Science Advisory Report 2013/050
Potential Effects Surrounding the Importation of European-Origin Cultured Atlantic Salmon to Atlantic Salmon Populations and Habitats in Newfoundland
- Atlantic salmon (Salmo salar) is a species with wide genetic diversity and variation in life history traits. North American and European salmon populations diverged about 500,000 years ago, and the resulting genetic divergence between these populations is significant, as illustrated by their different chromosome number and structure. There is also phenotypic and genetic divergence between Newfoundland and mainland North American salmon, including populations from the Saint John River, which is the primary source of strains used by the Newfoundland aquaculture industry. Because of intensive selection and domestication, farmed salmon are genetically distinct from wild populations.
- Native Newfoundland salmon populations are characterized by a large diversity of anadromous and non-anadromous phenotypes that have adapted to unique environmental conditions in fluvial (riverine) and lacustrine (lake) habitats and are often found in small population units. Recent reviews of the spawning escapement trends for south coast Newfoundland stocks have estimated population declines of an average of 40% over the last three generations (COSEWIC, 2010; DFO, 2012).
- Farmed European-origin Atlantic salmon strains can successfully breed with wild Atlantic salmon throughout the native species range and it is expected that potential fertile farmed European-origin Atlantic salmon escapees in Newfoundland waters would breed successfully with Newfoundland wild Atlantic salmon.
- There is a risk of substantive genetic and phenotypic consequences for native populations if they were to interbreed with escaped farmed European-origin Atlantic salmon (direct genetic effects). Although genetic and phenotypic consequences are difficult to predict, interbreeding would likely result in a subsequent reduction in genetic diversity among populations and reduced fitness within populations that would affect the character, abundance and viability of the native Newfoundland populations.
- These risks would be proportional to the frequency and number of escapees relative to the size and status of the native populations that would be exposed to potential interbreeding. The ability of escaped fish to survive and successfully interbreed with wild Atlantic salmon is influenced by the life-history stage (size) of escapees, their sex, the duration they remain in the culture environment, the time of year (season) they escape, timing of maturation and entry into freshwater.
- The fitness consequences of a given amount of interbreeding across multiple generations may vary depending on whether interbreeding is continuous or episodic.
- Indirect genetic effects (i.e., genetic changes that do not occur through interbreeding but because of changes in the environments that the organisms experience) have been documented in salmonids. Such environmental changes could occur through the presence of farmed European-origin Atlantic salmon in net-pens, whether or not the farmed fish escape. This effect may result in reduced adaptive genetic diversity and altered survival and reproduction in native populations, but a better understanding of the nature and magnitude of indirect genetic effects is required.
- Escaped farmed European-origin Atlantic salmon might also pose ecological threats to native populations. Competition between native juvenile Atlantic salmon and juvenile farm strain origin Atlantic salmon in fresh water will occur due to space limitations, and due to traits associated with farm strains, such as fast growth, large size at age and aggressive behaviour. It is not expected that there would be significant competition in the marine environment. Competition between the two types of fish might also occur on the spawning ground. To the extent that these interactions occur, they are expected to result in reduced fitness in native populations.
- Through technological and operational improvements, a significant reduction in the number of reported escapees of farmed Atlantic salmon has been documented in several jurisdictions, such as Maine and Norway where operational standards and independent verification of sites and equipment have been required since 2006.
- While the reported number of escaped farmed fish has decreased in Norway over the last decade, the in-river proportion of escaped farmed fish to returning wild salmon has remained relatively constant (Anon, 2011). The in-river proportion of escaped farmed Atlantic salmon to returning Atlantic salmon influences the scale of potential genetic and ecological consequences to wild Atlantic salmon populations.
- Despite improvements in technology and operational procedures, escapes of farmed salmon reared in marine net pens are inevitable, and based on current recapture methods, attempts to recover them are generally not successful.
- To assess the success of physical containment technologies and operational practices in mitigating escape events, information on the number and frequency of events is required. Challenges remain in detecting escapes from net-pens, particularly small-scale escapes and those involving smaller fish. Techniques used by the industry to estimate the number of fish in a net pen at a given point in time are not free of error, which may result in inventory discrepancies between number of fish stocked, reported deaths and harvesting. Improvements in tools and techniques to assess the number of fish within net pens during grow-out would allow for an increased ability to assess escape events involving small numbers of fish. Escapes tend to be linked primarily to structural damage of sea cages through weather events and operational errors. Examples of effective physical containment systems take into account site-specific information and include standards, auditing, and training.
- The only method currently available for effective reproductive containment of farmed fish on a commercial scale is the production of all-female triploids. The use of sterile female triploids in Atlantic salmon aquaculture would significantly reduce the proportion of escaped farm adults returning to rivers and the possibility of interbreeding with wild stocks. Although triploid Atlantic salmon can perform well in culture, further comparisons with diploids would be required to evaluate marine performance, pathogen resistance, disease transmission, ecological effects, costs of monitoring and husbandry techniques to optimize performance for use at a commercial scale.
This Science Advisory Report is from the March 26-28, 2013 advisory meeting on the Potential effects surrounding the importation of European-origin cultured Atlantic salmon to Atlantic salmon populations and habitats in Newfoundland. 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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