W&M ScholarWorks VIMS Articles Virginia Institute of Marine Science 1992 Stock Structure Of The Bluefish Pomatomus-Saltatrix Along The Mid-Atlantic Coast John Graves Virginia Institute of Marine Science Jan McDowell Virginia Institute of Marine Science Ana M. Beardsley Virginia Institute of Marine Science Daniel Scoles Virginia Institute of Marine Science Follow this and additional works at: https://scholarworks.wm.edu/vimsarticles Part of the Aquaculture and Fisheries Commons Recommended Citation Graves, John; McDowell, Jan; Beardsley, Ana M.; and Scoles, Daniel, Stock Structure Of The Bluefish Pomatomus-Saltatrix Along The Mid-Atlantic Coast (1992). Fishery Bulletin, 90(4), 703-710. https://scholarworks.wm.edu/vimsarticles/604 This Article is brought to you for free and open access by the Virginia Institute of Marine Science at W&M ScholarWorks. It has been accepted for inclusion in VIMS Articles by an authorized administrator of W&M ScholarWorks. For more information, please contact scholarworks@wm.edu. Abstract.- Restriction-fragment length polymorphism analysis of mitochondrial DNA (mtDNA) was used to investigate the genetic basis of stock structure of the bluefish Pomatomus saltatrix along the U.S. mid-Atlantic coast, and to determine the degree of genetic differentiation between mid-Atlantic bluefish and Australian conspecifics. A total of 472 young-ofthe-year (YOY) and yearling bluefish collected in New Jersey, Virginia, and North Carolina over a period of 3 years, and 19 YOY bluefish collected in New South Wales, Australia were analyzed with 9 informative restriction endonucleases. Despite considerable mtDNA variation within samples of U.S. mid-Atlantic bluefish, no significant genetic differentiation was detected among springspawned and summer-spawned (YOY) bluefish, YOY and yearling bluefish from different geographic locations along the mid-Atlantic coast, or yearling bluefish collected at the same location in different years. MidAtlantic bluefish differed from their Australian conspecifics by three or more restriction site differences, or a mean nucleotide sequence divergence of 1.96%. In addition, Australian bluefish demonstrated greatly reduced levels of mtDNA variation relative to the mid-Atlantic samples. The results of this study suggest that bluefish along the mid-Atlantic coast comprise a single genetic stock and that significant differentiation occurs among geographically disjunct populations of this widely distributed marine fish. Manuscript accepted 29 July 1992. Fishery Bulletin, U.S. 90:703-710 (1992). Stock structure of the bluefish Pomatomus saltatrix along the mid-Atlantic coast* John E. Graves Jan R. McDowell Ana M. Beardsley Daniel R. Scoles Virginia Institute of Marine Science. School of Marine SCience College of William and Mary. Gloucester Point. Virginia 23062 The bluefish Pomatomus saltatrix is bluefish within the estuaries during broadly distributed in temperate and the middle and late summer (Nyman warm-temperate coastal waters of and Conover 1988, McBride 1989), a the world's oceans (Briggs 1960), difference that is still evident in yearalthough it is absent from the eastern ling fish and may persist until fish Pacific (Smith 1949). In the United reach 4 years of age (Lassiter 1962). States, bluefish occur along the At- The extent to which each of the lantic and Gulf coasts, supporting major spawning events contributes large recreational and commercial juveniles to specific areas appears to fisheries. vary annually (Chiarella and Conover The movements and biology of the 1990). bluefish, like many fishes along the A general mixing of bluefish from Atlantic coast, are closely tied to different coastal areas may occur at large seasonal fluctuations in water the end of the first summer. Tagging temperature (reviewed in Wilk 1977). studies indicate that as water temSpawning appears to be concentrated peratures cool, young bluefish move in two spatially and temporally dis- out of the estuaries in a southerly tinct events: a spring spawn at the direction and probably overwinter in inside edge of the Gulf Stream in the the south Atlantic bight (Lund and south Atlantic bight, and a summer Maltezos 1970, Wilk 1977), while spawn in the shelf waters of the mid- adults move further offshore (Wilk Atlantic bight (Kendall and Walford 1977). As temperatures along the 1979). However, the presence of eggs mid-Atlantic coast warm in the and larvae indicates that some spawn- spring, there is a general movement ing occurs throughout the year, espe- of bluefish up the Atlantic coast, with cially in the southern portion of the larger bluefish making more extensouth Atlantic bight (Kendall and sive migrations into northern waters Walford 1979, Collins and Stender (Wilk 1977). 