The tribal approach to using artificial propagation for supplementation and reintroduction is based on scientific principles and practicality. This approach minimizes genetic risks to the natural populations by applying specific protocols for spawning, rearing, release, and disease prevention (Cuenco et al. 1993). Whenever possible, salmon spawning in a given stream will serve as the source of the artificially propagated fish planted in that stream. Source populations for all supplementation efforts will be chosen to best match the characteristics of the natural population. The intention of tribal supplementation proposals is to increase the abundance of naturally reproducing populations through out planting while keeping genetic risk at acceptable levels. The increase in survival and reproductive capacity gained through the use of artificial propagation in supplementation and reintroduction programs is necessary to recover stocks in a timely fashion.
Many salmon populations are at such a small fraction of their historical size that the risk of losing a whole population outweighs the risk of losing genetic variation. In the case of small populations, factors related to demographics, survival rates, and spatial structure become more important than genetic factors in determining the long-term viability of the population. Increasing the size of a population to permit it to breed effectively is necessary for that population to survive environmental changes.
Restoring the spatial structure through supplementation and habitat actions will help reestablish the natal gene flow between adjacent spawning populations and reduce the risk of localized extirpations. In comparison to other organisms, the “stray” rate of salmon is relatively high and therefore the inherent rate of gene flow is relatively high for salmon. Straying is part of the natural life history of salmon (Ricker 1972), and it promotes gene flow and extends the range of the population. In the past, salmon habitat, which was continuous, provided links between local populations, creating a metapopulation. But human activity partitioned the habitat and fragmented the populations, increasing the possibility of inbreeding depression by reducing natural gene flow and effective population size. The links between local populations have been broken and they must be reestablished.
It is important to evaluate changes resulting from supplementation actions and the tribal plan includes monitoring for genetic change. Although not meaningful by itself, the level of heterozygosity will be tracked and evaluated. For anadromous fish returning to natal areas to spawn, levels of heterozygosity would be expected to be low (Nevo 1978). Population fragmentation due to land use practices and other ecological and genetic factors also can alter the character of fish populations. These factors will also be considered in determining the consequences of enhancement actions. Should there be significant loss of heterozygosity, alternative restoration actions can be applied.
Supplementation is a short-term solution that would not be necessary over the long-term if other factors such as habitat restoration and passage improvement were to promote salmon survival. Unless the high mortality levels throughout the life cycle of salmon are reduced, losses of salmon populations and their associated genetic diversity will continue. Then, the survival advantage provided by supplementation efforts will simply delay the demise of salmon populations in the Columbia basin.