GRANTS PASS, Ore. (AP) – For the second time this year, federal officials are releasing additional water from a Northern California reservoir to combat a parasite that threatens to kill thousands of salmon in the drought-parched Klamath River.

The U.S. Bureau of Reclamation says it doubled the outflows from Lewiston Dam into the Trinity River on Tuesday, and will keep them up for seven days.

The decision was made following the discovery of a parasite known as Ich, which attacks salmon in stagnant water conditions. The agency increased flows last month to combat the same problem.

Sampling this week found the parasite in nine of the 20 fish tested.

“This is the only possible means of preventing or reducing the severity of a parasite outbreak,” David Murillo, the bureau’s regional director, said in a statement.

He added the agency was concerned about the effect on water levels in Trinity Reservoir, which also sends water into the Sacramento River for salmon and agriculture.

A federal judge denied an irrigation district’s bid to stop earlier releases for Klamath salmon.

Yurok Tribe Chairman Thomas P. O’Rourke Jr. said in a statement there have been no confirmed salmon deaths linked to the parasite, but the tribe is very concerned.

Tribal biologists last week delivered imprints of the gills of fish believed stricken by the parasite to the U.S. Fish and Wildlife Service Fish Health Center, and federal biologists sampled fish Monday.

During a 2002 drought, tens of thousands of salmon died in the Klamath, primarily from Ich, short for Ichthyophthirius multifiliis. The parasite attacks the gills of the fish, suffocating them.

The Trinity is the Klamath’s primary tributary and a source of much cleaner, colder water than the upper reaches of the Klamath River. Water in the upper reaches goes through a series of reservoirs where it is warmed by the sun and affected by algae and agricultural runoff.