1988). Presumably, eggs and larvae Although the seasonal movements are transported by cross-shelf cur- of bluefish may be conducive to a rents to estuaries along the Atlantic mixing of fish from different coastal coast which serve as nursery grounds areas, mark and recapture studies for the young bluefish. suggest that a large fraction of blueThe discrete temporal nature of the fish are recaptured in the same two spawning events is evidenced by general area in which they were a bimodal size distribution of juvenile tagged (Lund and Maltezos 1970, Wilk 1977). The degree to which this * Contribution 1750 of the Virginia Institute fidelity affects stock structure is not of Marine Science. known. 703 Fishery Bulletin 90(4). J 992 704 Table I NEW JERSEY Sample size, date, location, and age of bluefish PmnatQ7II;/J,8 saltatrix collected and analyzed in this study. YRL = yearling; YOY = young-ofthe-year. Sample VA88 VA89 VA90 NCSS NC89 NC90 NJ90-Sp NJ90-Su AU91 100 102 39 83 57 40 26 25 19 Date Location Age 7/88 7/89 7/90 7/88 7/89 7/90 8/90 8/90 2/91 York River VA York River VA York River VA Hatteras NC Hatteras NC Hatteras NC southern NJ southern NJ Port Stephens, N.S.W., Australia YRL YRL YRL YRL YRL YOY YOY YOY YOY SPRING-SPAWNED 10- I SUMMER·SPAWNED 8 6 4 2 o NORTH CAROLINA - 16.--------------------, 12 The genetic basis of population structure of the bluefish is poorly understood. Based on studies of morphological and scale characteristics, 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 recapture analysis that several populations are present along the midAtlantic 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 consistent 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 location 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. Materia's and methods II: W ~ 8 z 4 AUSTRALIA 3 2 ~-. 0'+_ :~-h + 1- + -r _. r-+ 50 100 150 200 250 STANDARD LENGTH (mm) Figure I Frequency distribution of standard lengths among YOY bluefish PmnatQ1nus sa-ltat-rix collected in New Jersey, North Carolina, and Port Stephens, N.S.W.• Australia. The New Jersey fish were separated into spring- and summerspawned groups based upon their standard length on the date of capture relative to a standard length of 125mm (Nyman and Conover 1988, McBride 1989). 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 collected by trawl on New Jersey state survey cruises during August 1990 (NJ90-Sp, NJ90-Su, Table 1). Fish were classified as spring- 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 in August (Nyman and Conover 1988, McBride 1989). The distribution of lengths is presented in Figure 1. 705 Graves et al.: Stock structure of Pomatomus saltatrix along the mid-Atlantic coast To obtain an estimate of the VIRGINIA NORTH CAROLINA degree of temporal genetic varia35 tion between bluefish year1988 1988 30 classes at a single collection loca15tion, 1-year-old (yearling) blue25 fish were purchased from com2010 mercial fishermen on the York 15 River, Virginia during July 1988 10 (VA88), 1989 (VA89), and 1990 5 (VA90), and in Hatteras, North 5 Carolina during 1988 (NC88) and 0 0 1989 (NC89). The distrib4tion of lengths of the Virginia and North 2_ -Carolina samples is presented in 1989 1989 Figure 2. 