# Review of Major and Minor Pathogens of Adult Pacific Salmon (Oncorhynchus spp.) in Freshwater in the Pacific Northwest of North America

## TL;DR

This review explores pathogens affecting adult Pacific salmon during their freshwater spawning phase in the Pacific Northwest, highlighting their impact on salmon health and population sustainability.

## Contribution

The paper provides a systematic categorization of pathogens based on their role in prespawn mortality and transmission risks to offspring.

## Key findings

- Pathogens are categorized by their impact on prespawn mortality and disease severity.
- The review integrates literature and field observations to assess pathogen effects on multiple salmon species.
- Understanding these pathogens is critical for the sustainability of ecologically and economically important salmon populations.

## Abstract

This comprehensive review examines pathogens affecting adult anadromous Pacific salmon (Oncorhynchus spp.) during their terminal freshwater migration and spawning across populations from California through Alaska, including Oregon, Washington, and British Columbia. We systematically reviewed selected pathogens based on their significance to adult salmon health or role in epizootiology, categorizing them by their impact on prespawn mortality (PSM), disease severity, and maternal or ‘egg-associated’ transmission risks to progeny. Our analysis encompasses macroparasites, microparasites, bacteria, and viruses affecting anadromous Pink (O. gorbuscha), Chum (O. keta), Coho (O. kisutch), Sockeye (O. nerka), and Chinook Salmon (O. tshawytscha) and Steelhead Trout (O. mykiss), integrating extensive literature analysis with direct field observations and case studies from representative geographic regions. Understanding pathogen impacts during the spawning life stage is crucial for salmon population sustainability, as the unique semelparous nature of Pacific salmon makes this terminal phase critical for reproductive success and the continuation of these ecologically, economically, and culturally vital species.