Reduction in fitness limits the useful duration of supplementary rearing in an endangered salmon population

Supplementation programs based on captive breeding and rearing are increasingly being used in recovery planning for endangered or threatened salmonid populations. However, it is largely unknown if increased abundance from these programs can offset deleterious genetic changes from the captive environment and lead to viable populations in the wild. In this paper, we developed a life-history-based population viability analysis that explicitly incorporates declines in fitness attributable to captive breeding and rearing using the breeder's equation as part of the projection model. Using endangered inner Bay of Fundy Atlantic salmon as a case study (a population assemblage for which supplementation is a major component for the recovery plan), we evaluated how genetic changes influence abundance trajectories and extinction risk. Based on the population projections, continual supplementation enables the population to build from critically low abundance levels, even under high rates of fitness loss. However, beyond 4–6 generations, loss of fitness (>15%) outweighs any increase in abundance and causes the population projection to start to decline. For the majority of the scenarios, abundances were predicted to increase and remain in excess of the current population size for 10 to upward of 30 years, albeit at progressively lower population-level fitness as compared to current values. Although the captive breeding and rearing program does prevent extinction in the short term in this case study, associated fitness costs limit the population's overall probability of recovery, as well as increase the length of time to recovery. Under the assumption of interbreeding between wild and captive-reared individuals, declines greater than 10% in relative fitness at a population level are sufficient to counter abundance increases resulting from supplementation. Although extinction risk over the short term can be reduced by increasing the proportion of the population that is reared in captivity, this comes at a cost to the probability of recovery for the population over the longer term, particularly as environmental conditions change. Generalizing from this case study, the useful duration of supplementation programs may be limited to short-term population increase (i.e., to prevent extinction) and may not be a workable strategy for longer term recovery planning.