1515 An analysis of geographic population structure of highly vagile 10 10 fishes, like the bluefish, is problematic. The presence of an adult bluefish in one geographic loca5 5 tion is not very meaningful, as the fish could easily travel to r 0 0 150 200 250 300 350 another location several hundred 14 -. - - ~ STANDARD LENGTH (mm) kilometers away within a few weeks. If discrete geographic 1990 12stocks of bluefish exist, such lDstocks might be expected to sep8 arate at the time of spawning. However, collection of adults at 6 this critical time is difficult since 4 bluefish spawn at the edge of the continental shelf during the spring and in the middle of the 150 200 250 300 350 shelf during the swnmer (Kendall STANDARD LENGTH (mm) and Walford 1979). Thus we decided to focus our study on their Figure 2 . products, YOY bluefish. Although Frequency distribution of standard lengths among yearling bluefish Pomatomus ;wrt~la8 some mixing probably occurs collected in (left) the York River, VA during summer 1988, 1989, and 1990, and (rIght) during cross-shelf transport, the Hatteras, NC during summer 1988 and 1989. genetic composition of YOY bluefish should reflect the composition of the offshore spawning population. . To determine genetic differentiation among bluefIsh mtDNA analysis along the mid-Atlantic coast, samples of YOY individDepending on size and quality of the bluefish, three uals were collected during summer 1990 in New Jersey different procedures were used to analyze bluefish (described above) and purchased from commercial mtDNA. The rapid isolation procedure of Chapman and fishermen in Hatteras, North Carolina (NC90). In adPowers (1984) was used to obtain mtDNA from dition to obtain an estimate of the degree of mtDNA samples of lateral red muscle from the yearling bluefish dif er~nt a o between isolated bluefish populations, collected in 1988 and 1989. After digestion, restriction a sample of 19 YOY bluefish was collected by hookfragments were separated electrophoretically on and-line in Port Stephens, N.S.W., Australia during 0.8-1.5% agarose gels run at 2 volts/em overnight and February 1991 (AU91). The size composition of all YOY visualized directly with ethidium bromide staining. For collections is presented in Figure 1. those samples in which there was not sufficient mtDNA -- nf rth :I+-rlf±~n'" rhD 0 n- Un 706 Fishery Bulletin 90141. J 992 for direct visualization, restriction digestions were endlabeled before electrophoresis with a mixture of all four 35S nucleotide triphosphates using the Klenow fragment (Maniatis et al. 1982). After electrophoresis, gels were treated with a scintillation enhancer, dried, and autoradiographs exposed at - 70°C for 5 days. Mitochondrial DNA was purified from YOY and yearling bluefish collected in 1990 and 1991 following the protocols of Lansman et al. (1981) and 35S-end- labeled restriction fragments were visualized autoradiographically after electrophoresis. Due to the thermal history of many of these specimens, yields of supercoiled mtDNA were low. In those instances, the nuclear band containing both nuclear DNA and relaxed mtDNA. was collected and dialyzed as described for mtDNA bands in Lansman et al. (1981), or mtDNA was reisolated following the Chapman and Powers (1984) protocol. For these samples, the Southern transfer and Table 2 Distribution of mtDNA genotypes among bluefish Pomatam11.S saltat1i.x samples. Each letter represents the fragment pattern for a particular restriction endonuclease: from left to right. AvaI, HindIlI. PvuII. DmI, Ec.()RV, SstI, PstI, SstII, and Neil. A description of all fragment patterns and sizes is available from the authors upon request. Composite genotype AAAAAAAAA AAAAAAAAB AAAAAAAAC AAAAAAAAD AAAAAAAAG AAAAAAAAH AAAAABAAA AAAABAAAA AAAABAAAB AAAABAABA AAAACAAAA AAAACAAAC AAAACAABA AAAADAAAA AAABAAAAA AAACAAAAA AAAEEAAAD AAAEFAAAD AABAAAAAA AABABAAAA AABABAAAB AABABAAAC AABABAAAE AACAAAAAA AACACAAAA BAAAAAAAA BAAAAAAAC BAAACAAAA BAAACAAAD BAAACBAAA BAAADAAAA BADAAAAAA BADACAAAA CAAAAAAAA CAAAAAAAC CAAABAAAC DAAAAAAAA DAAACAAAA DACAAAAAA EAAAAAAAF FAAAAAAAA Totals VAS8 VAS9 VA90 NCS8 NC89 NC90 NJ90-Sp NJ90-Su AU91 Total 44 0 1 6 0 0 0 45 0 2 1 1 0 0 1 1 0 3 0 0 0 0 0 0 2 0 0 4 0 1 0 0 1 2 0 0 2 2 0 4 0 2 0 0 1 0 1 0 0 0 1 0 0 1 1 0 0 2 0 0 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 6 0 0 13 0 1 0 0 2 4 0 0 2 0 2 2 1 1 0 1 0 0 0 1 1 1 2 1 0 0 1 17 1 0 0 0 0 0 5 0 0 2 1 1 