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Fishery Bulletin 1 10(4) 
tification, eggs from the 2 development studies were 
viewed as references. Eggs were positively identified 
as cobia if their diameter fell within the range noted 
in the literature (1.0-1. 4 mm) and if the morphologi- 
cal characteristics, with the exception of number and 
diameter of oil droplets, matched the hatchery-reared 
eggs and published description of cobia eggs (Ditty and 
Shaw, 1992; Ditty, 2006). The number and diameter 
of oil droplets were excluded as positive identifying 
characters because of damage incurred during preser- 
vation. For further analyses, eggs were labeled as early 
stage or late stage. The early stage comprised eggs from 
fertilization to blastopore closure, with the embryo not 
visible in preserved eggs (Ahlstrom and Moser, 1980). 
The late stage comprised eggs from blastopore closure 
to hatching, with the embryo evident in preserved eggs, 
including eggs at mid and late stages as described in 
Ahlstrom and Moser (1980). 
For secondary evidence for correct identification of 
eggs, an analysis of covariance (ANCOVA) was con- 
ducted to test for differences in the relationship of egg 
and oil-droplet diameters between known cobia eggs 
(early and late-stage eggs from the 2007 and 2008 egg- 
development studies) and eggs identified from plankton 
samples. If the relationship was not significantly differ- 
ent between hatchery-reared and field-collected eggs, 
the field-collected eggs were confirmed as cobia. Only 
measurements from eggs with single, intact oil droplets 
were used in the ANCOVA. 
For plankton samples that contained more than one 
undamaged egg, ages of cobia eggs were estimated 
through side-by-side comparison to eggs reared at 
the closest matching temperature from either the 
2007 or 2008 development study. In early-stage eggs 
that turned opaque with preservation, it was diffi- 
cult to discern their stage of cell division or cleav- 
age; therefore, age was estimated only for eggs that 
were not opaque. For a given sample, time of spawning 
was approximated on the basis of the age of an egg, 
which was estimated by back-calculating from the 
time of sample collection. When field-collected eggs 
resembled an intermediate stage between 2 sampling 
time periods within the egg development study, ages 
were estimated to be halfway between the 2 periods. 
Numbers of cobia eggs and larvae from split samples 
were estimated from the fraction of the sample sorted. 
Concentrations of cobia eggs and larvae collected in 
the ichthyoplankton survey were obtained by using 
calculated and estimated sample volume filtered for 
the corresponding sample. 
All statistical analyses were conducted using R sta- 
tistical software (R Development Core Team, 2009). 
The significance level of a=0.05 was used for all tests. 
Results 
Between April 2007 and June 2008, 554 cobia (275 
females, 279 males) were collected. Of these fish, 262 
came from carcass donations, 261 from fishing tourna- 
ments, and 31 from SCDNR. Specimens ranged in size 
from 850 to 1425 mm FL (mean=1042 mm) for females 
and 386 to 1215 mm FL (mean=930 mm) for males. 
Weights ranged from 6.7 to 38.3 kg (mean=15.0 kg) for 
females and 0.5 to 23.0 kg (mean=9.9 kg) for males. Most 
of the largest fish were female: 80% of the fish 1000 mm 
FL or longer and 79% of the fish 10 kg or greater in total 
weight. Capture locations were available for 183 fish (44 
offshore, 101 PRS, 39 SHS). 
Reproductive biology 
GSIs were calculated for 278 cobia (164 females, 114 
males) and were combined for April, May, and June of 
both years because of small sample sizes collected in 
April and June. GSI for all cobia ranged between 0.7 
and 22.5 (mean=6.1), and females had a significantly 
higher mean GSI (7.3) than males (4.4; P<0.05). Ovarian 
growth in other group-synchronous, batch-spawning spe- 
cies is isometric with body growth (Taylor et ah, 1998; 
Somarakis et al., 2004), and previous work on cobia 
ovaries with FOM has shown that there is no relation- 
ship between relative fecundity and either body weight or 
fork length (Brown-Peterson et al., 2001; van der Velde et 
al., 2010); therefore, female GSI was deemed appropriate 
as a proxy for ovarian maturation for statistical pur- 
poses. The average GSI from females collected inshore 
(mean=7.8; n = 64) was significantly higher than the 
average GSI from females collected offshore (mean=5.6; 
n=34; p=0.002), indicating that ovaries were in a more 
developed state in inshore fish than in offshore fish. The 
mean inshore GSI (7.4) remained statistically higher 
than the mean offshore GSI (5.6; P= 0.003) with removal 
of the actively spawning females. Therefore, the higher 
mean GSI from females collected inshore was not biased 
by the collection of actively spawning females inshore or 
spent females offshore. 
Histological analysis of 213 ovaries showed that fe- 
male cobia were in the middle of their reproductive sea- 
son and capable of spawning during May and June. All 
ovarian phases, except the immature and regenerating 
phases, were represented in the female samples exam- 
ined (Fig. 2). Immature specimens were unavailable be- 
cause current fishing regulations impose a minimum FL 
(84 cm), which is larger than the approximate length at 
first maturity for female cobia (80 cm). Females in the 
regenerating state were likely absent because sampling 
was done during the spawning season. 
Further histological analysis of female ovarian sam- 
ples was limited to 98 specimens for which capture 
location was known (64 inshore and 34 offshore). The 
majority (72%) of female cobia collected from both in- 
shore and offshore waters had ovaries in the late de- 
velopment subphase of the spawning capable phase 
(Table 2). Two females with ovaries in the developing 
phase were collected offshore in early April 2007 and 
inshore in early May 2007. Both of these females (89 
cm FL and 93 cm FL, respectively) were larger than 
the typical size at maturity, had no evidence of prior 
spawning (POFs), and were, therefore, likely maturing 
