lefebvre and Denson: Inshore spawning of Rachycentron canadum in South Carolina 
407 
ANCOVA showed that the slopes of the regression 
lines were not statistically different (P=0.35; overall 
coefficient of determination [r 2 ] = 0.61; Fig. 6). This 
result supports our visual identifications of cobia 
eggs in the plankton collections. 
Most early-stage eggs turned opaque: ages were 
estimated from a single sample collected 5 June 
2008 in PRS, and eggs were ~2-3 hr old. The ages 
of late-stage eggs were estimated at between 18 and 
26 hr old. Time of spawning was estimated to range 
from the late afternoon (1530 hr) to late evening 
(2145 hr), and most spawning occurred between 1530 
and 1800 hr (Table 5). The 3 exceptions were from 
one day in SHS, when spawning probably occurred 
near midnight. Although age estimates of the field- 
collected eggs were admittedly subjective, eggs col- 
lected from multiple stations on the same day were 
aged independently, and all of them were estimated 
to be nearly the same age. 
Over 8 sampling days, 42 cobia larvae (18 PRS; 24 
SHS) were collected. Larval concentrations ranged 
between 0.17 and 1.99 larva/100 m 3 (Table 5). Five 
yolksac cobia larvae were collected: 2 on 14 May 
2008 (stations PA05 and PA08), 1 on 29 May 2008 
(station PA07), and 2 on 21 May 2008 (station SA01). 
Discussion 
The results of this study indicate that cobia spawn 
within PRS and SHS, South Carolina, as evidenced 
by the high mean GSIs of females, the collection of 
actively spawning females, the presence of recently 
fertilized eggs, and the presence of larvae in these 2 
estuaries. Through the use of the estimated ages of 
field-collected eggs, in combination with time and loca- 
tion of capture from adult cobia collected at fishing 
tournaments, this study provides evidence of spawning 
of wild cobia in estuarine waters in the late afternoon 
and early evening hours. 
The mean GSI of female cobia collected in this study 
peaked at a higher value (7.3) than was previously re- 
ported for cobia in the Gulf of Mexico by Biesiot et al. 
(1994; 5.5), Lotz et al. (1996; 5) and Brown-Peterson et 
al. (2001; 4.5); in North Carolina by Smith (1996; 5.7); 
and along the Atlantic coast of the southeastern United 
States by Brown-Peterson et al. (2001; 5.5). The higher 
mean GSI value reported in this study is not biased 
by the collection of 2 actively spawning females: when 
these females were removed from analyses, the mean 
GSI values were still well above the values reported 
elsewhere. In addition to a higher GSI value, the size 
range of oocytes (500-850 pm) in ovaries in the late de- 
veloping subphase (the phase that dominated collections 
in this study) was greater than the size range reported 
elsewhere for the group with the most developed oocytes 
(500-650 pm [Lotz et al., 1996]). Fully developed and 
fertilized cobia eggs in this study and in Ditty and 
Shaw (1992) ranged in size from 1.15 to 1.42 mm; there- 
fore the larger oocyte sizes found in developing ovaries 
suggest that oocytes were more highly developed in our 
current study. Brown-Peterson et al. (2001) and van der 
Velde et al. (2010) reported that monthly GSI values 
corresponded with histological evidence of spawning 
times, supporting GSI as a proxy for the developmental 
state of ovaries. In our current study, the larger size 
range of oocytes, together with the higher GSI values, 
indicate that female cobia collected in South Carolina 
were closer to actually spawning than were specimens 
from previous studies. 
Differences in the stage of cobia ovarian development 
between this and other studies may stem from locations 
where females were captured. In previous work, females 
were collected from coastal waters in the Gulf of Mexico 
(Lotz et al., 1996; Brown-Peterson et al., 2001) and off 
the Atlantic coast of the southeastern United States 
(Smith, 1996; Brown-Peterson et al., 2001). For the 
samples we examined, the mean GSI of females col- 
lected offshore (5.6) was closer to the means reported 
elsewhere (Biesiot et al., 1994; Lotz et al., 1996; Smith, 
1996; Brown-Peterson et al., 2001). The dissimilarity of 
GSI values between female cobia collected inshore and 
those caught offshore was observed likely because those 
fish caught inshore were part of a spawning aggregation 
and those fish caught offshore were caught before or af- 
ter spawning or were intercepted as they were migrat- 
ing to spawning grounds. This hypothesis is supported 