4 0 0 0 0 0 3 0 0 1 0 0 2 0 0 0 0 0 0 0 0 2 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 IS 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 250 3 7 7 3 11 33 1 1 0 1 0 0 3 1 0 3 0 0 2 1 0 0 0 1 1 0 1 0 1 0 3 0 0 0 0 1 0 0 1 0 0 1 0 1 0 0 18 0 2 0 0 0 1 0 0 6 0 0 7 0 1 0 0 0 3 1 0 0 1 2 6 0 5 1 0 0 1 0 50 1 1 0 0 0 0 5 0 20 0 0 0 0 0 0 11 24 0 0 0 1 1 1 1 0 0 3 0 0 3 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 100 102 36 83 57 40 26 25 19 469 1 1 1 1 1 2 38 1 3 24 1 1 35 2 3 18 1 4 14 1 1 5 4 4 18 1 S 1 1 2 1 1 3 8 1 5 1 1 1 2 Graves et al.: Stock structure of Pomatomus saltatnx along the mid-Atlantic coast hybridization protocols of Maniatis et al. (1982) were followed after digestion and electrophoresis. Highly purified bluefish mtDNA, nick translated with biotin7-dATP, was used as a probe for mtDNA fragments. Hybridization filters were visualized after strigency washes using the BRL BlueGene Nonradioadtive Nucleic Acid Detection System. All mtDNA samples were digested with the following nine restriction endonucleases used according to the manufacturers' instructions: Aval, Dral, EcoRV, HindHI, Neil, Pst!, PvuH, Sstl, and SstH. The different restriction-fragment patterns produced by each restriction endonuclease were assigned a letter, and a composite mtDNA genotype, consisting of nine letters representing the fragment patterns generated by each of the restriction endonucleases, was constructed for each individual. The nucleon diversity (Nei 1987) was calculated for each sample and for the pooled samples. The nucleotide sequence divergence among mtDNA genotypes was estimated by the site approach of Nei and Li (1979). The mean nucleotide sequence diversity within samples and mean nucleotide sequence divergence between samples were calculated following the method of Nei (1987), with the latter value being corrected for within-group diversity (Nei 1987). The distribution of genotypes was evaluated for homogeneity among collections using the G-test (Sokal and Rohlf 1981); however, as several of the genotypes were represented by one individual, we employed the Roff and Bentzen (1989) Monte Carlo approach to estimate the significance of heterogeneity X2 values determined from the raw data. 707 from completely additive changes in fragment patterns. Considerable RFLP variation was detected within Atlantic bluefish samples (Table 2). The most common mtDNA genotype, AAAAAAAAA, ranged in frequency from 0.43 (NC 1990 YOy) to 0.75 (NJ 1990 YOY). The large number of variant genotypes resulted in nucleon diversities ranging from 0.416 to 0.798 (Table 3). Because many of the variant genotypes differed from the common genotype by several site changes, the within-sample mean nucleotide sequence diversities were also relatively high, varying from 0.63% to 1.490/0. In contrast to the mid-Atlantic bluefish, the Australian sample was quite depauperate of variation. Of the 19 fish in the sample, 18 shared a common mtDNA genotype (AAAEEAAAD), and one fish had a genotype differing from the common type by a single site change (Table 2). The lack of variation in the Australian sample was reflected in a low nucleon diversity (0.105) and a within-sample mean nucleotide sequence diversity of 0.07%. Significant genetic differentiation was not found between the samples of spring- and summer-spawned YOY bluefish collected in New Jersey during the summer of 1990. The corrected mean nucleotide sequence divergence between the two samples was extremely small (0.02%), indicating that average sequence divergence between two individuals randomly drawn from either the spring- or summer-spawned sample was the same as the divergence between two individuals randomly drawn from each group. Table 3 Results The analysis of 472 mid-Atlantic bluefish with 9 restriction endonucleases revealed 40 mtDNA genotypes, and 2 mtDNA genotypes were encountered among 19 Australian bluefish. A total of 77 restriction fragments was visualized, and the average individual was scored for 34 fragments, accounting for approximately 1.4% of the mtDNA genome. The restriction endonucleases, HindHI and PstI, revealed no