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Document Type: Journal Article
Center: NWFSC
Document ID: 6882
Title: Pacific Salmon, Oncorhynchus spp., and the definition of "species" under the Endangered Species Act
Author: Robin S. Waples
Publication Year: 1991
Journal: Marine Fisheries Review
Volume: 53
Issue: 3
Pages: 11-22

For purposes of the Endangered Species Act (ESA), a "species" is defined to include "any distinct population segment of any species of vertebrate fish or wildlife which interbreeds when mature."  Federal agencies charged with carrying out the provisions of the ESA have struggled for over a decade to develop a consistent approach for interpreting the term "distinct population segment."  This paper outlines such an approach and explains in some detail how it can be applied to ESA evaluations of anadromous Pacific salmonids. 

The following definition is proposed:  A population (or group of populations) will be considered "distinct" (and hence a "species") for purposes of the ESA if it represents an evolutionarily significant unit (ESU) of the biological species.  A population must satisfy two criteria to be considered an ESU:

  1. It must be substantially reproductively isolated from other conspecific population units, and
  2. It must represent an important component in the evolutionary legacy of the species.

Isolation does not have to be absolute, but it must be strong enough to permit evolutionarily important differences to accrue in different population units.  The second criterion would be met if the population contributes substantially to the ecological/genetic diversity of the species as a whole.

Insights into the extent of reproductive isolation can be provided by movements of tagged fish, natural recolonization rates observed in other populations, measurements of genetic differences between populations, and evaluations of the efficacy of natural barriers.  Each of these methods has its limitations.  Identification of physical barriers to genetic exchange can help define the geographic extent of distinct populations, but reliance on physical features alone can be misleading in the absence of supporting biological information.  Physical tags provide information about the movements of individual fish but not the genetic consequences of migration.  Furthermore, measurements of current straying or recolonization rates provide no direct information about the magnitude or consistency of such rates in the past.  In this respect, data from protein electrophoresis or DNA analyses can be very useful because they reflect levels of gene flow that have occurred over evolutionary time scales.  The best strategy is to use all available lines of evidence for or against reproductive isolation, recognizing the limitations of each and taking advantage of the open complementary nature of the different types of information. 

If available evidence indicates significant reproductive isolation, the next step is to determine whether the population in question is of substantial ecological/genetic importance to the species as a whole.  In other words, if the population became extinct, would this event represent a significant loss to the ecological/genetic diversity of the species?  In making this determination, the following questions are relevant:

  1. Is the population genetically distinct from other conspecific populations?
  2. Does the population occupy unusual or distinctive habitat?
  3. Does the population show evidence of unusual or distinctive adaptation to its environment?

Several types of information are useful in addressing these questions.  Again, the strengths and limitations of each should be kept in mind in making the evaluation.  Phenotypic life-history traits such as size, fecundity, and age and time of spawning may reflect local adaptations of evolutionary importance, but interpretation of these traits is complicated by their sensitivity to environmental conditions.  Data from protein electrophoresis or DNA analyses provide valuable insight into the process of genetic differentiation among populations but little direct information regarding the extent of adaptive genetic differences.   Habitat differences suggest the possibility for local adaptations but do not prove that such adaptations exist. 

The framework suggested here provides a focal point for accomplishing the major goal of the Act—to conserve the genetic diversity of species and the ecosystems they inhabit.  At the same time, it allows discretion in the listing of populations by requiring that they represent units of real evolutionary significance to the species.  Further, this framework provides a means of dressing several issues of particular concern for Pacific salmon, including anadromous/nonanadromous population segments, differences in run-timing, groups of populations, introduced populations, and the role of hatchery fish.


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