 historical records of occurrence of coho in 582 California streams, ranging from the Smith River near the Oregon border to the Big Sur River on the Central Coast. More recent surveys available for 42% of these streams indicate that 46% have lost their populations (Brown and Moyle, 1991). The Wilderness Society estimates in California, that coho salmon are extinct in 26% of their range, endangered in 22%, and threatened in the remaining 52% of the historic range.
Generally, the further south a stream is located on the California coast, the more likely it is to have lost its coho population (Brown and Moyle, 1991). Coho Salmon are rare in the Sacramento River even though several attempts have been made to establish runs (Hallock and Fry, 1967). It is likely that runs occurred at one time at least in tributaries to San Francisco Bay, if not in more interior streams. In the 1960s coho salmon were also widely distributed on the central coast surveys in the 1970s showed 12 streams from San Mateo to Monterey County, had documented runs of coho salmon (Hassler, Sullivan, Stern 1991). By the 1980s these populations had been extirpated in 8 streams, and with reestablishment of coho in Gazos Creek, there are now coho in 5 streams with small populations (Dave Hope personal communication).
EVOLUTIONARY SIGNIFICANT UNIT
The existence of stocks, as defined by Ricker (1972), is no longer in doubt. The subdivision of a species into local populations which posses genetic differences that are adaptive is the fundamental basis of the stock concept, and it is this concept that must be incorporated into management if fishery reserves are to be restored and maintained. (MacLean and Evans, 1981.) We recognize that many instances will arise were there is doubt. In those cases, we believe the prudent manager will recognize the stock in question until such time that enough evidence is collected to show otherwise. Since the loss of a stock is an irreversible loss, its existence should be given the benefit of any doubt. (Nehlsen, Williams and 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 