Science Advisory Report 2019/018
Advice from the assessment of the risk to Fraser River Sockeye Salmon due to Yersinia ruckeri transfer from Atlantic Salmon farms in the Discovery Islands area, British Columbia
Yersinia ruckeri Transfer Risk Assessment
- The assessment was conducted using farm-related data from 2002–2017, the current fish health management practices, and considering relevant scientific information.
- The assessment concluded that Y. ruckeri 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 reasonable certainty to high certainty (see below and Table 1).
- Two main factors influenced the attribution of minimal risk:
- it is extremely unlikely that at least one Sockeye Salmon would become infected with Y. ruckeri released from an Atlantic Salmon farm in the Discovery Islands area. because Y. ruckeri is primarily a freshwater pathogen (low survival in saltwater); and
- even in the extremely unlikely event that wild Sockeye Salmon would become infected with Y. ruckeri due to Atlantic Salmon farms in the Discovery Islands area, the magnitude of consequences to both Fraser River Sockeye Salmon abundance and diversity would be negligible because the infection would not be expected to spread within wild populations.
- The overall likelihood assessment, including separate farm infection, release, exposure, and infection assessments, was supported by the following key findings:
- Yersinia ruckeri is primarily a freshwater trout pathogen with survival greatly reduced in the marine environment;
- limited presence of Y. ruckeri and/or enteric redmouth disease (ERM) on Atlantic Salmon farms in the Discovery Islands area (4 of 16 years);
- Atlantic Salmon are susceptible to Y. ruckeri infections and ERM;
- there is no information on Sockeye Salmon susceptibility;
- Yersinia ruckeri has been isolated from Sockeye Salmon, but disease or outbreaks have not been confirmed in either freshwater or in seawater;
- there is temporal overlap of out-migrating juvenile lake-type Fraser River Sockeye Salmon with reports of Y. ruckeri on farms in the Discovery Islands area; however, there is no temporal overlap with adults.
- Uncertainty: Certainty in this risk assessment is limited by the lack of knowledge about:
- the persistence, infectivity and virulence of Y. ruckeri in seawater or, in salmonids in BC once outside of freshwater;
- shedding rates in Y. ruckeri infected Atlantic Salmon;
- the susceptibility, and the minimum infectious and lethal doses of Y. ruckeri in Sockeye Salmon.
- 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 Y. ruckeri infection on that farm;
- infected Atlantic Salmon shed Y. ruckeri into the surrounding environment;
- Sockeye Salmon are susceptible to Y. ruckeri; and
- current management practices are followed and will be maintained, including vaccination in hatcheries supplied with surface water, surveillance for early detection and biosecurity measures.
Characterization of Yersinia ruckeri and enteric redmouth disease (ERM)
- Infection with Y. ruckeri can lead to the development of ERM in a broad range of host-species including several salmonid and non-salmonid species, but primarily in freshwater.
- Rainbow Trout (O. mykiss) is considered to be the most susceptible species to Y. ruckeri and Atlantic Salmon considered less susceptible. All salmonid life history stages are susceptible, but the disease is most acute in Rainbow Trout fry and fingerlings (freshwater) and presents as chronic in older larger fish.
- Yersinia ruckeri is horizontally transmitted, therefore, susceptible fish can become infected through contact with infected fish, contaminated water and/or contaminated equipment.
- The incubation period of Y. ruckeri reported in Rainbow Trout and Atlantic Salmon ranges from 5 to 10 days at 13–15 °C under experimental conditions conducted in freshwater. There are no available saltwater immersion challenges, and no studies on Sockeye Salmon.
- Under experimental conditions, carrier fish (steelhead trout (O. mykiss)) did not shed enough bacteria to cause infection in other steelhead trout unless they were stressed.
- Minimum infectious or lethal doses of Y. ruckeri in Sockeye Salmon have not been determined.
- There are no morbidity or mortality data associated with Y. ruckeri infection in wild fish.
- The survival of Y. ruckeri in the aquatic environment is dependent on salinity. Y. ruckeri has been reported to survive in saltwater for up to 32 days (Barnes, 2011). At lower salinities (0–20 ppt), Y. ruckeri has been reported to survive for four months.
This Science Advisory Report is from the November 6–8, 2018 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.
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