|Abstract or Summary
- Spring Chinook salmon, Oncorhynchus tshawytscha, are transported above dams in the Willamette River to provide access to blocked spawning habitat. However, 30-95% of these transplants may die before spawning in some years. To varying degrees, salmon in other tributaries--both blocked and unblocked--have similar prespawn mortality (PSM). Our study determined if holding in constant temperature, pathogen free conditions prior to spawning increased survival to spawn. In addition, we evaluated pathogens as a potential cause of PSM.
Adult Chinook were captured early and late in the season from the lower Willamette River and from upper river tributaries and held in constant, cool temperature (13 ˚C), pathogen-free water at Oregon State University. Additional fish were sampled at time of transport from each of the collection sites. Finally, recent mortalities were collected from river surveys on holding and spawning reaches above traps. Necropsies were performed on all fish, and samples were processed for histology. Held fish were spawned to determine if progeny were viable.
Held fish were less likely to be a PSM than fish that were outplanted to the river. However, bacterial infections were more prevalent in held fish than outplanted fish. Consistent with these observations, PSM in held fish was more likely to have higher burdens of pathogens than spawned fish. Held spawned fish were more likely to have Myxobolus sp. brain infections and less likely to be infected with the kidney myxozoan Parvicapusla minibicornis than spawned outplanted fish. The equal likelihood of other pathogens for held and outplanted spawned fish suggests interactive effects determine survival and that holding at 13 ˚C prevented expression of lethal pathogenesis. Progeny of held fish from all locations and collection dates were viable. Overall, holding could be a viable method to reduce PSM, but issues of transport stress, proliferative disease such as those caused by bacteria, and antibiotics remain.