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Infectious agents are associated with survival and condition in free-ranging Pacific salmon

A school of fish swimming together.
Juvenile Coho salmon

Many wild salmon populations in the northeast Pacific Ocean, particularly towards the southern extent of their range, have experienced substantial declines in the last three decades.

Over the same period, marine infectious agents (viruses, bacteria, microscopic parasites, fungi) have received increased attention across various plants and animals (corals, starfish, pinnipeds, sea grass). However, few attempts have been made to quantify the impacts of infectious agents on wild salmon population-level survival.

Read the open access paper on the FACETS website.

Most studies on salmon pathogens are conducted in controlled environments such as laboratories or open netpens.

Outside of such controlled environments, in addition to resisting infection, salmon must contend with variable environmental conditions, the energetic demands of foraging, and a plethora of predators. Thus, infection need not cause direct mortality — it is enough to, in tandem with the aforementioned stressors, weaken fish and make them more vulnerable to predators.

This complex ecology is not readily captured in controlled studies, especially when survival is the primary endpoint to document disease.

Researchers studying free-ranging salmon model marine survival with consideration of factors including ocean temperature, predators, fisheries, food availability, and inter-species competition.

Utilizing a similar modeling framework, we conducted an exploratory analysis to determine whether any pathogens found in Chinook and Coho salmon along the British Columbia (BC) coast were correlated with survival or physical condition.

By conducting a study assessing dozens of pathogens in thousands of salmon sampled over a decade, we could (for the first time ever) identify pathogens associated with survival of free-ranging wild salmon in the ocean.

Two of the pathogens ranked highest in our analysis were:

Tenacibaculum maritimum, a bacterium that causes disease in salmon and other cultured marine fish worldwide.

○ Piscine orthoreovirus (PRV), a virus that causes disease in Pacific and Atlantic salmon worldwide, but whose impact on wild salmon in BC is vigorously debated. This is the first empirical evidence that PRV is negatively impacting wild Pacific salmon in BC.

These two pathogens are common on salmon farms in BC, and recent studies provide evidence of transmission from farms to wild salmon.

While other pathogens not linked to salmon aquaculture were also identified, those impacted by human activities such as aquaculture offer the greatest means of control.

Many studies show that high ocean temperatures impact salmon survival, but our research showed that in some cases pathogen presence can be more important. However, changes to climate may alter pathogen–host dynamics and thus increase incidence and impact of some salmon marine diseases. We are already observing some examples of this in freshwater systems.

While researchers have long known that infectious agents exert strong control over the demography of all animal populations, our study is the first to provide evidence of population-level impacts from infectious agents on free-ranging Pacific salmon.

Our results demonstrate that infectious agents deserve a seat at the table in the ongoing effort to determine the factors causing the decline of Pacific salmon populations.

Read the paper — Identification of infectious agents in early marine Chinook and Coho salmon associated with cohort survival by Arthur L. Bass, Andrew W. Bateman, Brendan M. Connors, Benjamin A. Staton, Eric B. Rondeau, Gideon J. Mordecai, Amy K. Teffer, Karia H. Kaukinen, Shaorong Li, Amy M. Tabata, David A. Patterson, Scott G. Hinch, and Kristina M. Miller.




FACETS is a multidisciplinary open access science journal. www.facetsjournal.com

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