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WHITE CITY — Two small ponds infested with Chinese mystery snails and non-native aquarium plants are being drained in the latest attempt to kill off these pesky invaders before they spread.

An experimental 2011 chemical treatment of the ponds at the Jackson County Sports Park knocked back the population of these illegal aquarium denizens, but they have rebounded and are threatening to spread into nearby Denman Wildlife Area or the Rogue River, biologists say.

Invasive species technicians from the Oregon Department of Fish and Wildlife on Wednesday afternoon opened a valve and began draining the lower of the two ponds.

Once the ponds are dry, the plan is to let the now-dormant snails dry out and die — unless their trapdoor-like shells thwart eradication efforts yet again.

"This is almost, I guess, our last-ditch effort," says Rick Boatner, ODFW's aquatic invasive species coordinator. "This is totally experimental. Nobody's done this before."

Compound-ing matters is that another nasty aquarium-dweller — a plant called Brazilian elodea — was found in the lower of the two ponds last year. If allowed to spread, the plant could clog the pond with enough vegetation to completely block sunlight in the water.

Plans are to remove as much of the elodea by hand as possible after the draining, says ODFW invasive species technician Sam Dodenhoff.

The snails will be left in place for raccoons, birds and other scavengers to finish off. The bass and other panfish in the popular fishing ponds will be killed and not relocated, Dodenhoff says.

Cipangopaludina chinensis, as these snails are known by scientists, are often found in aquariums and outdoor water features because they eat algae. Yet they are banned in Oregon, along with hundreds of other exotic species that are illegal to possess or sell here.

Though prohibited, they occasionally are found for sale in pet stores.

Mystery snails are large, with adult shells measuring 21/2; inches tall. They were discovered in the ponds off Kershaw Road by an angler in 2009.

ODFW crews in 2011 used copper sulfate to poison both ponds, which together cover slightly more than 3 acres. Workers returned to the 11/4-acre lower pond a few months later and applied more than a double-dose of copper sulfate.

Surveys showed the snail densite went from 122 animals per square meter to 13 per square meter, Boatner says.

The Brazilian elodea was discovered last year by Dodenhoff while he was doing a snail inventory.

As with the snails, the fear is that the elodea will travel down an old irrigation canal and into streams that feed Denman and, eventually, into the Rogue.

"A plant like Brazilian elodea is pretty treacherous," says Dan VanDyke, ODFW's Rogue District fish biologist. "It can wreak havoc in warm-water ponds where people want to fish. Where I've seen it, you can't even cast."

Both likely found their way into the ponds from people dumping aquariums, VanDyke says.

"This is just an example of the problems that happen when people do that," he says.

The ponds are fed from area runoff, but they could also be fed by springs, Boatner says.

After the lower pond is drained, water from the upper pond will be pumped down to it so it can drain through a screened diversion, Dodenhoff says.

The holes will finally be sucked dry with a sump pump and could be left dry into summer.

"It may take a while before we get them dry and feel like we have a good kill on those non-natives," VanDyke says.

Crews then will inspect the empty ponds to see whether other species might be present.

"We'll find out what else is in the pond," Boatner says. "That will be interesting."

Reach reporter Mark Freeman at 541-776-4470 or mfreeman@mailtribune.com.

Rick Boatner, ODFW's aquatic invasive species coordinator, removes non-native plants from a pond at the Jackson County Sports Park Wednesday. Mail Tribune / Jamie Lusch - Jamie Lusch