708 



Fishery Bulletin 90(4), 1992 



Considerable genetic differen- 

 tiation was not detected among 

 samples of yearling bluefish col- 

 lected at the same site in differ- 

 ent years. The mean nucleotide 

 sequence divergences (Table 4) 

 among the VA88, VA89, and 

 VA90 collections, and between 

 the NC88 and NC90 samples, 

 were of the same magnitude as 

 the within-sample mean nucleo- 

 tide sequence diversities (Table 

 3). Consequently, when adjusted 

 for within-sample diversity (Nei 

 1987), the corrected mean nu- 

 cleotide sequence divergences 

 among samples were nearly zero 

 (Table 4). 



Analysis of YOY bluefish from 

 the northern and southern mid-Atlantic bight revealed 

 little mtDNA genetic differentiation. The corrected 

 mean nucleotide sequence divergence between the com- 

 bined NJ90 YOY sample and the NC90 YOY collection 

 was 0.11%, suggesting little population structuring 

 along the mid- Atlantic coast. This inference was fur- 

 ther supported by an analysis of heterogeneity which 

 demonstrated no significant differences in the distribu- 

 tion of six major mtDNA genotypes (those occurring 

 in 10 or more of the 472 fish) and the pooled rare 

 genotypes among the seven mid-Atlantic collections 

 (Gh=39.5, 0.25<P<0.50). Heterogeneity x" analysis 

 of the distribution of all genotypes, including those 

 represented by a single individual, was performed using 

 the Monte Carlo simulation of Roff and Bentzen (1989). 

 A total of 320 of the 1000 randomizations produced x" 

 values greater than the original data set, indicating no 

 significant heterogeneity. 



The low levels of mtDNA differentiation among mid- 

 Atlantic bluefish collections contrasted with the sub- 

 stantial difference encountered between the combined 

 mid-Atlantic bluefish and the Australian sample. The 

 average mid-Atlantic bluefish could be distinguished 

 from its Australian conspecific by three or more restric- 

 tion-site changes. Two of the site changes were unique 

 to the Australian sample, and the third (A^dl pattern 

 D) occurred at a low frequency (0.01) in the combined 

 mid-Atlantic sample. The corrected mean nucleotide se- 

 quence divergence between the Australian sample and 

 the combined mid-Atlantic bluefish samples was 1.95%. 

 Significant heterogeneity was noted among the pooled 

 samples when the Australian sample was included with 

 the mid-Atlantic bluefish (Gh = 177, p<0.001). 



A sample of 10 yearling bluefish was analyzed from 

 the northeast Gulf of Mexico (Panama City, FL). Unlike 

 the Australian bluefish, all of the mtDNA genotypes 



found in the Gulf of Mexico mtDNA individuals were 

 also present in the mid-Atlantic samples, and 7 of the 

 10 Gulf of Mexico bluefish had the common mid- 

 Atlantic mtDNA genotype. Because of the small size 

 of the Gulf of Mexico sample, it was not appropriate 

 to test for frequency differences between bluefish from 

 the mid- Atlantic coast and the Gulf of Mexico. 



Discussion 



Mid-Atlantic bluefish demonstrated considerable mtDNA 

 genotypic variation. It is difficult to directly compare 

 the nucleon diversities calculated in this study with 

 those from other studies because the value is sensitive 

 to the number of restriction sites surveyed, and 

 analyses employing larger numbers of restriction endo- 

 nucleases typically have higher nucleon diversities. The 

 value of 0.696 for the pooled mid-Atlantic bluefish 

 samples is higher than those reported for many marine 

 fishes surveyed with a larger number of enzymes (A vise 

 et al. 1989, Gold and Richardson 1991), and indicates 

 a relatively high degree of genetic variation within the 

 bluefish. This trend becomes more apparent when 

 mean nucleotide sequence diversities, a measure of 

 intrasample diversity that is much less sensitive to the 

 number of restriction sites surveyed, are compared. 

 The value calculated in this study for the pooled mid- 

 Atlantic samples, 1.23%, is higher than values reported 

 for many other marine fishes (Ovenden 1990). 



The Australian bluefish demonstrated much less 

 variation than their mid-Atlantic conspecifics. The 

 sample of 19 Australian bluefish had a nucleon diver- 

 sity five times lower than the combined Atlantic 

 samples, and a mean nucleotide sequence diversity that 

 was an order of magnitude lower (Table 3). A similar 

 difference in the level of mtDNA variation between 



