Research Document 2018/019
Genetically Based Targets for Enhanced Contributions to Canadian Pacific Chinook Salmon Populations
By Withler, R.E., Bradford, M.J., Willis, D.M., and Holt, C.
Abstract
The Wild Salmon Policy (WSP) establishes conservation of wild Pacific salmon and their habitat as the highest priority for Pacific salmon resource management decision-making. Hatchery production in the Salmon Enhancement Program of DFO is used as a conservation tool for wild populations and can increase the availability of fish for harvest but is a risk factor to wild genetic diversity that requires management and mitigation to safeguard Pacific salmon biodiversity in Canada. We recommend use of the proportionate natural influence (PNI) and associated metrics developed by the U.S. Hatchery Scientific Review Group to evaluate and monitor the adaptive state of integrated hatchery populations, and to identify hatchery-influenced populations for WSP assessments. We develop a classification system for Canadian Pacific salmon populations that reflects the adaptive state of the population based on constituent proportions of natural- and hatchery-origin fish. Among the biological categories, increased genetic risk is associated with increasing hatchery influence and a decreasing proportion of wild fish. We modelled the population dynamics of a Chinook Salmon population including the genetic impacts on fitness from hatcheries to evaluate the use of three management measures - hatchery program size, proportion of hatchery fish marked, and proportion of marked fish selectively removed - in managing to a target PNI level. Except for populations at risk of extirpation, limiting hatchery size by scaling the size of the hatchery program to natural production is an effective way to minimize genetic risk of enhancement to wild populations. Limiting hatchery program size also limits the production of fish for harvest, resulting in a trade-off between genetic risk and socioeconomic benefit in enhancement programs implemented for harvest augmentation. Genetic risk associated with higher levels of hatchery production can be minimized by reducing the proportions of hatchery-origin fish included in the hatchery broodstock and/or allowed to spawn in the natural environment. Manipulation of proportions of natural- and hatchery-origin fish is dependent upon some type and level of marking that allows pre-spawning differentiation of fish originating from the two spawning environments. In conservation programs, the risk of domestication occurring at low PNI values must be balanced against the genetic and demographic risks of small population size in the absence of high proportions of hatchery-origin fish. We provide recommendations for the classification and management of enhanced populations consistent with the principles of developing explicit biological goals for hatchery-influenced populations, implementing scientifically defensible hatchery programs and using adaptive management of hatchery programs to meet objectives in a risk averse manner.
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