoirs dangerous to eat, according to the New Mexico Fish Consumption Advisory. New Mexico State University fish biologist is trying to find out where the toxic metal is coming from and how it's affecting the ecosystem.


"There is no reason why we should have mercury in our diet, because it is toxic at high levels and non-essential," said Colleen Caldwell, an adjunct professor with NMSU's fishery and wildlife sciences department. "The main t7oute that humans get mercury in their systems is by eating fish."

Mercury becomes a problem because it "biomagnifies" through the food chain. "That means plants that accumulate mercury are eaten by small fish, then bigger fish eat the smaller fish," Caldwell explained. "The mercury concentration increases up through the food chain and eventually gets into humans.

At high concentrations, mercury can cause nerve tissue degeneration and birth defects. According to the Fish Consumption Advisory, mercury levels are high enough that pregnant and nursing women should avoid eating piscivorous fish (fish-eating fish) caught in New Mexico's Caballo and Elephant Butte reservoirs. Some levels are so high that grown men are advised to eat no more than one fish a year, Caldwell added.

The advisory is published by New Mexico's health, environment, and game and fish departments. Although scientists have known about the high mercury levels since the 1970s, they still aren't sure where the metal is coming from. "We can't pinpoint the entry of mercury into our reservoirs, but we do have theories about where it might be coming from." Caldwell said.

One possibility is that the mercury was left behind when an amalgamation process was used to extract gold from ancient mines. Since then, the mercury may have washed into the reservoirs, Caldwell said. Or, mercury from coal plants in north-central and western New Mexico may be moving through the atmosphere and depositing on vegetation and the soil, eventually ending up in the reservoirs.

"When we get a big rainfall in New Mexico, we get a scouring across our landscape in which all of the rain gets diverted to the arroyos, eventually making its way to the reservoir," Caldwell explained.

Since joining NMSU last September, she has worked to get her mercury research program up and running. In one project, Caldwell is directing a doctoral candidate who is looking at what environmental factors are influencing mercury in the water and sediments at Caballo. "We're looking at the dynamics of how mercury, transformed into an organic form, accumulates in the sediment and then in the animals."

In a second study, Caldwell and another graduate student will consider how the mercury gets transferred up the food chain and its biological effects on double-crested cormorants living at Caballo and Elephant Butte reservoirs.

"We've collected cormorant eggs and are going to analyze the mercury levels the eggs received from the mother birds," Caldwell said. "When the chicks hatch, we'll go back and take blood and feather samples to study mercury levels."

Once Caldwell gets an idea about how mercury is currently affecting aquatic ecosystems in New Mexico, she said she'll use that information to decide how best to monitor the mercury problem.