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26 RethJnking the Stock Concept TWO VIEWS OF INTRA-SPECIFIC STRUCTURE @_w@ SPECIES 1 ~ SPECIES 2 ~ SPECIES' @_w@) , , SPECIES 1 ~ SPECIES 2 ~ SPECIES' Figure 1. Two views of intraspecific structure, a "ty- pological" and a "biological" conception of popula- tions within species. The circles, labeled with lower- case leUers, are individual populations wbose geograpbical habitat is represented by its size and position witbin the species space. Wbere circles inter- sect, babitat is shared Tbe arrows represent exchange rates: infinitesimal between species, variable between populations within species. In the first view, individual populations within a species are perceived as isolated, allopatric entities that can be uniquely described Interchange between the populations, while greater than between species, is still very low, on tbe order of a few genomes per generation In the second view, tbe complexity and wide range of potential intraspecific structures are recognized Habitats range from completely separate to situa- tions where one is completely witbin another. Ex- change rates vary between various populations, in some cases with great temporal variability due to environmental and population density changes. ies-management questions (see Sinclair 1988; Gauldie 1991). Independently, Leopold (1933) recommended managing populationS of wildlife based on their "radius of mobility," "rate of spread into unoccupied range," and "minimum units of range and population." This ba- sic approach of managing a species at some subspecific level has therefore been a tenet in resource manage- ment for almost 100, years. In general, stock status is ConscrvadOD Biology Volume 6, No, I, Marcb 1992 Dizon et aI, conferred using a variety of proxies that imply repro- ductive isolation. With isolation com~genetic diver- gence through drift and through local adaptation via processes such as differential selection ~r character dis- placement. But because reproductiv~ barriers within species are fragile or incomplete, especially in the ma- rine environment, the degree of isolation of one popu- lation relative to others can be complex, varying both in space and time. A variety of other terms in the literature all refer to organizational levels below the species: subspecies, race, deme, stock, and management unit. The use of such terms in resource management literature is highly inconsistent. Only the term "subspecies" is recognized by the International Code of Zoological Nomenclature; it is applied to populations that have received a trino- mial name. The other terms, lacking such accepted us- age, are frequently used to describe a variety of popu- lations for analytical convenience or to provide status to a group because that is the most conservative approach when data are lacking and conservation issues predom- inate. Many so-called stocks are mere "ecological ab- stractions" (Sinclair 1988),.and a perusal of the manage- ment literature indicates that when a term such as "stock" is used, it is used in the simplest and least re- strictive sense. little qualification is made or assumed about its genetic, evolutionary, or ecological implica- tions, Misconceptions occur later when such stocks are expected to behave as if they were biological popula- tions, In 1980, an ambitious attempt to formalize the stock concept was made with an international symposium (Stock Concept International Symposium convened at Alliston, Ontario, September 29-Qctober 9, 1980; see Booke [1981] for proceedings citation). Although many speakers emphasized the importance of management by stock, in that very large symposium only one author ventured any formal definitions. Booke ( 1981 ) defined a "phenotypic stock" as any population that maintains characteristics that are expressed depending on the en- vironment, and a "genotypic stock" as a population maintaining Hardy-Weinberg equilibrium, that is, con- stant gene frequencies for a particular character in each generation, The definition of phenotypic stock is similar to a later definition by Brown et aI. ( 1987): a population whose average life history parameters are meaningful from the standpoint of management. The definition of genotypic stock is similar to the definition of Larkin ( 1972), who described a stock as a population having a degree of genetic uniqueness: "a population of organ- isms which, sharing a common environment and partic- ipating in a common gene pool, is sufficiently discrete to warrant consideration as a self-perpetuating system which can be managed" (p. 11). Hoelzel and Dover (1989) defined a genetic stock as one that is genetically