Klamath River Salmon can’t seem to catch a break. Yet again, they are fighting to navigate the very river that their spawning cycle depends upon. In addition to the physical obstacle of the dams, rapidly elevating water temperatures, and algal blooms that choke the river’s oxygen supply, an aggressive parasite has been added to the list of factors that increasingly threaten the health of juvenile salmon who hatch in the Klamath River. Ceratonova shasta is a myxosporean parasite ravishing juvenile salmon populations in the Klamath River, resulting in a widespread fish kill that will “impact fish runs for many years to come.”

C. shasta lives inside of and relies upon two separate animals during its lifecycle. It lives the first stage of its life inside freshwater worms, called polychaetes, that live under water latched onto rocks. After their spores are released from the worm hosts, C. shasta spores enter the bodies of juvenile fish. Infected fishes’ health rapidly deteriorates as the disease attacks organs and soft tissue. Infected fish die and their bodies release more C. shasta spores into waterways where they continue their lifecycle and, with the help of dam-related conditions, continue to impact future generations of Klamath River salmon.

Dams appear to be a critical component in the ongoing proliferation of C. shasta. Dams create river conditions that allow the disease to rapidly spread to juvenile fish. Elevated water temperatures caused by river flow being trapped in dam-held reservoirs allow the disease to move through the bodies of infected juvenile salmon more rapidly. Reduced sediment flow caused by the dams results in larger numbers of the freshwater worms that C. shasta depend on before infecting juvenile salmon hosts. Without dams, C. shasta might have remained a largely overlooked disease with little impact of Klamath fisheries. Unfortunately, conditions created by the installation of the dams have created the perfect environment for C. shasta to thrive at the expense of juvenile salmon populations.

Research conducted in recent years has led water managers to attempt to mitigate dam impacts on salmon and other anadromous fish by releasing bursts of water through the dams at key moments to flush out algae and polychaetes downriver and dilute the concentration of infectious C. shasta spores. This year’s extremely dry winter led water managers to decide against releasing flushing flows. This resulted in the disease outbreak we are witnessing right now.

There is some good news: in a few years the lower four Klamath dams will be removed. This will lead to more natural variation in flows and will greatly reduce populations of polychaete worms and in turn C. shasta. Klamath salmon populations will stand a fighting chance once the Klamath River is allowed to flow more freely and the disease cycle is broken.