336 
Fishery Bulletin 1 12(4) 
levels of mixing within each basin could be sufficient to 
largely homogenize these effects. However, we cannot 
rule out the possibility that some fine-scale structure 
exists at the within-basin spatial scale for southern 
flounder. Additional investigations to understand the 
extent of mixing and its impact on spatial variation in 
vital rates, such as growth, recruitment, and mortality, 
will be important to fully comprehend harvest effects 
and to select the appropriate scale for management. 
Within both basins, no interjurisdictional manage- 
ment plan for southern flounder exists, and each state 
manages its fishery separately. In North Carolina, on 
the basis of a recent stock assessment, southern floun- 
der are considered overfished with overfishing occur- 
ring (Takade-Heumacher and Batsavage 6 ), but other 
states (South Carolina, Georgia, and Florida) have 
yet to conduct comprehensive stock assessments for 
flounder in their waters. In North Carolina, the south- 
ern flounder has long supported a lucrative commer- 
cial fishery (NCDMF, http://portal.ncdenr.org/web/mf/ 
commercial-fishing-harvest-statistics), whereas land- 
ings with commercial gear have historically been much 
lower in the other states. At the same time, the recre- 
ational fishery has grown considerably throughout the 
entire basin; landings have more than doubled since 
the 1980s (National Marine Fisheries Service, http:// 
www.st.nmfs.noaa.gov/recreational-fisheries/index). 
Combined with any increase in participation in com- 
mercial harvest activities, the steady growth of the rec- 
reational fishery for southern flounder may necessitate 
that other states develop comprehensive fishery man- 
agement plans in the near future. In that event, the 
likelihood that interstate cooperation will be required 
through a federal fishery management plan (e.g., under 
the auspices of the Atlantic States or Gulf States Ma- 
rine Fisheries Commissions) may increase. 
Our findings indicate that southern flounder may 
exist as separate but well-mixed stocks within the 
South Atlantic and Gulf of Mexico basins. Alternatively, 
if population structure exists in the form of multiple 
stocks that differ in their response to harvest, failure 
to recognize that structure can lead to overexploitation 
and depletion of less productive and resilient stocks. 
Therefore, identification of stock structure is essential 
for accurate estimation of vital rates and stock assess- 
ments that promote effective fishery management. Cur- 
rently, this process is hindered by the lack of under- 
standing of the underlying structure within the popu- 
lation of southern flounder in the U.S. South Atlantic. 
Conclusions 
Previous studies have found genetic differences be- 
tween southern flounder occupying South Atlantic and 
Gulf of Mexico waters, and the results from analyses of 
otolith shape variation presented here provide further 
evidence of basin-level differentiation. At finer spatial 
scales, we detected only limited evidence of population 
structure inferred from variation of otolith shape to 
support the existence of separate stocks within basins 
or within specific regions of a basin (among estuaries 
in North Carolina). Additional investigation of stock 
structure will be necessary, integrating these findings 
with results from other approaches, such as genetic 
analyses, acoustic or archival tagging, and analysis of 
spatial variation in life history traits, to determine the 
appropriate spatial scales to promote effective manage- 
ment and conservation of southern flounder stocks. 
Acknowledgments 
Funding for this research was provided by the NCDMF 
through the Marine Resources Fund. C. Batsavage and 
R. Gregory, both of the NCDMF, provided assistance 
with otolith aging. L. Paramore, K. West, J. Rock, M. 
Seward, and C. Collier all helped with sample collec- 
tion in North Carolina as part of the NCDMF fishery- 
independent sampling program. B. Roumillat and S. 
Arnott of the South Carolina Department of Natural 
Resources provided fish from South Carolina, J. Page 
and E. Robillard of the Georgia Department of Natural 
Resources provided fish from Georgia, and R. Brodie 
of the Florida Fish and Wildlife Conservation Commis- 
sion assisted with collections in Florida. G. Fitzhugh, 
of the Panama City Lab of the NOAA Southeast Fish- 
eries Science Center, helped by providing otoliths from 
fish collected in the Gulf of Mexico, as did M. Nims, 
then of the University of Texas at Austin. W. White of 
the University of North Carolina Wilmington helped 
with aspects of the analysis, and H. Iwata, of the Na- 
tional Agricultural Research Center in Japan, assisted 
by providing not only SHAPE software but also ad- 
ditional R code and troubleshooting. Interpretation of 
these data and any conclusions drawn are those of the 
authors and do not necessarily represent the views of 
the NCDMF. 
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