Science Advisory Report 2020/044

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

Summary

Tenacibaculum maritimum Transfer Risk Assessment

• Tenacibaculum maritimum 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 2002 to 2018 fish health data on salmon farms; and the current state of knowledge of T. maritimum, including fish health data surveys and studies from enhanced and wild salmon in British Columbia.
• Between 2002 and 2018, clinical signs of the disease mouthrot (yellow mouth) have been reported on Atlantic Salmon farms in the Discovery Islands area 13 of 17 years through the Fish Health Audit Surveillance Program and/or as a Fish Health Event.
• Susceptibility of Sockeye Salmon is unknown, therefore it was assumed for the risk assessment that Sockeye Salmon are susceptible to T. infection.
• The overall likelihood assessment concluded that Fraser River Sockeye Salmon would become infected with T. maritimum released from Atlantic Salmon farms in the Discovery Islands area is unlikely for juveniles and very unlikely for adults. The uncertainties for the different steps ranged from high uncertainty to high certainty.
• The consequence assessment concluded that the potential magnitude of impacts on the abundance and diversity of Fraser River Sockeye Salmon is negligible given that mortality attributable to T. maritimum infection from Atlantic Salmon farms was estimated to be less than 1%. This conclusion was made with reasonable uncertainty.
• The main sources of uncertainties, the approach taken to address each of them, and their potential impacts on the results/rankings and the final risk estimates were included in the risk assessment. The most conservative assumptions and estimates (e.g., plausible worst-case scenarios) were applied to the final rankings/conclusions wherever possible.

This risk assessment was informed by a summary of the current state of knowledge on Tenacibaculum maritimum and mouthrot (Wade and Weber, 2020). The key elements of this review are summarized below.

Characterization of Tenacibaculum maritimum and mouthrot

• Tenacibaculum maritimum has a global distribution. It is a naturally occurring member of the marine bacterial community. This species had been found in association with fish (i.e., on the surfaces of fish) with and without signs of disease. It is considered to be an opportunistic pathogen of marine fishes.
• Mouthrot (yellow mouth) is a disease of juvenile farmed salmonids in British Columbia (BC) and the United States (US) Pacific Northwest. Clinical signs of this disease are yellow plaques in the mouth. This disease is caused by infection with Tenacibaculum maritimum.
• Mouthrot has not been reported in any wild salmon species in BC or Washington State. However, it has been rarely observed in farmed Chinook Salmon.
• Genetic material of T. maritimum was detected in samples from 5 of 2,006 wild juvenile Fraser River Sockeye Salmon. Note that this detection does not necessarily indicate infection or disease.
• Mouthrot is usually diagnosed in juvenile Atlantic Salmon in the first few months after seawater entry. Mouthrot has not been diagnosed in farmed salmonids after the first year at sea.
• The mechanisms by which juvenile Atlantic Salmon die from T. maritimum infection are unknown.
• Tenacibaculum maritimum causing mouthrot has been demonstrated to transfer between fish in a laboratory experiment. However, neither the timing of shedding nor the rate of shedding during infection have been described.
• There is no commercially available vaccine for T. maritimum in Atlantic Salmon. Treatment of mouthrot is the largest use of antimicrobials by the BC salmon farming industry and it is used with success.

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.

Science Advisory Report 2020/044 (PDF, 1,045 KB)

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