"We believe it's the end of the snakeheads in the pond, but we're going to come back again later this fall and do some additional sampling just to make sure," said state Department of Natural Resources spokesman John Surrick.
"What's left is monitoring to make sure that we've achieved what we believe we've achieved," Surrick said. "The pond was toxic for enough time that there wouldn't be any fish in it, and had there been any additional fish eggs there, the pond was toxic for long enough that they would have hatched and been killed."
The state poisoned the ponds on the recommendation of a panel of scientific experts that convened this summer to deal with the non-native fish. DNR officials were concerned that the fish - which can survive out of water for up to three days and slither short distances on land - would make their way to the nearby Little Patuxent River, multiply and devour native fish.
Scientists had expected the rotenone to decompose quickly, aided by warm water temperatures, but recent rain and clouds slowed the decomposition process.
Biologists with DNR's fisheries services applied potassium permanganate yesterday morning to the 4-acre Crofton pond to neutralize the poison, Surrick said. They will sample the pond today to determine whether the water quality has returned to normal.
"We're confident we got the mix right," said DNR spokeswoman Heather Lynch.
DNR crews treated the middle-sized pond - measuring three-tenths of an acre - with the same chemical Monday night, and the water quality had returned to normal by yesterday morning, Surrick said. The poison in the smallest pond decomposed without the addition of a neutralizer, he said.
State scientists are confident that the snakehead extermination project has succeeded, based on the results of an electroshock treatment Friday. After the introduction of an electrical current into the water, which stuns but doesn't kill the pond inhabitants, scientists observed that the only living creatures were tadpoles, frogs and turtles.
"Rotenone only affects creatures that breathe through gills," Surrick said. "Turtles don't have gills, tadpoles in some stages don't have gills, and frogs don't have gills."
Later in the fall, biologists will return for a final snakehead sweep.