Graves et al.: Stock structure of Pomatomus saltatnx along the mid-Atlantic coast 



705 



To obtain an estimate of the 

 degree of temporal genetic varia- 

 tion between bluefish year- 

 classes at a single collection loca- 

 tion, 1 -year-old (yearling) blue- 

 fish were purchased from com- 

 mercial fishermen on the York 

 River, Virginia during July 1988 

 (VA88), 1989 (VA89), and 1990 

 (VA90), and in Hatteras, North 

 Carolina during 1988 (NC88) and 

 1989 (NC89). The distribution of 

 lengths of the Virginia and North 

 Carolina samples is presented in 

 Figure 2. 



An analysis of geographic pop- 

 ulation structure of highly vagile 

 fishes, like the bluefish, is prob- 

 lematic. The presence of an adult 

 bluefish in one geographic loca- 

 tion is not very meaningful, as 

 the fish could easily travel to 

 another location several hundred 

 kilometers away within a few 

 weeks. If discrete geographic 

 stocks of bluefish exist, such 

 stocks might be expected to sep- 

 arate at the time of spawning. 

 However, collection of adults at 

 this critical time is difficult since 

 bluefish spawn at the edge of the 

 continental shelf during the 

 spring and in the middle of the 

 shelf during the summer (Kendall 

 and Walford 1979). Thus we de- 

 cided to focus our study on their 

 products, YOY bluefish. Although 

 some mixing probably occurs 

 during cross-shelf transport, the 

 genetic composition of YOY blue- 

 fish should reflect the composi- 

 tion of the offshore spawning 

 population. 



To determine genetic differentiation among bluefish 

 along the mid-Atlantic coast, samples of YOY individ- 

 uals were collected during summer 1990 in New Jersey 

 (described above) and purchased from commercial 

 fishermen in Hatteras, North Carolina (NC90). In ad- 

 dition, to obtain an estimate of the degree of mtDNA 

 differentiation between isolated bluefish populations, 

 a sample of 19 YOY bluefish was collected by hook- 

 and-line in Port Stephens, N.S.W., Australia during 

 February 1991 (AU91). The size composition of all YOY 

 collections is presented in Figure 1. 



VIRGINIA 



NORTH CAROLINA 



STANDARD LENGTH (mm) 



STANDARD LENGTH (mm) 



Figure 2 



Frequency distribution of standard lengths among yearling bluefish Pomatomus saltatrix 

 collected in (left) the York River. VA during summer 1988, 1989, and 1990, and (right) 

 Hatteras, NC during summer 1988 and 1989. 



mtDIMA analysis 



Depending on size and quality of the bluefish, three 

 different procedures were used to analyze bluefish 

 mtDNA. The rapid isolation procedure of Chapman and 

 Powers (1984) was used to obtain mtDNA from 

 samples of lateral red muscle from the yearling bluefish 

 collected in 1988 and 1989. After digestion, restriction 

 fragments were separated electrophoretically on 

 0.8-1.5% agarose gels run at 2 volts/cm overnight and 

 visualized directly with ethidium bromide staining. For 

 those samples in which there was not sufficient mtDNA 



