Science Advisory Report 2016/017

Risks and benefits of juvenile to adult captive-reared supplementation activities to fitness of wild Atlantic salmon (Salmo salar)

Summary

• This review was challenging due to the paucity of information available to assess the benefits and risks of juvenile/smolt to adult supplementation (SAS).
• SAS consists of the capture of juvenile salmon at freshwater stages, rearing them in a captive environment to maturity, and releasing the adults back to the rivers of origin to spawn. The anticipated benefits to salmon populations are by circumventing the marine phase of the life cycle thereby resulting in a demographic boost in abundance of salmon populations subjected to low marine survival.
• Currently, SAS is being used in areas where salmon populations are at high risk of extinction and in cases where very low numbers of adult salmon are putting the population at risk of loss of genetic diversity which could affect long-term population viability.
• SAS reduces some of the known risks associated with traditional supplementation of juvenile stages but introduces risks at other points in the anadromous life cycle that are not well understood.
• Adaptive genetic changes associated with captivity through unintentional selection, domestic selection, and relaxation of natural selection can occur rapidly, even within one generation.
• An immediate benefit resulting from an increased abundance of salmon as a result of the increase breeding/spawning of SAS fish may be offset by the expectation that mean fitness of the captive-reared progeny will be reduced relative to wild fish. The reduced fitness may be the result of phenotypic differences (body size, growth rates, maturation rates) and in reduced survival at sea of progeny inherited from the parents. Some of these effects may manifest themselves in the first generation and for several generations following release of adults.
• Genetic mixing by interbreeding of released captive-reared SAS fish with wild fish is expected to occur. As long as there is some risk that SAS will cause phenotypic and genetic changes that reduce fitness of progeny in the wild, there is a risk that genetically mixed progeny may have reduced fitness in the wild.
• The risks to wild population abundance and characteristics will in general be greater when (i) SAS generates reductions to fitness of progeny relative to wild fish, (ii) SAS is continuously practiced over successive generations, and (iii) SAS releases represent an increasing proportion of the total number of adults in the population at spawning time.
• River populations in which there is sub-basin structuring with local adaptation at the tributary level are more likely to be negatively impacted by SAS conducted on a basin wide scale, unless collection and release of adults is conducted on a tributary basis.
• Within limits, as the scale of the SAS activity increases, the extent of potential benefits will increase, monitoring capacity and the power of assessment due to larger numbers of animals will increase, but the risks overall to the wild populations of salmon will also increase. The scale dimensions are spatial (size of the basin), demographic (number of animals being monitored) and temporal (seasons and years) and these characteristics should be considered in any decisions on proposed SAS activities.
• Due to the large uncertainties on the benefits and risks of SAS activities to wild Atlantic salmon fitness, if a SAS activity is conducted, it should be at a geographic and demographic scale that allows and includes an adequate monitoring and assessment capability to address the vast knowledge gaps on benefits and risks to wild salmon population persistence and productivity.
• The monitoring and assessment capabilities using genomic tools are improving rapidly. The development of new genetic markers and high throughput technologies will augment the capacity to evaluate parentage of progeny and to assess relative fitness and contributions to future generations of wild, SAS , and wild/SAS interbred salmon.

This Science Advisory Report is from the December 14 to 16, 2015 regional science peer review meeting on the Review of risks and benefits of adult captive-reared supplementation activities to fitness of wild Atlantic Salmon. 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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