The spatial distribution of infectious agents in wild Pacific salmon along the British Columbia coast.

Although infectious agents can act as strong population regulators, knowledge of their spatial distributions in wild Pacific salmon is limited, especially in the marine environment. Characterizing pathogen distributions during early marine residence, a period considered a survival bottleneck for Pacific salmon, may reveal where salmon populations are exposed to potentially detrimental pathogens. Using high-throughput qPCR, we determined the prevalence of 56 infectious agents in 5719 Chinook, 2032 Coho and 4062 Sockeye salmon, sampled between 2008 and 2018, in their first year of marine residence along coastal Western Canada. We identified high prevalence clusters, which often shifted geographically with season, for most of the 41 detected agents. A high density of infection clusters was found in the Salish Sea along the east coast of Vancouver Island, an important migration route and residence area for many salmon populations, some experiencing chronically poor marine survival. Maps for each infectious agent taxa showing clusters across all host species are provided. Our novel documentation of salmon pathogen distributions in the marine environment contributes to the ecological knowledge regarding some lesser known pathogens, identifies salmon populations potentially impacted by specific pathogens, and pinpoints priority locations for future research and remediation.