FORTUNA -- A large portion of salmon in the Klamath River continue to become infected by pathogens and die each year, especially when the water is warm and they crowd into cool spots for relief.

Biologists gathered on Wednesday to give a prognosis on the health of the river's fish, and there was not much positive information. Scientists are trying to understand the complicated relationship between conditions in the river, a worm-like host that harbors two key pathogens and how fish handle infection and disease.

Studies show that the polychaete worm that is an intermediate host to the pathogens is most heavily infected below Iron Gate Dam, the lowest on the river. It's also prevalent far above all of the dams, in the Williamson River.

High numbers of parasite spores are released when adult spawning salmon die, and the most spawners are seen just below Iron Gate Dam, said Jerry Bartholomew, a researcher for Oregon State University.

The annual conference has drawn more people as attention on the Klamath has intensified. Commercial fishing was shut down last year and sport and tribal fishing slashed to protect a weak run of fish, causing tens of millions in economic losses along a long stretch of the West Coast.

As water temperature in the river increases, one parasite begins to replicate quickly, and fish infected by it die more rapidly. The incidence of infection in young salmon jumped quickly on May 1, 2006, one month later that a similar spike on April 1, 2005, according to a presentation by Ken Nichols with the U.S. Fish and Wildlife Service.

Nichols said it almost looks as if there were a trigger responsible for the sharp jump in infection at those times, but it had not yet been identified.

Scott Foott, a pathologist with the U.S. Fish and Wildlife Service said that the Klamath's native fish have evolved with the pathogens, but get overwhelmed when conditions are stressful. They crowd together in cool spots when water gets warm and that allows infection to spread rapidly, he said.

”It's kind of a death of a thousand cuts,” Foott said.

Fisheries biologist Pat Higgins asked if high pH and ammonia levels observed below Iron Gate Dam would suppress the immune response of salmon. Foott said any additional stress would probably make the fish more susceptible.

Rebecca Quinones with the U.S. Forest Service reported that hatchery bred fish are more and more often spawning in the river -- making up 13 percent of the chinook salmon that return to the Klamath. That percentage is as high as 40 percent in the Shasta River.

Quinones also said that 2006 was another bleak year for spring run salmon, which almost exclusively return to a single tributary of the Klamath -- the Salmon River -- not including the Trinity River. The number of adult steelhead that return to Iron Gate Hatchery is also decreasing, she said.