| Summary: | Identification of subpopulations within species of commercially harvested fishes is necessary to effective fishery management. Subpopulations may differ in their capacity to support harvests and, in order to maintain an optimum yield from all fish stocks, each subpopulation must be managed separately. Management without regard to subpopulation structure may result in under harvest of some stocks and over harvest of others. Further, failure to recognize the existence of subpopulations can result in the complete elimination of vulnerable substocks and subsequent loss of genetic variability within the population in general (Larkin 1977). The subpopulation can be defined as a fraction of a population that is itself genetically self-sustaining (Marr 1957). This definition is similar to the local population or deme defined by Mayr (1963). The subpopulation is also referred to as a substock (Larkin 1977) or unit stock (Parrish 1964). Subpopulations may have their own characteristics of growth, mortality, recruitment, migration and behavior, more or less independent from one another and, since they sometimes inhabit the same general locality, may be exploited together during part or all of their lifetime (Marr and Sprague 1963). Marr and Sprague (1963) group methods of studying subpopulations into four categories. They include (1)the study of fish movements as revealed by tagging or marking (2)the study of vital statistics (3)the study of phenotypic qualities (meristic and morphometric characteristics) and (4)genetic characteristics. Tagging experiments are costly in time and effort and are limited to areas where an effective recapture method exists. Vital statistics, according to Marr and Sprague (1963), refer more to a definition of subpopulations than to a tool for studying them. Phenotypic characteristics, although often environmentally modifiable, can be of practical importance in delineating subpopulations provided certain conditions are met during sample collection. The study of genetic characteristics, that is measurable characteristics which bear a direct relationship to the genotype and are not environmentally modifiable, can be a very useful tool in identifying subpopulations. In particular, gene frequency data based on allelic polymorphisms can serve to characterize discrete, reproductively isolated subpopulations (Wilkins 1972). In order to successfully employ any or all of the...
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