1991 (Table 7). Of the naturally spawning coho salmon, 3,880 were from tributaries in which supplementation occurs (Noyo River and coastal streams south of San Francisco). Only 160 fish in the range of this range ESU (all in Ten Mile River) were identified as native stock (lacking a history of supplementation with non-native hatchery stocks).
Populations south of San Francisco were estimated at around 10,000 returning adults in the late 1800s, these populations had fallen to 100 or fewer in 1990 (Hope 1993). These populations have rebounded due to efforts enhance survival through curing populations of Bacterial Kidney Disease and careful propagation and release of fry. The returning adults are now estimated to be between 500- 1000 on good brood years (D. Hope pers. com.). Evaluation of these streams by Nehlsen in 1991 identified stocks in small coastal streams north of San Francisco as at moderate risk of extinction, and those small streams south of San Francisco at high risk of extinction. Higgins et al. (1992) looking closely at streams in Sonoma County north identified the Gualala River at high risk of extinction.
Historic data on adult coho salmon runs are available for 186 streams. Recent data for 133 of those 186 streams show 71 no longer have runs of coho salmon, while 62 still maintained runs of adult coho salmon (Brown et al. 1994).
NATURE OF THE THREAT
In reviewing the following description of the factors that threaten the continued existence of coho salmon in California, it should be noted that this decline has been continuing for over 50 years. The cumulative effects of all human induced impacts has so depressed the populations that even factors such as natural predation have taken on critical significance. Human development and its associated impacts are primarily responsible for the decline of coho salmon populations. Habitat loss in both freshwater and ocean, overexploitation, disease and a host of other human related influences has created a critical situation for the continued viability of coho salmon 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 t