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Fishery Bulletin 98(4) 



and therefore, correlation should not be taken as conclu- 

 sive evidence for causation. 



Several reports have stated a small probability of en- 

 countering genetically "differentiated" stocks in Penaeus 

 species, mainly due to their low variability and the appar- 

 ent homogeneous allele distributions among the subpop- 

 ulations (Lester, 1979, 1983; Mulley and Latter, 1981a, 

 1981b; Richardson, 1982). However, Sunden and Davis 

 ( 199 1 ) could trace a slight geographic differentiation across 

 the range of P. uannamei. They detected at least one 

 unique allele in each wild population. Tarn and Chu (1993) 

 attributed the higher genetic similarity shown between 

 the species pair P. merguiensis and Metapenaeus ensis to 

 the genetic differentiation among populations of the same 

 species. In addition, some allozyme variation was obsei-ved 

 among populations of Metapenaeus benettae (Salini, 1987) 



in Australia and P. kerathurus in the Mediterraenean 

 (Mattocia et al., 1987). From later studies, it was noticed 

 that when the allozymes surveys were performed over 

 larger geographical scales, substantially higher signifi- 

 cant variation and differentiation among populations was 

 found. Such is the case for the wild populations of P. 

 monodon along the Australian coast (Benzie et al., 1992), 

 where expected heterozygosities have ranged between 

 0.053 and 0.103, and significant genetic differences have 

 existed among Australian populations. 



The use of more variable markers such as mtDNA genes 

 have confirmed the allozymic findings on the structuring 

 of P. monodon populations in Australia (Benzie et al., 

 1993), and P. notialis and P. schmitti in the South Ameri- 

 can Atlantic coast (Machado et al., 1993). 



From our data, subpopulations of both species appear 

 genetically differentiated in terms of significant values 

 of standardized variance of their genotypic distributions 

 (Table 3). Two reasons seem to account for such a result: 1) 

 private alleles exist in about every polymorphic locus (75% 

 in both species), and 2) the most frequent allele alternates 

 among subpopulations of the same species in several loci 

 (see Table 2). 



Heterogeneity among subpopulations is commonly 

 explained as a lack of genetic flow (see Slatkin, 1985, for 

 a review). Nonetheless, penaeids can be considered vagile 

 species because they have two dispersal phases, the plank- 

 tonic larvae and the vagile adult. Lester (1979) imputes 

 this characteristic to the high levels of genetic similarity 

 among the penaeids populations of the Gulf of Mexico. 



Although penaeids are capable of being displaced, geo- 

 graphical barriers must prevent their movement. The Gulf 

 of California is now considered as a much more complex 

 ecosystem than it was formerly believed: Santamaria-Del 

 Angel et al. (1995) proposed 14 biogeographic subdivi- 

 sions in the Gulf on the basis of satellite images describ- 

 ing concentrations of photosynthetic pigments. Likewise, 

 Gonzalez-Farias et al. (1995) described complicated pat- 

 terns of carbon turn-over throughout the Gulf from the 

 analysis of organic matter and heterotrophic bacteria. 

 The upper Gulf region is a very unique environment 

 where the Colorado River drained until about fifty years 



