Abstract
Infectious diseases are potential contributors to decline in Coho salmon (Oncorhynchus kisutch) populations. Although pathogens are theoretically considered to pose higher risk in high-density rearing environments like hatcheries, there is no direct evidence that hatchery-origin Coho salmon increase the transmission of infectious agents to sympatric wild populations. This study was undertaken to compare prevalence, burden, and diversity of infectious agents between hatchery-reared and wild juvenile Coho salmon in British Columbia (BC), Canada. In total, 2,655 juvenile Coho salmon were collected between 2008 and 2018 from four regions of freshwater and saltwater in BC. High-throughput microfluidics qPCR was employed for simultaneous detection of 36 infectious agents from mixed-tissue samples (gill, brain, heart, liver, and kidney). Thirty-one agents were detected at least once, including ten with prevalence >5%. Candidatus Brachiomonas cysticola, Paraneuclospora theridion, and Parvicapsula pseudobranchiocola were the most prevalent agents. Diversity and burden of infectious agents were substantially higher in marine environment than in freshwater. In Mainland BC, infectious burden and diversity were significantly lower in hatchery smolts than in wild counterparts, whereas in other regions, there were no significant differences. Observed differences in freshwater were predominantly driven by three parasites, Loma salmonae, Myxobolus arcticus, and Parvicapsula kabatai. In saltwater, there were no consistent differences in agent prevalence between hatchery and wild fish shared among the west and east coasts of Vancouver Island. Although some agents showed differential infectious patterns between regions, annual variations likely contributed to this signal. Our findings do not support the hypothesis that hatchery smolts carry higher burdens of infectious agents than conspecific wild fish, reducing the potential risk of transfer to wild smolts at this life stage. Moreover, we provide a baseline of infectious agents in juvenile Coho salmon that will be used in future research and modeling potential correlations between infectious profiles and marine survival.
Highlights
Coho salmon (Oncorhynchus kisutch) is one of seven Pacific salmon species with cultural, recreational, and economic significance to the residents of the North East Pacific region, especially in Canada [1]
This study provides a baseline dataset on the detection of infectious agents in wild and hatchery-reared Coho salmon, spanning a decade of sampling efforts
Our study does not support the hypothesis that hatchery fish carry higher diversity and burden of infectious agents compared to sympatric wild salmon species [15,16,17]
Summary
Coho salmon (Oncorhynchus kisutch) is one of seven Pacific salmon species with cultural, recreational, and economic significance to the residents of the North East Pacific region, especially in Canada [1]. Distinct populations of Coho salmon are found in the coastal streams and rivers of British Columbia (BC) and most of them have a three-year lifecycle [1,2]. Wild Coho salmon populations in the United States and Canada have been in decline for over three decades [3,4,5]. In the Interior Fraser River watershed, the abundance of Coho salmon have decreased by more than 60% since 1996, with marine survival reaching an alltime low of 0.1–0.3% in 2014 [2]. Returns for some of Coho salmon populations are so low that they have been listed as endangered or threatened by the Committee on the Status of Endangered Wildlife in Canada or through the Endangered Species Act in the United States [2,6]. Several factors are believed to have contributed to population declines, including climate change, food availability in the marine environment, infectious diseases, predation, fishing, land-use activities, and synergistic effects among these factors [4,5,7]
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