Cooperative Fish and Wildlife Research Units Program: Oregon
Education, Research and Technical Assistance for Managing Our Natural Resources

Prespawn mortality (PSM) presents a major problem for population recovery of spring Chinook Salmon (Oncorhynchus tshawytscha) of the Willamette River basin in Oregon. In certain reaches and years, pre-spawning mortality has exceeded 90%. An archived collection of histologic slides of intestines from over 664 spring Chinook salmon collected since 2009 were evaluated. These included tissues collected from live, PSM, as well as post-spawned fish from the river, and artificially spawned broodstock fish held in captivity that were collected earlier in the summer. One consistent change observed during histopathological evaluation was degeneration of the intestinal epithelium and loss of villous structure, with concurrent severe enteritis. We discovered a natural progression of decline in epithelial integrity (EI) spawning and subsequent death. The majority of apparently healthy fish collected from the river in the summer exhibited high EI scores, with a mean of 68% intact epithelium across the study. Sexually mature fish (“broodstock”) that were spawned in captivity in September exhibited variable EI scores, with a mean EI score of about 35%. Fish that spawned naturally in the river and died later in the year (post-spawn river; late September – October), exhibited severe loss of EI, with a mean EI score of about 14%. When accounting for calendar day, there was a prominent accelerated loss of the intestinal epithelium strongly associated with PSM fish – i.e., while collected mostly in August, the intestines had similar EI scores (mean 13.5) to fall-collected post-spawn river. Hence, we suggest this accelerated gut degeneration is linked to premature senescence and is a histologic marker of PSM. Regarding parasites, Ceratonova shasta and Enterocytozoon schreckii were common in all groups of the salmon, although neither were statistically linked to either PSM or a decline in EI.