MARB 403 Stock Identity
An excerpt from The Encyclopedia of Marine Mammals, 2nd ed. ISBN 978-0-12-373553-9
Definition of Stock
The term "stock" has been used to refer to both biological and management entities (although in many cases, they are combined or inseparable). A management stock is a group of conspecific individuals that are managed separately. The delineation of these stocks is very much dependent on the goals of managers and may not be based on biological discontinuities (e.g., International Whaling Commission management stock designations for baleen whales). With the exception of the definition by Moritz (1994), who described a “management unit” (MU) (which he synonymized with “stock” and appears to be equivalent to management stock) as having significant differences in allele frequencies at nuclear or mitochondrial DNA loci, the criteria for determining management stocks may have little or no biological rationale or consistency and may be influenced greatly by political interests. Nevertheless, management stocks have been used widely due to the paucity of biological information and will likely continue to play an important role in conservation. Developments in management strategies for situations with incomplete biological information should improve the success of conservation programs (Taylor, 1997). Although management stocks offer more flexibility in the sense that they can still be the focus of management programs without evidence of biological distinctiveness, conservation goals (e.g., maintaining genetic diversity) are more likely to be achieved if stocks are based on biological data. Therefore, this article focuses mainly on biological stocks.
Biological stocks are characterized by no or low levels of genetic exchange (which means that members of a biological stock tend to interbreed with each other more often than with other individuals). An entity with this property has also been called a population, subpopulation, evolutionary significant unit (ESU), deme, and subspecies (the only intraspecific taxon recognized by the International Commission on Zoological Nomenclature). When gene flow between two groups is absent, there is usually no disagreement that they represent separate biological stocks. However, it is more typical that some level of genetic exchange exists. Even low levels of genetic exchange can obscure stock boundaries and complicate the task of discriminating biological stocks. Although there is no consensus on the threshold level of gene flow above which stock status is no longer recognized, several approaches have been developed to make the identification of biological stocks more objective and explicit.