nd Lichatowich, 1991.)
The best biological science of the day and the Federal Endangered Species Act recognize the importance of listing distinct population segments of vertebrates. In the Definition of a Species paper (Waples 1991), salmon populations will be considered distinct for listing purposes if they represent an evolutionary significant unit (ESU) of a biological species. An ESU is defined as a population that 1) is substantially reproductively isolated from conspecific populations, and 2) represents an important component of the evolutionary legacy of the species.
The term evolutionary legacy is used in the sense of inheritance and is something received from the past and carried into the future. The evolutionary legacy of a species is the genetic variability that is a product of past evolutionary events and that represents the reservoir upon which future evolutionary potential depends. Conservation biology recognizes that these genetic resources will help ensure that the dynamic process known as evolution will not be constrained in the future if a species maintains this reservoir of its evolutionary past. DNA analysis has advanced enough in the last few years that it can now analyze gene flow that has occurred over evolutionary time scales, and therefore determine the lineage of a population of salmon.
The most important question to answer is, will the loss of population significantly affect the genetic diversity of a species? This too must be evaluated by looking at many types of information. Life history traits, size, migration patterns, fecundity, and timing of the spawning run age, all of which can represent a local adaptation to a local environment. Unfortunately, adaptive genetics is not well differentiated by past genetic studies. It is likely that future genetic studies with higher resolution may prove that habitat differences not only create local adaptation but also impart a genetic variation over time.
Many types of evidence should be considered when making a species determination. Isolation 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 sa