Merten et al. : Genetic structure and dispersal capabilities of Coryphaena hippurus in the western Atlantic 
425 
Table 3 
Mismatch parameters used in the estimation of population expansion of dolphinfish ( Coryphaena hippurus ) based on mi- 
tochondrial nicotinamide adenine dinucleotide (NADH) dehydrogenase subunit 1 (ND1) sequence data analyzed in this 
study. T=coalescence time in generations (x=2pT); p=mutation rate 1.2% per million years; T=years since expansion; 0=value 
used to estimate initial effective population size (N q) before and after ( 9i ) expansion; Ah=population size after expansion; 
M=scaled migration rate; m=immigration rate from adjacent population; PsSD=P r °bability of the expected mismatch dis- 
tribution to fit the expansion model. The bold value is significant at PcO.OOl. Modified following Diaz-Jaimes et al. (2010). 
USSE=southeastern United States; NEC=northeastern Caribbean Sea; EC=eastern Caribbean Sea; CNA=central North 
Atlantic. 
Region Demographic expansion parameters Spatial expansion parameters 
X 
T 
0 
N 0 
01 
Nt 
PSSD 
X 
T 
0 
N 
M 
m 
PSSD 
USSE 
2.21 
71,400 
0.00 
0.00 
99,999 
© 
T— 1 
X 
CM 
>0.001 
2.21 
71,400 
2.97 
64,000 
99,999 
0.78 
0.760 
NEC 
2.31 
74,700 
1.89 
40,700 
99,999 
o 
X 
CM 
0.370 
2.31 
74,700 
1.89 
40,900 
99,999 
1.22 
0.280 
EC 
1.59 
51,400 
5.25 
113,200 
1205 
2.5 x 10 7 
0.800 
1.61 
52,000 
5.23 
112,700 
298.00 
0.0013 
0.810 
CNA 
3.85 
124,500 
0.00 
0.00 
99,999 
2.1 x 10 9 
0.590 
3.85 
124,500 
0.007 
15.1 
99,999 
3311 
0.530 
Mean 
2.49 
80,500 
1.78 
38,475 
75,300 
1.6 x 10 9 
0.440 
2.50 
80,650 
2.52 
54,353 
75,073 
828.25 
0.595 
were large differences in 9q (twice the product of the 
effective population size [Nq] and mutation rate [p]) 
and in 6\ (2 (NY |: p)), indicating rapid demographic ex- 
pansions. The average value of 6q among regions (1.78) 
indicated a small initial size for the female breeding 
population (iVo -38,475) followed by very rapid popula- 
tion expansion (iVi=~1.6x 10 9 ). 
Discussion 
Analyses of the ND1 gene in dolphinfish revealed ge- 
netic homogeneity around Puerto Rico and shallow ge- 
netic heterogeneity across the western central Atlan- 
tic but failed to identify regional genetic differences 
among the southeastern United States, northeastern 
Caribbean Sea, eastern Caribbean Sea, and central 
North Atlantic. These results, when examined with 
tag data (Fig. 1), indicate that substantial mixing is 
occurring throughout the western central Atlantic and 
that the entire region could be identified as a single 
stock. It is clear that migration across the region is 
high and the degree of reproductive isolation is low. 
However, although migration and reproductive isola- 
tion are critical attributes considered for stock identifi- 
cation, biological data sources (e.g., catch data, growth 
rates, fecundity at age) should be considered in future 
studies to strengthen the support for or against the 
identification of individual dolphinfish stocks in the At- 
lantic (Begg and Waldman, 1999). 
Haplotypes were randomly distributed (Fig. 2) — a 
finding that indicates the presence of a single pan- 
mictic population. A possible explanation for this pan- 
mixis is Gulf Stream intensification (Coetlogon et al., 
2006) and the recirculation tendency of surface wafers 
around the North Atlantic subtropical gyre (Richard- 
son, 1993). This current pattern may facilitate signifi- 
cant migration of dolphinfish among locations in the 
North Atlantic by larval dispersal and as a result of 
the propensity of juvenile and subadult fish to ex- 
hibit strong fidelity with floating objects (e.g., Sargas- 
sum and flotsum) (Farrell et al., 2014; Merten et al., 
2014a) 
The dispersal capability of large pelagic species like 
dolphinfish is high in the absence of obvious barriers 
(e.g., temperature and land). Presumably, the highly 
migratory nature of dolphinfish, coupled with wide- 
spread spawning activity, resulted in the moderate lev- 
els of gene flow and low genetic differentiation found 
in this study. This pattern is characteristic of intra- 
basin distributions of other pelagic fishes, including 
blue marlin (McDowell et al., 2007), sailfish (Graves 
and McDowell, 2003), and wahoo (Theisen et al., 2008). 
However, oceanographic features, such as gyres, or pop- 
ulation isolation due to coastal geomorphology, could 
limit population mixing and lead to genetic heterogene- 
ity. In the case of bluefin tuna, Carlsson et al. (2006) 
observed a slight genetic separation of stocks in the 
Atlantic Ocean, likely a result of geographically sepa- 
rated spawning areas due to coastal geomorphology, 
and suggested the population consists of a mixed-stock 
fishery. 
The potential for year-round spawning behavior and 
the lack of reproductive isolation in dolphinfish are 
important life-history characteristics that contribute 
to the observed genetic homogeneity. Dolphinfish have 
been characterized as batch spawners because of the 
presence of several size classes of eggs in the ovaries 
occurring simultaneously at geographically separate 
regions (Palko et al., 1982; Ditty et al., 1994; Oxenford, 
1999), a reproductive characteristic that indicates that 
females spawn over broad times and locations as they 
migrate throughout the region. Female dolphinfish of- 
ten are active reproductively from November through 
July in the Straits of Florida and from May through 
July off Cape Hatteras, North Carolina (Palko et al., 
