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Dizon et al, genotypic (d) Information. Included are evidence both for and against designating population as a separate stock. In the designation "Type II albc, "jor example, information to the rigbt oj the solidus would be evidence jor "lumping," to the left would be for "splitting. " Missing letter abbreviations would signify lack oj reliable data Note that phylogeo- graphie designation depends on the results oj selection op- erating to produce a locally adaPted genome (indicated by difjerences in demographic, PhenotyPic, and genotyPic mea- sures) and on gene flow (indicated by differences in distri- bution or by movement data). Hierarchial stock categorization allows resource manag- ers to direct limited resources to tbe populations most de- seroing oj protection, that is, the populations that are most likely to be evolutionarily significant units. Using this com- prehensive classification of stock allows preliminary, con- servative splitting oj assemblages wbere data are lacking without the danger that these divisions will hecome en- trenched as biological dogma. Introduction A "population stock," as defined in the Marine Mammal Protection Act of 1972 (MMPA), is a group of animals that share a common space and interbreed; a "verte- brate species," as defined in the Endangered Species Act of 1973 (ESA), is a distinct population segment that interbreeds. The purpose of such definitions in both acts is to direct management efforts to taxon levels below that of the species to ensure that populations that are uniquely adapted to given areas are not irreversibly re- duced by harvest or habitat destruction. Although the stock concept is thus codified into law by these envi- ronmental acts, no useful working definitions of in- traspecific taxa are provided. At the same time, no uni- versally accepted definitions or even pro tem consensus exist among biologists. Thus, much confusion exists among researchers, regulators, users, and conservation- ists as to what taxon levels conservation efforts ace to be directed toward The problem with the definitions of stock in the acts is that it is not obvious how to define quantitatively what is meant by either "sharing" or "interbreeding," For example, do we extend protection to the gray squir- rels in Central Park in New York? They certainly meet the legislative criteria of sharing a common space and interbreeding. Or if members from two apparently dis- tinct populations of dolphins only interbreed during EL Nifto events, should these populations be managed as one management unit? What if their ranges overlap, or they interbreed once every 100 (or 1000) years? Should they then be managed as separate stocks? Of course, j ~ Rethinking the Stock Concept I 25 deberla ser incluU:ia usando abreviaciones con letras: dis. trihuciOn (a), respuesta poblacionaUb), injormacion jeno- tipica (c), e injOt'l'lUUi6n genotipica (d). Se incluirfa tanto evidencia afavor como en contra, por ejemplo, 'Tipo II a/bc" donde la injOt'l'lUUion a la derecba sgria. evidencia en javor del "agrupamiento': infOt'l'lUUiOn a la izquierda serla en javor de la "disociacion"; la ausencia de abreviaciones con letras indicaria ausencia de datos confiables. La designacion jilogenetica depende de los resultados de seleccion natura4 que opera para producir un genoma loealmente adaptado (indtcado par diferencias en mediciones demogrdficas, feno- tipicas, y genotipicas), y del flujo genico (indicado por difer- encias en distribuci6n 0 por datos sobre movimientos). La categorizacion jerdrquica del sotck permite a los ad- ministradores de recursos asignar los limitados recursos a aquellas poblaciones que mas necesitan proteccion, es decir a poblaciones que son, muy probablemente, unidades eval- utivamente significativas. EI uso de esta c/asificacion com- prensiva de stocks permite una subdivision preliminar con. servadora de /as agrupaciones cuando exista falta de datos, sin el peligro de que estas subdivisiones se transjonnen en dogma bio16gico, there ace no indisputable answers to these questions. It is absurd to apply a rigid, typological definition of stock in such situations, because at present there is no way to determine how unique and isolated a population must be before it is a "stock" Intraspecific structure theoret- ically can range from complete panmixia (no intraspe- cific structure) to isolation so complete for so long that speciation processes ace at work or almost complete (Fig, I). Rather than a universal definition that could be applied in the regulatory acena to decide if a population is or is not a "stock," what is required (and more prac- tical) is a means of describing the usually complex bi- ological population structure in a more informative manner, The purposes of this paper ace to show that currently used simplistic definitions of stocks ace not very useful in satisfying management objectives and to suggest a way to describe population structure that captures its complexity. Using examples from the marine mammal and fishery literature, we review the history of the stock concept and methods currently used to infer stock structure. We suggest how to classify the various qual- ities of stocks and conclude with a case study demon- strating how the suggested method can be applied. Background Around the turn of the century, pioneering fishery biol- ogists F. Heincke and J. Hjort established the local self- sustaining population as opposed to the typOlogically defined species as the preferred unit of study for fisher- Conservadoo BlolosY volume 6, No.1, March 1992