does not have to be absolute, but it must be at least significant enough to impart genetic differences that can be maintained in an evolutionary unit. Isolation must be considered in the backdrop of natural straying, recolonization, evaluations of natural barriers, and shear measurement of genetic differences between populations.
The National Marine Fisheries Service evaluated the stocks of coho salmon in California and the West Coast. They determined that at least two ESUs could be established in California, Central California coast (Map 1) and the Southern Oregon/Northern California ESU (Map2). The State of California has recognized the distinct stocks of coho salmon south of San Francisco as an ESU, it is likely that future studies and a better understanding of adaptive genetics will likely support this listing decision.
HISTORIC AND CURRENT ABUNDANCE
Historical information on state-wide coho salmon abundance are estimates made by fisheries managers, based on limited catch data, hatchery records and personal observations of runs in various streams. Estimates for the number of coho spawning in the state in the 1940s range from 200,000 500,000 (Brown et al. 1994) to close to 1 million (California Advisory Committee on Salmon Steelhead and Trout, 1988). According to some researchers (Brown and Moyle, 1991), coho populations held at about 100,00 in the 1960s and dropped to an average of 33,500 during the 1980s (Brown and Moyle, 1991). The reliability of these estimates is uncertain, and they must be viewed only as order-of magnitude approximations.
The total number of adult coho salmon entering California Streams in 1988-90 averaged about 31,000 fish per year. However, hatchery fish made up 57% of this total, and many main stem populations contain at least some fish of recent hatchery ancestry. The hatchery stocks, without exception, have in their ancestry fish from other river systems and often from outside California (Brown and Moyle, 1991). This may explain the overall lack of genetic differentiation of coho salmon from different California streams that have been extensively planted (Bartly, 1991). Recent studies of spawning escapement showed that 63 populations showed significant decline and only 2 showed increases. This data is similar to that found in Washington. ( Konkel and McInyre 1987). In 1890 Sacramento River summer and fall runs of coho salmon were noted in the press, but by 1950, no viable populations could be found (Hallock and Fry 1967).
The upper Klamath River had runs of 1,600 coho before the 1920s; these coho are now extinct (Klamath River Task Force). The California Department of Fish and Game summarized recent data and determined that most remaining coho salmon were hatchery fish although some wild coho may still remain in small tributaries. This same report concluded coho salmon in California, including hatchery stocks, could be less than 6% of their abundance in the 1940s, and have experienced at least a 70 % decline in numbers since the 1960s, and many populations have been eliminated and others have runs only 1 out of 3 years, indicating two brood years have been lost and extinction is imminent.
In 1993, Pacific Rivers Council, Brown and Moyle (1991) estimated that naturally spawned adult coho salmon (regardless of origin) returning to California streams were less than 1% of their abundance at mid-century, and indigenous, wild coho salmon populations in California were a probably 1,000 individuals. Brown and Moyle (1991) also found that 46% of the California streams that historically supported coho salmon no longer supported runs. Brown et al. (1994) found in central California that the proportion of streams with coho salmon present was higher in recent surveys (1995-96), moving from 47% to 57%, but northern California streams with coho present, fell from 63% to 52%.
Brown and Moyle (1991, Brown et al. 1994) estimated average coho salmon spawning escapement in the central California coast ESU as 6,160 naturally spawning coho salmon and 332 hatchery spawned coho salmon from the period from 1987 to 