Science Advisory Report 2019/019

Advice from the assessment of the risk to Fraser River Sockeye Salmon due to Renibacterium salmoninarum transfer from Atlantic Salmon farms in the Discovery Islands area, British Columbia

Summary

Renibacterium salmoninarum Transfer Risk Assessment

• The assessment was conducted using farm-related and environmental data from 2002–‍2017, based on the current fish health management practices, and considering relevant scientific information.
• The assessment concluded that R. salmoninarum attributable to Atlantic Salmon (Salmo salar) farms operating in the Discovery Islands area poses minimal risk to Fraser River Sockeye Salmon (Oncorhynchus nerka) abundance and diversity. During this assessment uncertainties were evaluated at each step and ranged from high uncertainty to high certainty (see bullets below and Table 1).
• Two main factors influenced the attribution of minimal risk:
• in the likelihood steps, it was determined that it is extremely unlikely that Fraser River Sockeye Salmon would become infected with R. salmoninarum released from an Atlantic Salmon farm in the Discovery Islands area with uncertainties ranging from high certainty to reasonable uncertainty; and
• in the consequence steps, it was determined in the very unlikely event that Fraser River Sockeye Salmon would become infected with R. salmoninarum due to Atlantic Salmon farms in the Discovery Islands area, the infection would not be expected to spread within wild populations, hence the magnitude of consequences to both Fraser River Sockeye Salmon abundance and diversity would be negligible. The associated uncertainty ranged from reasonable uncertainty to high uncertainty.
• The overall likelihood assessment, including separate farm infection, release, exposure, and infection assessments, was supported by the following key findings:
• Renibacterium salmoninarum and/or bacterial kidney disease (BKD) has been detected on Atlantic Salmon farms in the Discovery Islands area;
• Sockeye, Chinook (O. tshawytscha), Chum (O. keta), Coho (O. kisutch), Pink (O. gorbuscha) and Atlantic salmon are susceptible to R. salmoninarum infections and BKD;
• there is a temporal overlap of migrating juvenile and adult Fraser River Sockeye Salmon and other susceptible Pacific salmon species (Coho, Chinook Chum and Pink salmon) with reports of R. salmoninarum on farms in the Discovery Islands area;
• it was estimated that the potential maximum R. salmoninarum waterborne concentrations in net pens were two orders of magnitude below the lowest reported infectious doses for R. salmoninarum in Chinook Salmon (Rhodes and Mimeault, 2019).
• Uncertainty: Certainty in this assessment is limited by the lack of knowledge about:
• how long and how close populations of wild juvenile and adult Pacific salmon are in the vicinity of Atlantic Salmon farms;
• the prevalence of R. salmoninarum-infected Atlantic Salmon on farms in the Discovery Islands area;
• shedding rates of R. salmoninarum infected farmed Atlantic Salmon;
• the minimum infectious and lethal doses of R. salmoninarum for Sockeye Salmon;
• the proportion of the wild susceptible fish population that would be exposed and become infected with R. salmoninarum released from an Atlantic Salmon farm in the Discovery Islands area;
• BKD mortality in wild Sockeye Salmon and other susceptible Pacific salmon species;
• the consequences to the abundance and diversity of Fraser River Sockeye Salmon from sub-lethal effects of R. salmoninarum infection; and
• the consequences to subsequent generations related to presence of R. salmoninarum in spawning populations.
• For the purpose of the risk assessment, a number of key assumptions were made:
• positive detection of the pathogen on an Atlantic Salmon farm in the Discovery Islands area is evidence of R. salmoninarum infection on that farm;
• infected Atlantic Salmon shed R. salmoninarum into the surrounding environment;
• the minimum infectious dose and exposure time for Chinook Salmon was used as a proxy for the minimum infectious dose and exposure time for Sockeye Salmon; and
• current management practices are followed and will be maintained, including broodstock screening, surveillance for early detection and biosecurity measures.

Characterization of Renibacterium salmoninarum and bacterial kidney disease (BKD)

• Infection with R. salmoninarum can lead to the development of BKD in a broad range of salmonids.
• Sockeye, Chinook, and Chum salmon are considered most susceptible to R. salmoninarum, Coho and Atlantic salmon are considered to have an intermediate susceptibility and Lake, Brown, Bull and Rainbow and steelhead trout are considered the least susceptible. There are insufficient data to assign different susceptibilities to different life history stages.
• Renibacterium salmoninarum is both horizontally and vertically transmitted. Therefore, susceptible fish can become infected through contact with infected fish, contaminated water and/or contaminated equipment, as well as become infected with R. salmoninarum passed from female to ova.
• The incubation period of R. salmoninarum in Chinook Salmon under experimental conditions in freshwater ranges from 80 days to 12 months as reported in Chinook Salmon under experimental conditions conducted in freshwater. There are no available seawater immersion studies.
• The highest level of mean shedding by individual juvenile Chinook Salmon challenged with R. salmoninarum were estimated to be 6.5 x 106 and 3.1 x 106 cells per fish per hour at 8°C and 12°C, respectively (Purcell et al., 2016).
• Minimum infectious or lethal doses of R. salmoninarum for Sockeye Salmon have not been determined.
• In Chinook Salmon fry in freshwater, the lowest concentration of R. salmoninarum to cause infection is 7 x 108 cells/m³ at a 24-hour exposure; the lowest concentration required to cause mortality is 3 x 1010 cells/m³ when exposed for 15 minutes.
• BKD has been reported in wild salmonids.
• The survival of R. salmoninarum in the marine environment varies and depends on temperature and nutrient availability. R. salmoninarum has been reported to survive in raw seawater for seven days at 10°C although viability was reduced to and remained at 1% after 24 hours.

This Science Advisory Report is from the November 6–8, 2018 national advisory meeting on Assessment of the risk to Fraser River sockeye salmon due to bacteria causing systemic infections transferred from Atlantic salmon farms located in the Discovery Islands area, British Columbia. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2019/019 (PDF, 697 KB)

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