U.S. Department of Commerce


Date of this Version



Conservation Biology, Pages 718–721, Volume 12, No. 3, June 1998


Taxonomy and systematics have long been characterized by heated debates over methodology and pointed exchanges between “lumpers” and “splitters.” Therefore, when I wrote the paper (Waples 1991) outlining the concept of evolutionarily significant units (ESUs) for identifying “distinct population segments” of Pacific salmon ( Oncorhynchus spp.) under the U.S. Endangered Species Act (ESA), I realized that any attempt to identify conservation units below the species level would also generate a good deal of scientific discussion. This has proved to be the case (e.g., Dizon et al. 1992; Rojas 1992; Moritz 1994 a ; Vogler & DeSalle 1994; Nielsen 1995).

At the same time, a parallel discussion has taken place of which the primary focus is not biological issues but rather the degree to which the ESU concept satisfies the original intent of the U.S. Congress and the ESA itself (for divergent views on this issue see Gleaves et al. 1992; Rohlf 1994). The recent essay in these pages by Pennock and Dimmick (1997) follows in this tradition. Pennock and Dimmick argue that the ESU concept adopted as policy by the National Marine Fisheries Service (NMFS; 1991 Federal Register 56: 58612) for identifying distinct population segments of Pacific salmon under the ESA (1) is inconsistent with the original intent of Congress, (2) unduly narrows the scope of populations that can be considered distinct under the ESA, and (3) limits flexibility under the ESA.

The comments below address these criticisms as they apply to the NMFS ESU policy for salmon.


An examination of the legislative and legal history of the ESA does not support Pennock and Dimmick’s claim that the NMFS ESU policy is inconsistent with the intent of Congress or the Act. Three basic principles guided development of the policy: (1) the clear intent of the framers of the ESA to use it to protect genetic diversity (93rd Congress, 1st Session, 1973, H.R. Report 412); (2) the 1979 directive that the government agencies involved list populations “sparingly and only when the biological evidence indicates that such action is warranted” (96th Congress, 1st Session, 1979, Senate Report 151); and (3) the stipulation in the ESA itself (Sec. 4(b)(1)(A)) that listing decisions be based “solely on the basis of the best scientific and commercial data available.” This last point was re-emphasized during the 1982 amendment process, which added the word “solely” to the passage cited above. This addition was “intended to remove from the process of listing and delisting of species any factor not related to the biological status of the species” (97th Congress, 2d Session, 1982, H.R. Report 567, Part 1).