Research Document 2017/075

Assessment of the risk to Fraser River Sockeye Salmon due to Infectious Hematopoietic Necrosis Virus (IHNV) transfer from Atlantic Salmon farms in the Discovery Islands, British Columbia

By Mimeault, C., Wade, J., Foreman, M.G.G., Chandler, P.C., Aubry, P., Garver, K.A., Grant, S.C.H., Holt, C., Jones, R.M., Johnson, S.C., Trudel, M., Burgetz, I.J., and Parsons, G.J.

Abstract

An assessment was conducted to determine the extent of the risk to Fraser River Sockeye Salmon due to the potential transfer of Infectious Hematopoietic Necrosis Virus (IHNV) from Atlantic Salmon farms in the Discovery Islands of British Columbia under current fish health management practices..

The risk assessment was conducted in three main steps. First, an assessment of the likelihood that wild fish populations would be infected and become diseased due to IHNV released from Atlantic Salmon farms operating in the Discovery Islands under current fish health management practices (which includes assessments of the likelihood of disease, release, exposure and infection). Second, an assessment of the magnitude of consequences of IHNV attributable to Atlantic Salmon farms on Fraser River Sockeye Salmon (which includes assessments of the magnitude of consequences to abundance and diversity). Third, the combination of the likelihood and consequence assessments in risk matrices to obtain final risk estimates..

IHN outbreaks on Atlantic Salmon farms in the Discovery Islands are very unlikely given the current fish health management practices which include efficient vaccination for IHN. However, if an outbreak were to happen, IHNV would be released into the marine environment as infected farmed Atlantic Salmon shed the virus. In the event that IHNV is released from Atlantic Salmon farms, juvenile Fraser River Sockeye Salmon, a susceptible host, would be very likely to be exposed to the virus during their migration either by swimming into net pens holding farmed Atlantic Salmon or by swimming through IHNV plumes dispersed from the infected farm. However, the estimated maximum waterborne concentrations of IHNV under current fish health management practices would be several orders of magnitude lower than the minimum lethal dose of IHNV for juvenile Sockeye Salmon and infection would therefore be extremely unlikely. Overall, it was concluded that it is extremely unlikely that wild fish populations would be infected and become diseased due to IHNV released from Atlantic Salmon farms operating in the Discovery Islands under current fish health management practices.

There were no data to directly support the potential impact of IHNV on wild Fraser River Sockeye Salmon populations. Consequently, data from IHN epizootics in Sockeye Salmon smolts in Alaska which resulted in 8% mortality were used as a proxy for the potential magnitude of consequence on Fraser River Sockeye Salmon. It was concluded with high uncertainty that the potential magnitude of consequences to the abundance and diversity of Fraser River Sockeye Salmon resulting from IHNV infection attributable to Atlantic Salmon farms would be moderate representing between 5 and 10% reduction in the number of returning adult Fraser River Sockeye Salmon and no loss of Fraser River Sockeye Salmon conservation units.

Overall, the assessment concluded that IHNV attributable to Atlantic Salmon farms in the Discovery Islands poses minimal risk to Fraser River Sockeye Salmon abundance and diversity under the current fish health management practices. This risk assessment includes considerable sources of uncertainties, both due to intrinsic variability and knowledge gaps.

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