COS 183-3 - CANCELLED - Pathogen Risk to Juvenile Spring-run Chinook Salmon (Oncorhynchus tshawytscha) in the Upper Klamath Basin, to Inform Re-introduction Efforts

Abstract: The Upper Klamath Basin (UKB) consists of a series of tributaries and lakes in Southern Oregon. Historically inhabited by spring-run Chinook salmon (Oncorhynchus tshawytscha), dam development in the early-20th Century cut off all anadromous fish movement into the UKB. With the planned removal of the four lowermost dams in the Klamath River, spring-run Chinook will once again have access to UKB habitats. While pathogen presence is well documented in a variety of fish species, the absence of spring-run Chinook salmon in the UKB has led to a lack in understanding the risks associated with pathogens in spring-run Chinook. Using sentinel fish exposures, we describe the risks of potential pathogens that may be encountered upon re-introduction. In spring 2022 (once in April and once in May), four cages each containing twenty spring-run Chinook salmon, brood year 2020 smolts, were held at two sites, Klamath Lake outlet (KPP) and Williamson River (WNC). At 3-days and 7-days post-exposure two cages at each site were removed and the fish were transported to the J.L. Fryer Aquatic Animal Health Laboratory. Each removal was followed by an immediate necropsy of half of each group of fish. The other half were monitored for 28 days, followed by euthanasia and pathogen sampling. Fish tissues were tested using bacterial cultures, cell cultures for virus, molecular analyses, and wet mount observations for external parasites. Renibacterium salmoninarum, Ceratonova shasta, Ichthyophthirius multifiliis and Trichodina spp. were detected at both UKB sites with fluctuations in prevalence between months. Flavobacterium columnare was detected only at KPP during May, whereas Parvicapsula minibicornis, Nanophyetus salmonicola and viruses were not detected. C. shasta, F. columnare, and I. multifiliis were more prevalent in May, associated with higher temperatures. C. shasta was determined to be ITS-1 type II and histology of C. shasta-infected intestinal tissues indicated that Chinook are a dead-end host. Since climate models predict an increase in water temperatures and lower summer flows in the Klamath mainstream, the detection of F. columnare, and I. multifiliis raises concerns as disease is more severe at elevated temperatures and should be considered during re-introduction efforts.