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Research Document - 2015/030

The potential direct and indirect genetic consequences for native Newfoundland Atlantic Salmon from interbreeding with European-origin farm escapes

By E. Verspoor, P. McGinnity, I. Bradbury, and B. Glebe


Aquaculture usually involves the production of non-native populations and species.  As well, it is increasingly focused on a relatively small number of species and a few inbred, highly selected, and domesticated strains. Although delivering economic gains, this focus can potentially lead to negative interactions and impacts on the character, abundance, and viability of native populations in farming regions. The complexity of biological systems means these risks are difficult to predict and where impacts are found to occur, they can be difficult to mitigate. This makes it imperative that risks are carefully considered in advance.

These risks are considered here in respect of a request to import genetically improved Norwegian Atlantic salmon strains to Canada for use in farms on the south coast of the island of Newfoundland. The current production in the region is c. 15,000 tonnes and is based on strains from the Saint John River in New Brunswick. This request occurs against a background of depressed local wild salmon stocks. These have declined in abundance by ~45% from 1996-2010, particularly near the main farming area (~70% decline in the Conne River), and are designated as “threatened” by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC 2010). Marine mortality is seen as the main problem, with farm-wild interactions a possible contributing factor. As such, concern exists that the use of Norwegian farmed Atlantic salmon strains may lead to further impacts, exacerbating current declines.

Available scientific evidence related to the genetic, phenotypic, and demographic consequences of indirect and direct genetic interactions of such an introduction is reviewed. It specifically considers if:

  1. farmed European-origin Atlantic salmon were able to successfully breed and/or interbreed with native Atlantic salmon;
  2. the likelihood that European-origin aquaculture escapes will mate successfully with native wild salmon;
  3. the risks that such interbreeding would present to native populations; and
  4. how risk scales with the size of the interaction.

Norwegian farmed salmon and wild, native stocks of salmon in Newfoundland are encompassed within the species Salmo salar L. However, a compelling body of evidence shows that they are highly genetically divergent, and probably as divergent as any two sets of populations with the species. By many experts, they are viewed as belonging to different subspecies, even considering the evidence that some wild Newfoundland stocks have a naturally mixed European and North American ancestry; this is a legacy of the period after the last glaciation during which the region’s rivers were recolonized. However, a robust and detailed understanding of the adaptive divergence of Norwegian farm and Newfoundland wild salmon is lacking.

Existing knowledge indicates that a proportion of escaped farmed Norwegian salmon would survive and enter rivers on the south coast of Newfoundland, leading to direct genetic interactions, with numbers conditioned by the magnitude and timing of escapes. If they do escape, survive, and migrate into rivers, it is likely that they will be able to breed successfully given that they have done so with native populations across their wild range, including in other parts of eastern North America. This is particularly likely where local wild populations are depressed and competition for mates is limited. Existing studies indicate interbreeding will result in lower mean population fitness, depressed abundance, altered character, and reduced viability. Through increased gene flow engendered by direct interbreeding of farm fish, genetic mixing and impacts may be extended beyond directly impacted populations and could lead to increased genetic homogenisation of populations across a region. Whether these impacts will be greater than those arising from the use of North American farm strains is uncertain and will depend on the specific nature of the differences in adaptive divergence of the respective farm strains from wild Newfoundland populations. In both cases, the size and persistence of interactions can be expected to be crucial, with impact increasing with the amount of interbreeding and its persistence over a large number of generations. Even low levels of repeated interbreeding in already depressed populations could put local populations into an extinction vortex. The unusual phenotypic diversity of Newfoundland populations means that the consequences of impacts on salmon biodiversity may potentially be greater than elsewhere, and its preservation will be a key conservation concern.

Indirect genetic interactions may also occur. Caged or escaped farm fish can change the environment so as to alter selective pressures and long-term fitness. This can lead to decreased survival, reductions in population size, increased genetic drift, and a lowering of long-term adaptive capacity. The latter will also be the outcome of interbreeding where there is reduced hybrid fitness. This effect may be manifest beyond the first generation where some hybrids survive and lead to the introgression into wild populations of new maladaptive gene variants, changes to existing gene and genotype frequencies, and disruption of co-adaptive genomic structure, compromising the character, abundance, and viability of affected populations.

Indirect impacts from caged farm fish or freshwater rearing facilities that release waste water are of concern because of the potential for the introduction of exotic pathogens or increasing numbers of native pathogens. Again, this can cause increased or selectively-altered mortality, reducing a population’s adaptive capacity. Indirect genetic impacts can also arise if farm salmon escape as juveniles into rivers and compete with wild fish, if escaped farm adults ascend rivers and interfere with the reproduction of wild fish, reducing wild breeding success; if they spawn successfully, they may also produce offspring that compete with wild juveniles. What is not known is whether there is an increased or altered risk from using Norwegian as compared to North American farm Atlantic salmon strains. However, the risk can be expected to differ given their substantive evolutionary divergence, the presence of different pathogen strains and “species”, and host susceptibilities.

Risks of direct and indirect impacts can be expected to scale with the relative magnitude of the number of farm fish present compared to the abundance of wild populations. Small populations, or those experiencing declines or low abundance, will be more susceptible to genetic impacts than will large or healthy populations. Given the current status of populations along the south coast and near aquaculture activities, the risk of significant impact may be higher than elsewhere.

More research is required to address the knowledge gaps in current understanding. This should focus on increasing the capacity to predict impacts. Key in this respect are:

  1. accurate assessments of the actual numbers of farm fish that will escape and the proportions that will enter rivers under different escape scenarios;
  2. the development and refinement of genetic tools for identification farm escapes and hybridization;
  3. studies of the genomic basis of adaptive divergence within wild populations and among wild and farmed strains;
  4. the relative fitness of farm and wild salmon, and the various generations of hybrids, in the local environment under different conditions of wild demographics;
  5. a better understanding of wild population dynamics and, in particular, the way in which density dependence affects juvenile mortality; and
  6. the development of realistic, robust, virtual individual-based, stochastic population demographic models that incorporate realistic genetic models and the effects of environmental variation and change.

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