Health of Pacific salmon overwintering in the open ocean

Ecosystems on land and in the ocean around the North Pacific rely on abundant salmon. Similarly, salmon provide a sustainable source of nutrition, income, and cultural identity for communities.

Unfortunately, many wild salmon stocks have been declining in numbers, despite efforts to halt and reverse this trend by many organizations and governments. Most of these efforts focus on easily accessible coastal waters and rivers, while the factors influencing salmon survival in the remote open ocean are poorly understood, specifically during the winter.

Here we used large-scale molecular screening tools to detect infectious agents and shifts in gene expression of salmon overwintering in the open Pacific Ocean. We found that offshore salmon carry fewer pathogens compared with the coastal waters of Canada.

Coho salmon were an exception in this trend and had elevated levels of infection of three pathogens which resulted in a higher overall infection burden compared with their coastal counterparts.

Of these, two common parasites were identified as key pathogens of all salmon species, suggesting that they are transmitted in the open ocean. Salmon experiencing reduced prey availability, and in some cases warmer temperatures, in their environment showed signs of malnourishment and overall reduced activity of many genes, including ones relating to immunity, possibly to conserve energy.

This pattern was predominantly observed in smaller or lower condition salmon. Such salmon had higher levels of infections and were more often infected with opportunistic pathogens that take advantage of weak individuals, emphasizing a critical period for the survival of salmon.

Together, this study provides the first report on the health, condition, and infections of four Pacific salmon species in the Northeast Pacific during the winter.

We also highlight the cumulative interplay between pathogens and environmental conditions linked to climate. This much-needed information fills a gap in understanding of factors important to survival of salmon in the ocean.

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