Where do escaped farmed salmon go after unintentional release from aquaculture facilities?

The unintentional release of domesticated salmon poses a significant risk to wild Atlantic salmon populations. In their latest research, Mahlum and colleagues use a hierarchical species distribution model to determine the spread and potential impact of domesticated salmon, following escape events in aquaculture facilities.

Atlantic salmon is a culturally and economically important species that has been subjugated to intense anthropogenic pressures over the last century. These pressures have caused sharp and continued declines over the last 40 years, and created an economic opportunity that has resulted in the domestication of Atlantic salmon for aquaculture production.

Like many species that are cultivated for food consumption, the production of Atlantic salmon has seen substantial industrial growth since the 1970’s. Unfortunately, this growth has often been at the expense of wild salmon populations, with the introduction of new diseases, increased impacts of parasites, or the hybridization (i.e., genetic mixing) of domesticated salmon that escaped net pen facilities with wild salmon.

Hybridization is believed to be one of the main threats to wild populations and has been shown to reduce the overall fitness of the population. However, we currently have a limited understanding of how wild and escaped farmed salmon interact in nature. This has resulted in the use of abatement methods that are largely reactive (e.g., the physical removal of escaped salmon after they entered the river) and often implemented during the spawning season when the potential for hybridization is greatest

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Escaped farmed Atlantic salmon. Photo: Dr. Helge Skoglund, NORCE Norwegian Resource Centre

A better approach in reducing potential introgression would be to implement an ecosystem-based management strategy that considers the interaction of multiple stressors. To do this, however, we first need to understand how the production of aquaculture relates to the number of domesticated salmon found in rivers, to provide insight into the ability of the ecosystem to buffer against unwanted interactions. 

Since 2004, we have monitored important salmon rivers across western Norway and tracked the location and number of escaped farmed salmon. By analyzing which factors describe the number of escaped farmed salmon in rivers (e.g., river discharge or health of wild population), we can then provide evidence-based recommendations to governmental agencies responsible for regulating aquaculture.

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Drift diving personnel are used to quantify the total number of wild and escaped farmed salmon in rivers throughout Norway. Photo: Dr. Helge Skoglund, NORCE Norwegian Resource Centre

Our results show that production limits for salmon (e.g., allowable biomass) next to a river and the size of the wild population are important variables that can be used to improve aquaculture management in important Atlantic salmon areas.   

By understanding the relationship between these factors, we can begin to adjust allowable biomass limits based on the ability of the ecosystem to buffer against unintentional releases of domesticated salmon. In addition, these results can be useful for spatial planning activities that seek to establish new aquaculture sites in areas with existing facilities and in novel marine habitats where domesticated and wild salmon would overlap.

While our study allows us to provide important recommendations to resource managers, it is imperative that industry continues to develop new technologies that minimize their environmental footprint (e.g., triploid fish), in order to meet their ambitious economic goals of industry growth

Read the full Open Access article, Salmon on the lam: Drivers of escaped farmed fish abundance in rivers, in Journal of Applied Ecology

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