Abstract. - To investigate the 

 genetic basis of stock structure of the 

 weakfish Cynoscion regalis, a total 

 of 370 individuals was collected from 

 four geographic sites along the mid- 

 Atlantic coast of the United States 

 over a period of 4 years. Restriction 

 fragment length polymorphism 

 (RFLP) analysis of weakfish mito- 

 chondrial DNA, employing either 6 

 or 13 restriction endonucleases, dem- 

 onstrated a low level of intraspecific 

 mtDNA variation, with a mean nu- 

 cleotide sequence divergence of 0.13% 

 for the pooled samples. The common 

 mtDNA genotype occurred at a fre- 

 quency of 0.91-0.96 in all samples, 

 and no significant heterogeneity was 

 found among samples in the occur- 

 rence of the common mtDNA geno- 

 type or rare variants. The lack of 

 spatial partitioning of rare mtDNA 

 genotypes among collection sites sug- 

 gests considerable gene flow along 

 the mid-Atlantic coast. Together 

 these data are consistent with the 

 hypothesis that weakfish comprise a 

 single gene pool, and indicate that 

 the fishery should be managed as a 

 single, interdependent unit. 



A genetic analysis of \A/eal<fish 

 Cynoscion regalis stock structure 

 along the mid-Atlantic Coast* 



John E. Graves 

 Jan R. McDowell 



Virginia Institute of Marine Science, T-chool of Marine Science 

 College of William and Mary, Gloucester Point, Virginia 23062 



M. Lisa Jones 



Department of Biology, College of William and Mary, Williamsburg, Virginia 23185 



The weakfish Cynoscion regalis is 

 broadly distributed along the Atlan- 

 tic coast of the United States. It is 

 common from Long Island NY to 

 Cape Canaveral FL, and has occa- 

 sionally been reported from as far 

 north as Nova Scotia and as far south 

 as the Gulf of Mexico (Bigelow and 

 Schroeder 1953, Weinstein and Yer- 

 ger 1976). Weakfish abtmdance varies 

 considerably on both a spatial and 

 temporal basis, especially in the 

 northern part of the species' range 

 (Bigelow and Schroeder 1953). 



The life history of the weakfish has 

 been well studied (reviewed in Wilk 

 1979). Spawning occurs in estuarine 

 and coastal waters from late spring 

 through early fall, with a peak of ac- 

 tivity in late May and early June. 

 There appears to be little offshore 

 transport of the early-life-history 

 stages, and young-of-the-year remain 

 in shallow estuarine and coastal 

 waters during their first summer. 

 Like many fishes along the mid- 

 Atlantic coast, weakfish move off- 

 shore to overwinter as coastal waters 

 cool during the fall, returning in the 

 spring when inshore temperatures 

 increase. 



The seasonal inshore and offshore 

 movements of weakfish could lead to 

 significant mixing of fish from differ- 



Manuscript accepted 4 June 1992. 

 Fishery Bulletin, U.S. 90:469-475 (1992). 



'Contribution 1749 of the Virginia Institute 

 of Marine Science and School of Marine Sci- 

 ence, College of William and Mary. 



ent coastal areas. Tagging studies by 

 Nesbit (1954) showed that a large 

 proportion of weakfish tend to return 

 to the same coastal region in which 

 they were tagged, although many 

 fish were recaptured in areas distant 

 from the original tagging location. 

 The differential size distribution of 

 weakfish along the mid- Atlantic coast 

 is consistent with the hypothesis that 

 mixing of weakfish from different 

 coastal areas occurs. Larger (older) 

 weakfish are more predominant in 

 northern waters during the summer, 

 and the mean size of weakfish tends 

 to decrease as one moves down the 

 Atlantic coast (Wilk and Silverman 

 1976). Whether this represents an 

 ontogenetic change in seasonal move- 

 ments or differential survival or 

 growth of fish from different coastal 

 areas is not known. 



Weakfish support an important 

 commercial and recreational fishery. 

 Commercial landings over the past 

 110 years have undergone dramatic 

 fluctuations. Combined commercial 

 and recreational landings were at a 

 recent peak during 1980 at 36,400 

 metric tons (t) and subsequently 

 dropped to about 19.lt over a period 

 of 2 years (Vaughan et al. 1991). The 

 total catch has remained fairly con- 

 stant for the last 8 years, although a 

 significant decline in landings from 

 northern waters has been noted over 

 the period (Vaughan et al. 1991). For 

 example, the combined commercial 



469 