variant fragment patterns, while the remaining seven enzymes revealed from two (SstI and SstH) to eight (Neil) different fragment patterns. Restrictionsite gains or losses were inferred Genetic variation within bluefish Pcnnaromus saltatrix samples expressed as nucleon diversity and mean nucleotide sequence diversity. The spring- and summer-spawned NJ YOY bluefish collections were pooled (NJ90 combined) for comparison with the NC90 YOY sample. and all NJ, VA, and NC bluefish collections were pooled (mid-Atlantic combined) for comparison with the AU91 YOY sample. YRL = yearling; YOY = YOWlg-of-the-year. Sample Age n Nucleon diversity Mean nucleotide sequence diversity VA88 VA89 VA90 NC88 NC89 NJ90-Sp NJ90-Su YRL YRL YRL YRL YRL YOY YOY 100 102 36 83 57 26 25 0.781 0.777 0.565 0.632 0.663 0.416 0.467 1.34% 1.410/0 0.89% 1.15% 1.20% 0.72% 0.63% NJ90 combined YOY 51 0.438 ·0.67% NC90 YOY 40 0.798 1.49% 372 0.696 1.23% 19 0.105 0.07% -- - - -------- -- .. -- . - ----- . - .. --- -------- - - - .... - . - - - - - . - - . - - mid-Atlantic combined AU91 YOY 708 Fishery Bulletin 90(4). J 992 Considerable genetic differenTable 4 tiation was not detected among Mean nucleotide sequence divergences (%) among selected bluefish Pomatomus saltasamples of yearling bluefish coltrix collections. Values are presented with and without correction for within-sample variation. lected at the same site in different years. The mean nucleotide Corrected Uncorrected Collections sequence divergences (Table 4) Among collections at a single location over 2 or more years among the VAS8, VA89, and 0.11 1.39 VA88 vs. VA89 VA90 collections, and between 0.18 VA88 vs. VA90 1.20 the NC88 and NC90 samples, 0.05 1.20 VA89 vs. VA90 were of the same magnitude as 0.01 NC88 vs. NC89 1.18 the within-sample mean nucleoBetween spring- and summer-spawned bluefish tide sequence diversities (Table 0.69 0.02 NJ90-Sp vs. NJ90-Su 3). Consequently, when adjusted Between mid-Atlantic yay fish for within-sample diversity (Nei 1.19 0.11 NJ90-combined vs. NC90 1987), the corrected mean nuBetween mid-Atlantic and Australian bluefish cleotide sequence divergences 2.60 1.96 mid-Atlantic combined vs. AU91 among samples were nearly zero (Table 4). Analysis of YOY bluefish from found in the Gulf of Mexico mtDNA individuals were the northern and southern mid-Atlantic bight revealed also present in the mid-Atlantic samples, and 7 of the little mtDNA genetic differentiation. The corrected 10 Gulf of Mexico bluefish had the common midmean nucleotide sequence divergence between the comAtlantic mtDNA genotype. Because of the small size bined NJ90 YOY sample and the NC90 YOY collection of the Gulf of Mexico sample, it was not appropriate was 0.11%, suggesting little population structuring to test for frequency differences between bluefish from along the mid-Atlantic coast. This inference was furthe mid-Atlantic coast and the Gulf of Mexico. ther supported by an analysis of heterogeneity which demonstrated no significant differences in the distribution of six major mtDNA genotypes (those occurring Discussion in 10 or more of the 472 fish) and the pooled rare Mid-Atlantic bluefish demonstrated considerable mtDNA genotypes among the seven mid-Atlantic collections genotypic variation. It is difficult to directly compare (GH = 39.5, 0.25<P<0.50). Heterogeneity x.2 analysis the nucleon diversities calculated in this study with of the distribution of all genotypes, including those those from other studies because the value is sensitive represented by a single individual, was performed using to the number of restriction sites surveyed, and the Monte Carlo simulation of Roff and Bentzen (1989). 