almon in California.
Loss and Fragmentation of Historic Range
The coho salmon consists of a highly organized network of dynamically connected, locally adapted populations. Each population is locally adapted due to the unique effects of a stream environment and particular natural selection that is therefore unique to each population. Each population has accumulated a unique combination of adaptive traits that cannot be duplicated or replaced in the span of a human lifetime. Inbreeding depression and speciation were historically minimized or prevented by the infrequent but periodic exchange of individuals between neighboring populations. Since suitable habitats and thus coho populations are often small and relatively isolated, the occasional exchange of individuals is beneficial because geographically nearby streams have similar habitats and are thus likely to reproduce successfully. The maintenance of such a dynamic metapopulation structure, and the interpopulation diversity associated with it, is necessary to ensure the future of the species and its role in ecosystems. This means that each breeding population is geographically, evolutionarily, and ecologically distinct, perhaps at as fine a scale as individual spawning tributary streams and major river reaches. Either the cumulative depletion or extinction of many populations or the fragmentation and severing of natural linkages between populations can precipitate rapid extinction of the species across large portions of the its range (Frissell 1993). These types of impacts immediately threaten coho salmon.
Complex metapopulation structure is adaptive in a region subject to frequent catastrophic disturbance, such as volcanism, earthquakes, sea level change, large landslides, flooding, and wildfire (Frisell 1993). When such disturbances cause localized extinctions, coho from adjacent populations colonize and eventually re-establish populations. Natural colonists of local origin, unlike hatchery fish or foreign stocks transferred from distant locations, are likely to be relatively well adapted to the empty habitat by virtue of geographic proximity and environmental similarity. Given sufficient time they can successfully restore the former range of the species. Because the life span of a single population is likely to be less than a few thousand years or perhaps centuries, maintenance of a broad distributional range and an expansive network of such population is critical for the long-term survival of the species as a whole. Large-scale fragmentation and collapses of range, such as coho have exhibited in California and the Columbia Basin this century, indicate that metapopulation structure and function is breaking down catastrophically, and that remaining populations face greatly increased risk of extinction (Frissell 1993).
Southern populations of coho salmon may be unusually well adapted to future, warmer and intensively human-altered environments. This indicates that the populations of coho salmon disappearing most rapidly are those which may be of greatest utility to ecosystems, fish managers, and fish harvesters in the future.
The loss of the coho salmon over significant portions of its range curtails or eliminates the functional role of the species in the ecosystem. In freshwater, coho and other salmon species are prey items for numerous aquatic and terrestrial predator and scavenger species and may contribute significantly to the nutrient budget of aquatic and riparian ecosystems (Houston 1983, Cederholm et al. 1989). At sea, and in freshwater, coho salmon function as a significant, if not primary food resource for a incredible array of predatory animals. Thus the loss of coho salmon is felt throughout the full range of the aquatic and terrestrial ecosystem.
For the present, our lack of understanding of how subtle natural genetic signatures create success or failure in a specific stream, makes it imperative that we save all existing runs of coho salmon to preserve the genetics for future conservation efforts. Loss of any population causes an irreversible blow to the ge