Science Advisory Report 2019/017
Advice from the assessment of the risk to Fraser River Sockeye Salmon due to Aeromonas salmonicida transfer from Atlantic Salmon farms in the Discovery Islands area, British Columbia
Aeromonas salmonicida 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 A. salmonicida 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 certainty to reasonable uncertainty (see bullets below and Table 1).
- Two main factors influenced the attribution of minimal risk:
- in the likelihood assessment, it was determined that it is very unlikely that Fraser River Sockeye Salmon would become infected with A. salmonicida released from an Atlantic Salmon farm in the Discovery Islands area. The uncertainties for the different likelihood steps ranged from high certainty to reasonable uncertainty; and
- in the consequences steps, it was determined in the very unlikely event that Fraser River Sockeye Salmon would become infected with A. salmonicida 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 with reasonable uncertainty.
- The overall likelihood assessment, including separate farm infection, release, exposure, and infection assessments, was supported by the following key findings:
- evidence of A. salmonicida and/or furunculosis on Atlantic Salmon farms in the Discovery Islands area;
- the temporal overlap of migrating juvenile and adult Fraser River Sockeye Salmon with reports of A. salmonicida on Atlantic Salmon farms in the Discovery Islands area;
- infected Atlantic Salmon shed A. salmonicida into the surrounding environment at all stages of infection and after death;
- the estimated maximum A. salmonicida waterborne concentration on farms or dispersed from farms is not expected to cause infection in migrating Sockeye Salmon; and
- an infection with A. salmonicida attributable to Atlantic Salmon farms, would not be expected to spread within Fraser River Sockeye Salmon.
- 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 minimum infectious dose of A. salmonicida for Sockeye Salmon and other Pacific salmon species;
- furunculosis-related mortality in susceptible Pacific salmon;
- the spread of infection within migrating populations of Pacific salmon; and
- the consequences to abundance and diversity of Fraser River Sockeye Salmon resulting from sub-lethal effects of A. salmonicida infection.
- For the purpose of the 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 A. salmonicida infection on that farm;
- Atlantic Salmon and Sockeye Salmon are equally susceptible to A. salmonicida infections and furunculosis; and
- current management practices are followed and will be maintained, including vaccination of 100% of farmed Atlantic Salmon to prevent furunculosis, surveillance for early detection and biosecurity measures.
Additionally, this risk assessment was informed by a summary of the current state of knowledge related to A. salmonicida and furunculosis (Boily et al., 2019) of which the most relevant elements are summarized below.
Characterization of Aeromonas salmonicida and furunculosis
- Infection with A. salmonicida can lead to the development of furunculosis, a septicemic bacterial disease, in salmonid fishes in freshwater, brackish and marine environments.
- Information about interspecific and intraspecific variability in susceptibility is limited; however, all salmonid species are considered susceptible to A. salmonicida infection and disease.
- Infection with A. salmonicida and the development of disease can occur in all life stages of fish; however, there are insufficient data to assign different susceptibilities to different life history stages of Pacific salmon.
- A. salmonicida is transmitted horizontally, therefore, susceptible fish can become infected through contact with: infected fish, contaminated water and/or contaminated equipment.
- The bacterium is shed at all stages of infection including dead hosts. Shedding rates have been reported in Atlantic Salmon in seawater.
- The survival of A. salmonicida in raw seawater varies from 2 to 26 days and is dependent upon physical conditions including temperature, salinity and visible and ultraviolet radiation.
- Minimum infectious or lethal doses of A. salmonicida in Sockeye Salmon could not be found in the literature.
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 Scheduleas they become available.
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