2 analyses employing larger numbers of restriction endoA total of 320 of the 1000 randomizations produced x. nucleases typically have higher nucleon diversities. The values greater than the original data set, indicating no value of 0.696 for the pooled mid-Atlantic bluefish significant heterogeneity. samples is higher than those reported for many marine The low levels of mtDNA differentiation among midfishes surveyed with a larger number of enzymes (Avise Atlantic bluefish collections contrasted with the subet al. 1989, Gold and Richardson 1991), and indicates stantial difference encountered between the combined a relatively high degree of genetic variation within the mid-Atlantic bluefish and the Australian sample. The average mid-Atlantic bluefish could be distinguished bluefish. This trend becomes more apparent when from its Australian conspeci:fic by three or more restricmean nucleotide sequence diversities, a measure of intrasample diversity that is much less sensitive to the tion-site changes. Two of the site changes were unique to the Australian sample, and the third (Neil pattern number of restriction sites surveyed, are compared. The value calculated in this study for the pooled midD) occurred at a low frequency (0.01) in the combined Atlantic samples, 1.23%, is higher than values reported mid-Atlantic sample. The corrected mean nucleotide sequence divergence between the Australian sample and for many other marine fishes (Ovenden 1990). the combined mid-Atlantic bluefish samples was 1.95%. The Australian bluefish demonstrated much less Significant heterogeneity was noted among the pooled variation than their mid-Atlantic conspecifics. The samples when the Australian sample was included with sample of 19 Australian bluefish had a nucleon diverthe mid-Atlantic bluefish (GH = 177, p<O.OOl). sity five times lower than the combined Atlantic A sample of 10 yearling bluefish was analyzed from samples, and a mean nucleotide sequence diversity that the northeast Gulf of Mexico (Panama City, FL). Unlike was an order of magnitude lower (Table 3). A similar the Australian bluefish, all of the mtDNA genotypes difference in the level of mtDNA variation between Graves et al.: Srock structure of Pomatomus saltatrix along the mid-Atlantic coast . conspecific populations has been noted between Atlantic and Pacific blue marlin (Graves and McDowell, unpubl. data). The striking lack of variation within the Australian sample could be the result of a smaller effective population size of females resulting from population bottlenecks, or may simply reflect a period of isolation sufficient for the sorting of gene trees (Nei 1987, Avise et al. 1988, Chapman 1990, Bowen and Avise 1990). We found little evidence to support the hypothesis that genetically distinct stocks of bluefish exist along the mid-Atlantic coast. Although appreciable mean nucleotide sequence divergences were found between sampling locations (Table 4), when corrected for withingroup variation the values became extremely small, indicating that most of the observed differentiation could be accounted for by variation within the samples. The lack of population structuring was also supported by the homogeneous distribution of all genotypes and the fact that the level of genetic divergence among sampling locations was not appreciably greater than the level of divergence among samples taken at anyone location in different years. The extent of gene flow among populations can also be inferred from the frequency distribution of rare alleles (Slatkin 1989). An inspection of Table 2 indicates that almost all mtDNA genotypes that occurred more than once were found in different collections, suggesting significant gene flow among sampling locations. For example, the genotype AAAABAABA, which was present in three individuals, occurred in the VA89, Ne88, and NJ90-Su collections. An exception to this pattern was presented by the genotype AAAAAAAAD, which occurred seven times: in six individuals of the VA88 sample and one individual of the VA89 sample. However, an examination of bluefish mtDNA genotypes not included in this analysis-because the individuals were greater than one year old, or because they came from a sample that was too small for inclusion in this analysis-suggests that the observed distribution of the AAAAAAAAD genotype may be an artifact of sampling error. The genotype was present in two bluefish collected in 1988 (one in New York and one in Connecticut) and in six bluefish collected in 1989 (two in New York, two in Virginia, and two in North Carolina). In contrast to the genetic similarity among