Lynch et al.: A genetic investigation of the population structure of Brevoortia tyrannus 
95 
suite of microsatellite markers used in this study were 
sufficient for detecting even weak levels of differentia- 
tion (F st >0.0025). 
Although none of the five COI <P ST AMOVAs or five 
microsatellite R ST AMOVAs were significant, two of the 
five microsatellite F ST AMOVAs showed a small but 
statistically significant partitioning of genetic varia- 
tion between YOY and yearling menhaden collected in 
Chesapeake Bay in successive years (following the 2006 
year class, 1.80%, P=0.0176) and YOY and yearling 
menhaden (combined) from the four geographic regions 
along the U.S. Atlantic coast (0.575%, P- 0.0000). 
The pairwise comparisons between sample locations 
corroborate the <P ST and R ST AMOVA results. No pair- 
wise comparison revealed a statistically significant 
difference between any two of the four geographic re- 
gions of Atlantic menhaden after Bonferroni correction. 
These findings support the hypothesis that the sig- 
nificant results from the ^st AMOVAs were a result of 
random processes and not biologically meaningful (for 
a discussion see Waples, 1998). The collective results 
indicate no significant partitioning of genetic varia- 
tion between the sampling regions of Atlantic menha- 
den, and the null hypothesis that Atlantic menhaden 
comprise a single stock along the U.S. Atlantic coast 
cannot be rejected. 
The lack of statistically significant genetic differ- 
ences among Atlantic menhaden sampling regions is 
consistent with the life history traits of the species. Of 
all the North American Brevoortia, Atlantic menhaden 
undertake the longest coastal migrations and have the 
most temporally and geographically protracted spawn- 
ing season (Whitehead, 1985). Atlantic menhaden are 
batch spawners, spawning multiple times during a year. 
Additionally, Atlantic menhaden larvae are found in 
waters from Maine to Mexico and are the most widely 
distributed larvae of any clupeoid in the western North 
Atlantic; (Kendall and Reintjes, 1975). The larvae can 
take up to 90 days to cross the continental shelf and 
are affected by along-shore transport, coastal storms, 
freshwater discharge from estuaries, and wind-forcing 
(Checkley et al., 1988). Menhaden also undergo an on- 
togenetic shift in migration, where larger fish migrate 
farthest north (Dryfoos et al., 1973). These characteris- 
tics appear to keep Atlantic menhaden — and their gene 
pool — well mixed. 
Population structure has not been found in genetic 
analyses of other clupeids including Atlantic herring 
( Clupea harengus ) (Grant, 1984), twaite shad ( Alosa fal- 
lax) (Volk et al., 2007), and European pilchard ( Sardina 
pilchardus) (Gonzalez and Zardoya, 2007). In contrast, 
some clupeid species exhibit significant stock structure, 
often attributed to the presence of geographic barri- 
ers or temporal reproductive isolation. These include 
Pacific herring (Clupea pallasi) stocks in the eastern 
North Pacific and Bering Sea (Grant and Utter, 1984), 
in the Bering Sea and Gulf of Alaska separated by the 
Alaska Peninsula (O’Connell et al., 1998), and from 
Honshu and Hokkaido Islands (Sugaya et al., 2008). 
Shaw et al. (1999) also found significant genetic struc- 
turing between Icelandic summer-spawners, Norwegian 
spring-spawners, and Norwegian fjord stocks of Atlantic 
herring. 
Implications for management 
Loss of unique genetic variation due to fishing pressure, 
habitat degradation, and hatchery stocking has been 
reported for Pacific cod (Gadus macrocephalus), leopard 
darter ( Percina panterina), Japanese flounder (Paralich- 
thys olivaceus ), and American shad (Alosa sapidissima) 
(Grant and Stahl, 1988; Echelle et al., 1999; Brown et 
al., 2000; Sekino et al., 2003), and there is concern that a 
concentration of fishing effort in and around Chesapeake 
Bay could result in the loss of unique genetic variation in 
Atlantic menhaden. In this study we have demonstrated 
high genetic variability and a homogeneous distribution 
of genetic variation within Atlantic menhaden from four 
sampling locations along the U.S. Atlantic coast — a 
result consistent with the current management practice 
that recognizes a single stock of Atlantic menhaden. The 
apparent genetic connectivity between New England, 
mid-Atlantic, Chesapeake Bay, and U.S. South Atlantic 
samples indicates that loss of unique genetic variation 
due to the consolidation of fishing pressure in Chesa- 
peake Bay is not likely. Our analysis of mitochondrial 
and nuclear loci revealed significant allele frequency 
differences between Atlantic and Gulf menhaden, sup- 
porting independent management of these resources. 
However, the small magnitude of these differences found 
in this and previous studies would indicate that a re- 
evaluation of the specific status of the two putative 
species, based on analyses of morphological and genetic 
characters, is warranted. 
Acknowledgments 
This project was funded by the Virginia Marine 
Resources Commission (RF-07-06). We thank J. Ander- 
son, J. Archambault, H. Corbett, B. Dailey, P. Lynch, G. 
Nelson, and T. Tuckey for providing menhaden samples. 
This is Virginia Institute of Marine Science contribu- 
tion no. 3049. 
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