Assessment of the risk posed to wild Atlantic Salmon population abundance and genetic character

Direct genetic interactions, which arise from the interbreeding between wild and escaped farmed Atlantic Salmon and their offspring, pose a threat to the genetic integrity and abundance (fitness) of wild Atlantic Salmon populations. The risks posed to wild Atlantic Salmon population abundance and genetic character in Atlantic Canada by direct genetic interaction with escapes from Atlantic Salmon aquaculture were assessed in three main steps: a likelihood assessment, a consequence assessment, and a risk estimation. The risks were assessed for wild salmon populations in the six designatable units (DUs) in Atlantic Canada in proximity to Atlantic Salmon net pen aquaculture. These populations were assessed in 2010 by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) for recommended listing as Endangered or Threatened under Canada’s Species at Risk Act. The Inner Bay of Fundy DU has been listed as Endangered since 2003. The risk assessment used multiple peer-reviewed analytical approaches and both domestic and international data sources. These included the use of available demographic and dispersal modeling, reports of escape events by industry, detections of escapees from in river monitoring or opportunistic sampling, and results of genetic screening for interbreeding. The likelihood assessment was carried out in three steps: the likelihoods of release (escape), exposure (freshwater entry), and interbreeding. The results were then combined to determine an overall likelihood. Each step assumed that 2011 to 2021 management practices were maintained and production levels used in the models followed either transfer records or maximum allowable production levels, recognizing the maximum allowable production levels were not achieved and wild population sizes were likely overestimated. The consequence assessment determined the potential impacts to wild Atlantic Salmon population abundance and genetic character, depending on the number of escaped Atlantic Salmon estimated to have entered rivers and interbred with wild Atlantic Salmon. The risk to abundance across modeled escape rates was low for Nova Scotia Southern Upland East, Inner Bay of Fundy, and South Newfoundland East, low to high for Nova Scotia Southern Upland West and South Newfoundland West, and medium to high for Outer Bay of Fundy DUs. The risk to genetic character across modeled escape rates was low for Nova Scotia Southern Upland East and South Newfoundland East, medium for Inner Bay of Fundy, medium to high for Nova Scotia Southern Upland West, and high for Outer Bay of Fundy and South Newfoundland West DUs. Among the model variables, the level of risk related to direct genetic interactions increased by proximity to, and intensity of, aquaculture operations, and by declining wild population size and worsening conservation status; risk decreased by farming sterile fish. Based on peer-reviewed literature and available data, certainty at each step of the likelihood and consequence assessments ranged from high certainty to reasonable certainty. Results and conclusions of this analysis were generally consistent with observations from other jurisdictions spanning the North Atlantic. To assess what level of mitigation may be required to reduce risk, simulations estimated the effects of reducing escapees for the level of production assumed here. In general, where risk was assessed to be high, simulations estimated that a reduction of 50% or greater of escapees was required to reduce the risk to low. The conclusions of this risk assessment should be regularly reviewed and revised as new and relevant information becomes available, such as changes to industry practices, production levels, escape rates, or sizes of wild salmon populations. Further research studies and monitoring are needed to improve model accuracy and validate the model assumptions and outputs. An overview of potential mitigation measures led to a conclusion that eliminating all human errors and equipment failures associated with Atlantic Salmon net pen escapes is not realistic. Therefore, no single measure will necessarily eliminate risk, but cumulative effects of multiple measures will contribute to an effective reduction in escapes and risk. Technologies such as fully closed containment or fully sterile populations would eliminate direct genetic interactions between escaped and wild Atlantic Salmon but the utility of these approaches has not been widely demonstrated and assessed. Demonstration of efficacy of implemented mitigation measures will require that experts evaluate results from comprehensive monitoring programs.

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