Lefebvre and Denson: Inshore spawning of Rachycentron canadum in South Carolina 
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460 
U H 1 1 1 1 1 
1100 1150 1200 1250 1300 1350 1400 
Egg diameter (pm) 
Figure 6 
Regression lines for egg (ED) and oil-droplet (OD) diameters from 
preserved hatchery-reared cobia ( Rachycentron canadum) eggs 
from the egg-development studies in 2007 and 2008 (circles, solid 
line; y=0.23x+98.46) and eggs identified as cobia from plankton 
collections conducted in Port Royal Sound and St. Helena Sound, 
South Carolina, in 2008 (triangles, dotted line; y = 0.28jc-7.63). 
Results of the analysis of covariance show no significant differ- 
ence between the relationship of ED and OD of known cobia and 
suspected cobia eggs (P=0.35; overall coefficient of determination 
[;- 2 ] =0.61). 
resented likely because of the time of day 
anglers were targeting fish. Evidence from 
both the histological analysis and plankton 
collections of eggs in this study suggests that 
wild cobia in South Carolina inshore waters 
spawn from mid-afternoon to late evening, 
with most activity occurring between 1530 
and 1800 hr. Unfortunately, fish collected 
at tournaments were probably caught from 
morning to mid-afternoon, before the peak 
in daily spawning activity. 
The best evidence produced from histologi- 
cal analysis that cobia were spawning in- 
shore in South Carolina was the collection 
of actively spawning females. The duration 
of FOM is unknown for cobia, although this 
hormonally controlled “point-of-no-return” on 
the path to spawning commences 6 to 13 hr 
before spawning in spotted seatrout (Brown- 
Peterson et ah, 1988; Roumillat and Brou- 
wer, 2004), a co-estuarine inhabitant with 
cobia in the southeastern United States that 
spawns the same time of year. Because the 
earliest stages of FOM were unavailable to 
us, we were unable to estimate the time of 
FOM onset in cobia. However, both of the 
actively spawning females that we collected 
had oocytes that were in the late stages of 
FOM (near the mid-point of hydration), indi- 
cating that they would have spawned in the 
mid-afternoon and likely in the proximity of 
their capture. 
Female cobia undergoing hydration have been col- 
lected in few studies, although there have been excep- 
tions (Smith, 1996; van der Velde et al., 2010). Actively 
spawning females may compose a larger percentage of 
the inshore cobia population in South Carolina than 
the percentage reported here, but prespawners may not 
take a baited hook. As oocytes hydrate during FOM, 
their volume can nearly quadruple (Wallace and Sel- 
man, 1981). The ovaries of an actively spawning female 
caught in PRS composed one-sixth of the total body 
weight of the fish, nearly filling its entire body cav- 
ity. It is possible that cobia cease feeding during egg 
hydration. 
Collection of cobia eggs in PRS and SHS also sup- 
ports the idea that these areas are inshore spawn- 
ing sites. Morphological features of wild cobia eggs 
matched closely those of hatchery-reared cobia eggs 
and were supported with the quantitative comparison 
of egg characteristics. Cobia eggs found in plankton 
collections had oil-droplet diameters within the re- 
ported range but smaller than the diameters reported 
for the hatchery-reared eggs. The hatchery eggs came 
from the spawns of only 2 females that had been hor- 
monally induced to spawn, and maternal condition is 
known to affect egg and larval quality in fishes, in- 
cluding the size of oil droplets (Berkeley et al., 2004; 
Gagliano and McCormick, 2007; Sogard et al., 2008). 
Eggs collected in the plankton samples likely came 
from several females that encountered a variety of 
environmental conditions and had variable physiologi- 
cal conditions compared with the eggs from females 
reared in a controlled environment with a regular, 
strictly controlled diet. 
The collection of eggs and larvae within PRS and 
SHS alone, although highly indicative of spawning, 
does not alone positively confer evidence of spawning 
habitat. Origin of late-stage larvae often cannot be de- 
termined; active dispersal or transport may be possible 
because of their increased swimming ability (Clark et 
al., 2005) or ability to control their vertical position in 
the water column, which allows migration by selective 
tidal stream transport (Boehlert and Mundy, 1988; 
Hogan and Mora, 2005). Cobia eggs were found in 
PRS from 12 to 20.5 km inshore. On the basis of the 
measured current speeds, a floating object could have 
traveled 7-15 km during a single flood tide, making 
it improbable that all eggs were spawned beyond the 
estuary. The most conclusive evidence for spawning 
within PRS comes from eggs that were estimated to be 
only 2-3 hr old and collected in the Broad River, 15.0 
km inshore. With an average current speed of 0.7 m/s 
measured during the time of their collection, these eggs 
must have come from this estuary. In SHS, one par- 
ticular collection of 496 early-stage eggs 9.7 km inshore 
lends compelling evidence that spawning occurred near 
the time and in the immediate vicinity of collection. 
