The state Department of Natural Resources destroyed 20,000 hatchery trout today, bringing to at least 156,000 the number of fish the agency has destroyed this year in hopes of curbing the spread of whirling disease, an illness fatal to trout.

The brown and rainbow trout killed today were from net pens below Jennings Randolph Dam in the North Branch of the Potomac River, Assistant Secretary Michael Slattery said. The DNR has known since 1995 that the Jennings Randolph Culture Station harbored the parasite that causes whirling disease, but decided in March to close the operation after the disease was found at two trout-rearing stations in Garrett County.

The rearing station at the Mettiki coal mine near Table Rock has been closed. The state-owned Bear Creek Rearing Station near Accident has been emptied of fish. Last month, the whirling disease parasite also was found in Bear Creek, which supplies water to the Bear Creek station.

Slattery said the trout destroyed today were taken to a rendering plant in Winchester, Va., and converted to chicken feed. The rendering process kills the whirling disease parasite, he said.

The parasite deforms trout skeletons, causing them to swim in circles until they die. All species of trout and salmon may be susceptible, but rainbow and cutthroat trout appear to be more susceptible than other trout species, according to the Whirling Disease Initiative in Bozeman, Mont.

The disease isn't a threat to humans, but it has decimated wild trout populations in some Western rivers. The disease is found in 25 states, including Maryland neighbors Pennsylvania, West Virginia and Virginia, according to the group's Web site,