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Evaluation of Pathogen Risks and Testing Considerations for Chinook Salmon Egg Movements Between New Zealand and California

Open-File Report 2026-1065
Land Management Research Program and Species Management Research Program
Prepared in cooperation with California Department of Fish and Wildlife, Anchor QEA, and HDR
By: , , and 
https://doi.org/10.3133/ofr20261065

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Executive Summary 

Oncorhynchus tshawytscha (Walbaum in Artedi, 1792; Chinook salmon) were historically abundant in the McCloud River but are now extirpated from this tributary owing to dam construction and lack of passage. Planning efforts to restore populations above Shasta and Keswick Dams are currently underway, including an evaluation of potential source populations. One potential source is New Zealand Chinook salmon, which are believed to have originated from tributaries of the Sacramento River. These fish could be returned to California if reintroduction risks, including risks of pathogen introduction, could be sufficiently mitigated. The U.S. Geological Survey was contracted to provide scientific support for reintroduction efforts, including evaluating the risks of pathogen transmission via the movement of Chinook salmon eggs from New Zealand to the McCloud River. This report estimates pathogen risks associated with egg movement and considers epidemiological and biosecurity measures to minimize these risks.

Pathogen risks associated with the movement of Chinook salmon eggs from New Zealand were evaluated based on pathogen virulence, transmission route, and geographic distribution. These criteria identified 14 moderate- and high-risk pathogens out of the 30 pathogens evaluated. Pathogen species and strains were considered high risk if they have the potential for vertical transmission (that is, transmission from parent to offspring), are moderately or highly virulent, and are exotic to the Sacramento River Basin. According to these criteria, we identified the following pathogens as high risk:

  • New Zealand rickettsia-like organisms 1 and 2.—These bacterial pathogens have been associated with mortality events in farmed Chinook salmon from the South Island of New Zealand but have not been detected in other regions.
     
  • Pilchard orthomyxovirus (POMV).—POMV has been detected in Sardina pilchardus (Walbaum, 1792; pilchards) and Salmo salar (Linnaeus, 1758; Atlantic salmon) from the coasts of southern Australia and Tasmania. POMV can cause relatively high mortality rates and may be indirectly transmitted via contaminated water sources.
     
  • Infectious pancreatic necrosis virus (IPNV).—IPNV has a wide geographic distribution and is present in the Sacramento River Basin, but the IPNV-like viruses detected in Australia and New Zealand are unique from those found in the United States.
     
  • Yersinia ruckeri.—This bacterial pathogen is the causative agent of enteric redmouth disease and has a widespread geographic distribution. However, the strains that are present in Australia and New Zealand are unique from those found in North America.

Strategic use of testing and biosecurity measures can minimize pathogen risks associated with the movement of eggs. The most effective measures include iodophor treatment of eggs to remove external pathogens, testing of all the adult fish from which gametes are obtained, and a quarantine period after transport to confirm pathogen testing results. Additional measures to enhance biosecurity could include testing the quarantined fish following emergence and (or) developing a fish health history of the source population through pathogen monitoring.

Suggested Citation

Couch, C.E., Powell, D.B., and Lovy, J., 2026, Evaluation of pathogen risks and testing considerations for Chinook salmon egg movements between New Zealand and California: U.S. Geological Survey Open-File Report 2026–1065, 18 p., https://doi.org/10.3133/ofr20261065.

ISSN: 2331-1258 (online)

Table of Contents

  • Acknowledgments
  • Executive Summary
  • 1. Introduction
  • 2. Risk Assessment Criteria for Fish Pathogens
  • 3. Relative Risk Categories for Fish Pathogens
  • 4. Profiles of High- Risk Pathogens
  • 5. Risk Reduction Approaches
  • 6. Combined Measures to Minimize Risk
  • 7. Conclusions
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Evaluation of pathogen risks and testing considerations for Chinook salmon egg movements between New Zealand and California
Series title Open-File Report
Series number 2026-1065
DOI 10.3133/ofr20261065
Publication Date March 03, 2026
Year Published 2026
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Fisheries Research Center
Description Report: vi, 18 p.
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Additional publication details