Science Advisory Report 2020/043
Advice from the assessment of the risk to Fraser River Sockeye Salmon due to Moritella viscosa transfer from Atlantic Salmon farms in the Discovery Islands area, British Columbia
Summary Moritella viscosa Transfer Risk Assessment
- Moritella viscosa released from Atlantic Salmon (Salmo salar) farms operating in the Discovery Islands area was assessed to pose minimal risk to Fraser River Sockeye Salmon (Oncorhynchus nerka) abundance and diversity under current farm practices.
- The assessment relied on 2012 to 2018 fish health data on salmon farms; and the current state of knowledge of M. viscosa, including fish health data surveys and studies from enhanced and wild salmon in British Columbia.
- Between 2012 and 2018, evidence of M. viscosa infection has been reported on Atlantic Salmon farms in the Discovery Islands area by industry, the Fish Health Audit Surveillance Program and/or as a Fish Health Event in six of seven years.
- Throughout 2012 to 2018, all evidence of M. viscosa on Atlantic Salmon farms in the Discovery Islands area occurred only between December and February. Fraser River Sockeye Salmon are known to migrate through this area between May and October.
- In the overall likelihood assessment, it was concluded that the likelihood that Fraser River Sockeye Salmon would become infected with M. viscosa released from Atlantic Salmon farms in the Discovery Islands area was extremely unlikely for both juveniles and adults because exposure was extremely unlikely given no temporal overlap between M. viscosa occurrences on Atlantic Salmon farms and Fraser River Sockeye Salmon migration timing. Each step of the assessment was ranked with reasonable certainty.
- The consequence assessment was not performed because the outcome of the overall likelihood assessment was extremely unlikely.
- Although there are a few sources of uncertainty related to this risk assessment, they are not expected to change the final risk estimates (outcomes of the assessment).
- If winter ulcer outbreaks occur on Atlantic Salmon farms in the Discovery Islands area during the migration window of Fraser River Sockeye Salmon, a new risk assessment would be warranted.
This risk assessment was informed by a summary of the current state of knowledge related to Moritella viscosa and winter ulcer (Wade and Weber, 2020). The key elements of this review are summarized below.
Characterization of Moritella viscosa and Winter Ulcer
- Winter ulcer is a disease caused by infection with M. viscosa, a bacterium found in the marine environment and commonly reported in farmed Atlantic Salmon in the Northern Atlantic Ocean.
- In BC, winter ulcer has only been reported in farmed Atlantic Salmon. However, molecular evidence of M. viscosa has been detected in two of 2,006 juvenile Fraser River Sockeye Salmon sampled in June, 2013. Note that this detection does not necessarily indicate infection or disease.
- Winter ulcer typically occurs when water temperatures are below 10oC.
- The evidence suggests limited horizontal transmission of M. viscosa between fish.
- Vaccines are available for M. viscosa in Atlantic Salmon which can reduce the incidences of winter ulcer.
- Between 2012 and 2018, winter ulcer was diagnosed at the farm-level in 17 of the 715 fish health audits conducted on Atlantic Salmon farms in BC. Between the end of 2011 and 2018 (excluding the years 2013 to 2015), 13 Fish Health Events were attributed to winter ulcer on Atlantic Salmon farms in BC. Between 2011 and 2018, one mortality event was attributed to winter ulcer on an Atlantic Salmon farm in BC.
This Science Advisory Report is from the December 3-5, 2019 National Peer Review Meeting on the Assessment of the risk to Fraser River Sockeye Salmon due to bacteria causing erosive lesions 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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