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Disease Effects on Wild Populations

Myxozoan Parasites in Klamath River Salmon

Disease Effects on Wild Populations

Myxozoan Parasites in Klamath River Salmon

Disease reduction in Klamath River Salmon


Problem: The effects of declining Pacific NW salmon populations on coastal economies and tribal communities have been felt for several decades, but the reduction of the commercial catch by 90% in 2006 was a direct result of the weak returns of Chinook salmon to the Klamath River (KR). Severe infection by the myxozoan parasite Ceratomyxa shasta has, in large part, been responsible for the declining numbers of juvenile KR fall Chinook and coho salmon and subsequent impacts on later adult returns. Compared with infection of juvenile Chinook salmon from other Pacific NW rivers, the numbers in the KR are striking. For example, incidence of Ceratomyxa shasta infection in juvenile Chinook salmon, collected in the Klamath River above the confluence with the Trinity River between May and the end of July, examined histologically has varied between 21 - 37% (2004 - 2008 data); this infection rate is much higher (31 - 62%) in samples analyzed by QPCR for the same time period. (Juvenile salmonids outmigrate during this period but also at other times of the year when infection prevalence is lower). Whereas, in the Fraser River, Canada, C. shasta infection prevalence was estimated to be 3.3% and in the Columbia River estuary, C. shasta was detected in 9.6% and 12% of juvenile Chinook salmon in 1983 and 2001, respectively. The high prevalence and severity of C. shasta infections in KR fish indicates this parasite is a key factor limiting salmon recovery in that system.

Map of the Klamath River.

Fig. 1 Map of the Klamath River which straddles the border of Oregon and California in the Pacific North West of USA. It flows westward over 400 km into the Pacific Ocean.

Ceratomyxa shasta has a complex life cycle, involving an invertebrate (polychaete worm) host as well as salmon (see Fig. 2). Salmon in the KR have evolved with C. shasta and are relatively resistant to infection compared to salmon from rivers where the parasite is absent, thus the current severity of ceratomyxosis in these fish suggests a shift in the host:parasite balance. Research conducted by OSU and the US Fish and Wildlife Service (USFWS) have identified a stretch of the river in which high parasite densities and large numbers of infected invertebrate hosts result in severe disease and high mortality, thus providing a target for management actions.

Life cycle of the myxozoan parasite Ceratomyxa shasta.

Fig 2. Life cycle of the myxozoan parasite Ceratomyxa shasta: the actinospore released from the tiny invertebrate host, Manayunkia speciosa, infects salmonids via the gills and in the intestione develops into the myxospore stage. Clinical disease signs in the fish include swollen abdomen from ascites accumulation, inappetance, hemorhaging intestinal lining. Infection is seasonal and progression is temperature dependent. Each spore stage is 10µm or less in size.

Solution: In 2007, a multidisciplinary panel of fish disease experts and fishery managers met to develop a research plan focused on management actions to reduce disease (ceratomyxosis) in natural juvenile salmon of the KR. This proposed research effort is in addition and complementary to on-going monitoring in the basin. The reach from Iron Gate Dam to the Scott River was identified as the primary management area, based on juvenile salmon infection status and data from fish exposures, invertebrate host surveys and water sampling studies.

Potential management actions would have the goal of

  1. Reducing polychaete host populations in the selected KR reach
  2. Reducing the effects of the infectious actinospore on juvenile salmon
  3. Reducing the input of myxospores from specific salmonid fishes
  4. Decreasing fish exposure/Increasing fish resistance

A preliminary ranking of management actions was made based on the likelihood that the action would result in a reduction in ceratomyxosis in natural salmon populations of the Klamath River. Other considerations included the effect on non-target organisms and the ability to monitor effects. The panel recommended several actions as being the most likely to cause a biological effect and as high priority for further research. We propose to test the feasibility of the highest priority management actions through controlled laboratory and field experiments. These studies would be conducted over 2-3 years, leading to large-scale tests or implementation.

Impacts: Salmon losses in the Klamath basin have had devastating effects on tribal communities along the river and the coastal communities of Oregon and California. The recent closure of the salmon troll industry in 2006 was a management decision in response to the low contribution of the Klamath basin - only 5% of the fishery - with disease being a primary factor. The cost to Oregon coastal communities was estimated to be $28 million dollars in 2006 and Governors of both Oregon and California declared fishery disasters that resulted in allocation of $60 million dollars in federal assistance. Salmon trollers estimate that reducing disease effects by as little as 10% would increase the number of Klamath River adult salmon to the point that fishing on that population could resume and allow that industry to survive.

Research collaborators

  • Russ Perry, USGS
  • Susan Fricke, Karuk Tribe
  • Barry McCovey and Mike Belchik, Yurok Tribe
  • Nicholas Som and Nicholas Hetrick, USFWS Arcata
  • Scott Foott, Anne Bolick, and Kim True, USFWS Cal-Nev Fish Health Center
  • Ted Wise, Sarah Bjork, Bill Tenniswood, ODFW

Funding for KR research and monitoring at OSU has been provided through the Bureau of Reclamation, Oregon Sea Grant, the US Fish and Wildlife Service, the Karuk, Yurok and Hoopa Tribes, PacifiCorp, a federal appropriation (through NOAA) and Oregon Commercial Restoration and Enhancement (through ODFW) .