Quantifying Pathogen <scp>eDNA</scp> at a Trap‐and‐Haul Facility for Adult Chinook Salmon ( <i>Oncorhynchus tshawytscha</i> )

Adult Chinook salmon (Oncorhynchus tshawytscha) trap-and-haul operations exist in rivers across the Pacific Northwest with the objective of re-establishing salmon populations above impoundments. These efforts are complicated by high prespawn mortality (PSM), which can hinder restoration by reducing reproductive success. Recent improvements at trapping facilities in the Willamette River Basin, Oregon, USA, may reduce PSM by streamlining trap-and-haul operations to reduce handling stress. However, studies suggest that PSM is also related to factors associated with haul operations, including fish density and exposure to external stressors such as pathogens. Environmental DNA (eDNA) can help detect pathogens in confined systems with high fish-to-water ratios. In this work, pathogen eDNA samples were collected from anaesthesia and transport tank waters at a trap-and-haul facility in 2017 and 2021 and analysed to assess pathogen presence. Pathogen DNA was detected in both tanks prior to fish addition, including Aeromonas salmonicida, Renibacterium salmoninarum, Salmincola californiensis, and Ceratonova shasta. After fish entered the anaesthesia tank, average pathogen concentrations increased in magnitude for A. salmonicida (73%), S. californiensis (3165%), C. shasta (1619%), and decreased for R. salmoninarum (-57%). Results suggest shedding of horizontally transmitted pathogens, particularly A. salmonicida, during transport.