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.

Omid Nekouei1*, Raphael Vanderstichel1, Karia H. Kaukinen2, Krishna Thakur1, Tobi Ming2, David A. Patterson3, Marc Trudel4, Chrys Neville2, Kristina M. Miller2,5

1 Department of Health Management, University of Prince Edward Island, Charlottetown, PE, Canada

2 Pacific Biological Station, Fisheries and Oceans Canada, Nanaimo, BC, Canada

3 Fisheries and Oceans Canada, Cooperative Resource Management Institute, School of Resources and Environment Management, Simon Fraser University, Burnaby, BC, Canada

4 St. Andrews Biological Station, Fisheries and Oceans Canada, St. Andrews, NB, Canada

5 Forest and Conservation Sciences, University of British Columbia, Vancouver, BC, Canada

More details on sample collection

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Sampling of juvenile Coho salmon was carried out between 2008 and 2018 (11 year-classes), over the first nine months of early migration. In freshwater, sampling of wild fish was carried out using beach seining within natal lakes or dip netting at smolt fences, generally before hatchery releases. In hatcheries, fish were sampled just prior to release. In the marine environment, fish were collected via mid-water trawl sampling from the Canadian Coast Guard vessel, WE Ricker, or by trawl or purse seine sampling from smaller fishing vessels contracted by DFO. In general, all fish propagated artificially from eggs and milt from the group of spawners and maintained in a controlled hatchery environment until their release are defined as “hatchery fish”. Wild fish are defined as fish that developed from eggs produced by parents that spawned naturally in the river bed. Overall, 2,655 juvenile Coho were used in our study, including fish sampled in freshwater drainages in the BC Mainland (i.e., Fraser River watershed and central coast) and Vancouver Island, and marine samples within the Salish Sea on the inside waterway between Vancouver Island and the Mainland, and on the west coast of Vancouver Island.