Prespawn Mortality in Spring Chinook Salmon in the Upper Willamette River: Potential Causes and Management Strategies

In an effort to reintroduce spring Chinook to blocked spawning habitat, salmon have been transported above both Fall Creek and Dexter Dams. However, around 60-90 % of the transplanted salmon died before spawning in the past few years.  Prespawn mortality is not solely a problem for these transplants; it has also been observed in other mainstem tributaries, although not to these levels.  By examining potential causes of prespawn mortality, we hope to discover if Chinook salmon are physiologically predisposed to pathogens or if an environmental factor such as temperature or parasite burden is responsible.

Freshly dead prespawn mortalities and postspawn fish were collected by Oregon Department of Fish and Wildlife survey crews from Fall Creek and the North Fork Middle Fork of the Willamette River.   Live fish were sampled at Dexter and Fall Creek traps and below Willamette Falls.  Spawned fish from Willamette Hatchery were also sampled.  In addition, fish were held in a cool temperature, pathogen free facility to determine if survival to spawning could be improved. Fish from Fall Creek and Dexter traps were transported to the Fish Performance and Genetics Lab in both June and July. Necropsies were performed on all fish and samples were taken for histology. 

For each of the four parasites most commonly found, prespawn mortalities had a higher severity of infection when compared to successful spawners. Furthermore, the fish sampled at Willamette Falls showed no infection of Nanophyetus in the kidney while fish collected in the upper watershed had massive infections. This suggests that infection with this parasite was likely in the Willamette River above the Falls. 

Holding seemed to reduce prespawn mortality. The fish held longer had a higher survival rate(90% vs 70%), perhaps because they had less exposure time to pathogens and likely never experienced elevated temperatures.