Identification of genetic populations of the Pacific blue shrimp Penaeus stylirostris of the Gulf of California, Mexico

Species are often composed of discrete breeding units (i.e. populations or stocks) which, while not reproductively isolated from other such groups, ma y have limited opportunities to exchange genetic material because of geogra phic distance, barriers to migration, or spawning asynchrony. Low levels of gene flow between stocks may result, over time, in their genetic divergenc e, and species that are subdivided into morphologic or genetically distinct stocks are said to be structured. The aim of our investigation was to test whether or not Penaeus stylirostris from the Gulf of California (Mexico) w as structured into genetically distinct populations. Shrimp samples were co llected in 1996 from six regions of the Gulf where specimens with distinct morphologic characteristics had previously been identified. Statistical ana lysis of 324 RAPD loci (randomly amplified polymorphic DNA). resolved throu gh polyacrylamide gels and scored for each of 78 specimens, permitted the q uantification and comparison of between-stock genetic differences. The find ing that genetically discrete stocks of P. stylirostris call be found in a small portion of the geographic distribution range of the species, disagree s with the long-held perception that this resource is panmictic in nature. This new evidence is not only of interest for selective breeding programs i n the shrimp aquaculture industry, but is also relevant to the management o f the Mexican shrimp fishery which, at present. is perceived and managed as a single stock.