704 



Fishery Bulletin 90(4). 1992 



The genetic basis of population structure of the bluefish is poorly 

 understood. Based on studies of morphological and scale char- 

 acteristics, Wilk (1977) suggested that two populations exist along 

 the mid-Atlantic coast. These populations correspond to the fish 

 which spawn off North Carolina in the spring, and those that 

 spawn in the northern mid- Atlantic during the summer. Lund and 

 Maltezos (1970) also concluded on the basis of mark and recap- 

 ture analysis that several populations are present along the mid- 

 Atlantic coast. Chiarella and Conover (1990) used scales from 

 summer-spawning fish in the New York Bight to back-calculate 

 length at age-1 and found that most summer-spawning fish had 

 lengths corresponding to a spring birthdate, a result not consis- 

 tent with spring- and summer-spawning stocks. They concluded 

 that the morphological and life-history differences found between 

 spring- and summer-spawned bluefish are probably ecophenotypic 

 in nature, and suggested that a direct genetic analysis of stock 

 structure was warranted. 



In this paper, we present the results of a restriction-fragment 

 length polymorphism (RFLP) analysis of bluefish mitochondrial 

 DNA (mtDNA) among bluefish collected along the mid-Atlantic 

 coast over a period of 3 years. We employed RFLP analysis of 

 mtDNA to evaluate genetic differentiation between spring- and 

 summer-spawned bluefish collected at a single location at the same 

 time, among similarly-sized bluefish collected at the same loca- 

 tion over several years, and among bluefish collected during the 

 same year from the north and south mid-Atlantic coast, as well 

 as from a disjunct population in Australia. 



Materials and methods 



Experimental design and collections 



Bluefish were collected along the mid-Atlantic coast during 

 1988-90, and in Australia during 1991 (Table 1). To test the 

 hypothesis that spring- and summer-spawned bluefish represent 

 genetically distinct stocks, young-of-the-year bluefish were col- 

 lected by trawl on New Jersey state survey cruises during August 

 1990 (NJ90-Sp, NJ90-SU, Table 1). Fish were classified as spring- 



10 



8- 



NEW JERSEY 



SPRING-SPAWNED 



i 



SUMMER-SPAWNED 



-1 — t^ — I — i- 



NORTH CAROLINA 



AUSTRALIA 



r ~l ~ 



"50 100 150 200 



STANDARD LENGTH (mm) 



250 



Figure 1 



Frequency distribution of standard lengths 

 among YOY bluefish Pomatomus saltatrix col- 

 lected in New Jersey. North Carolina, and Port 

 Stephens. N.S.W-, Australia. The New Jersey 

 fish were separated into spring- and summer- 

 spawned groups liased upon their standard 

 length on the date of capture relative to a stan- 

 dard length of 125 mm (Nyman and Conover 

 1988. McBride 1989). 



or summer-spawned based on the date of 

 capture using a standard length of 125 mm 

 used as the cut-off between the two groups 



August (Nyman and Conover 1988, 



in 



McBride 1989). The distribution of lengths 

 is presented in Figure 1. 



