CROFTON, Md. -- Environmental officials in yellow plastic jumpsuits and gas masks sprayed fish poison into a pond, spelling the end for a school of notorious northern snakeheads.

Snakeheads and other fish jumped and flopped around shortly after crews began applying the poison Wednesday morning. The poison, rotenone, is absorbed through the gills and kills fish by interfering with the ability of their cells to use oxygen.

"It feels to them like they simply can't breathe. So, they're looking for air," said Michael Slattery, a biologist with the U.S. Fish and Wildlife Service, pointing to a 4-inch snakehead popping its head above the surface.

The snakehead is a native of the Yangtze River in China that grows up to 3 feet long and devours smaller fish and other aquatic animals.

A pair of snakeheads were plopped in the pond two years ago by a Maryland man who had bought them at a New York market.

Since then, Maryland officials have found hundreds of 2- to 4-inch long snakeheads -- apparently the offspring of the original pair.

It's expected to take several days for all the fish to die, said Eric Schwaab, director of the fisheries service for the Maryland Department of Natural Resources.

By the afternoon, more than 120 dead or dying snakeheads were netted, all 3 to 4 inches long, except one about 18 inches long, which Schwaab said he suspects was one of the original pair dumped into the pond. In July, a local angler caught another adult that was about 26 inches long.

"We knew exactly what we had as soon as we saw it," said DNR biologist Kinte Thompson, who plucked the dead snakehead out of 2 feet of water, later displaying the wide-mouthed, toothy fish to the gathered media.

The voracious appetite of the snakehead was apparent when one 4-inch juvenile was cut open and a whole inch-long sunfish and a whole mosquito fish, similar to a minnow, were found in its stomach.

Rotenone is commonly used to control fish populations because it breaks down into the environment within days and does not affect humans or other land animals, even if they eat the fish.