midAtlantic samples, a large, consistent genotypic difference was noted between the mid-Atlantic bluefish and a conspeci:fic population in Australia. The corrected mean nucleotide sequence divergence of almost 2% is more than an order of magnitude larger than the values detected among mid-Atlantic samples, and is similar to values reported between northwest Atlantic and Barents Sea capelin populations (Dodson et al. 1991) 709 or among populations of freshwater fishes of different river systems (Bermingham and Avise 1986). While significant genetic differentiation was found between mid-Atlantic and Australian bluefish, no major differences were detected between mid-Atlantic bluefish and a small sample from the Gulf of Mexico. Consistent restriction-site differences have been reported between Gulf of Mexico and mid-Atlantic populations of a number of marine organisms, including horshoe crabs Limulus polyphemus (Saunders et al. 1986), oysters Crassostrea virginica (Reeb and Avise 1990), and black sea bass Centropristis striata (Bowen and Avise 1990). These preliminary results suggest that bluefish from the Gulf of Mexico and the niid-Atlantic are not as genetically isolated as many other coastal marine species, although much larger samples will have to be surveyed to determine if significant mtDNA genotypic frequency differences exist between the two areas. Considering the high vagility of bluefish and their continuous distnbution around Florida, this result is not unexpected. The lack of significant genetic differentiation between spring- and summer-spawned bluefish is consistent with the results of Chiarella and Conover (1990), who found no correlation between the season in which an adult bluefish spawned and the hatch-date of an individual. These data suggest that the bimodal distribution of YOY bluefish in mid-Atlantic estuaries results from two major spawning events of the same population of bluefish, rather than the participation of different stocks. The morphological differences found between spring- and summer-spawned bluefish are probably ecophenotypic, resulting from early-lifehistory development in appreciably different environments. Similar morphological plasticity has been demonstrated in many other marine fishes (Barlow 1961). The high degree of genetic homogeneity detected within mid-Atlantic bluefish is also consistent with the results of tag and recapture studies. While many bluefish return to the same site for several years (Lund and Maltezos 1970), migratory habits appear to change with age (Wilk 1977). Thus, the potential exists for considerable interchange, and it is important to note that even small levels of exchange can prevent the accumulation of genetic differentiation (Hartl 1988). The results of this study cannot disprove the null hypothesis that bluefish along the mid-Atlantic coast share a common gene pool. There appears to be sufficient gene flow to prevent the accumulation of even slight genetic differences. Determining the magnitude of exchange between geographic regions would require an extensive tag and recapture program. Until such data are available, the resource should be managed as assumed in the Fishery Management Plan for the Bluefish-as a single, genetically homogeneous stock. 710 Acknowledgments Bluefish were kindly provided by Hunt Howell, Alice Webber, Raoul Castaneda, Bill Andrews, Katy West, Debbie Fabel, Steve Battaglane, and R. Bill Talbot. This project resulted from a study initiated by Herb Austin and Brian Meehan of the Virginia Institue of Marine Science. Robert Chapman provided helpful advice with the statistical analysis. Critical reviews of the manuscript were provided by John Olney and John Musick. Funding for this research was provided by the U.S. Fish and Wildlife Service (F-60-R) and the Commonwealth of Virginia. Citations Avise, J.C., R.M. Ball. and J. Arnold 1988 Current versus historical population sizes in vertebrate species with high gene flow: A comparison based on mitochondrial DNA lineages and inbreeding theory for neutral mutations. Mol. BioI. Evol. 5:331-344. 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