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Fishery Bulletin 1 10(4) 
Figure 4 
Micrographs of embryonic development of cobia (Rachycentron canadum ) eggs (A, B, C, and D) from the 2007 egg-devel- 
opment study of hatchery-reared eggs (26.5°C) at 20x magnification and of eggs (E and F) from the 2008 development 
study (26°C) at 40x magnification. (A) Stage I (1 hr postfertilization [pf] ): newly fertilized egg with single oil droplet 
(OD) and first cellular divisions occurring. (B) End of stage I, start of stage II (7 hr pf): germ ring (GR) has formed. 
(C) Stage II (13 hr pf): optic cups (OC) distinguish head and tail regions of embryo. (D) Stage III (17 hr pf): embryo is 
acquiring pigmentation, and somites are visible; free melanophores (FM) in yolk begin to migrate toward OD. (E) Stage 
IV (21 hr pf): embryo is heavily pigmented and is >50% the internal circumference of the egg; flexion (FX) of embryo is 
evident. (F) Stage V (25 hr pf): embryo fills more of the egg and greater movement is evident. 
Ichthyoplankton survey 
Between 6 May and 18 June 2008, 52 anchored plank- 
ton-net samples (26 PRS, 26 SHS) were collected. All 
samples from PRS were sorted completely. Of the 26 
samples from SHS, 17 were split to one-half (1 sample), 
one-quarter (14 samples), or one-eighth (2 samples) the 
original volume. Measured current speeds ranged from 
0.08 to 0.93 m/s (0.29 to 3.35 km/hr), and most speeds 
ranged between 0.14 and 0.71 m/s (0.50 to 2.56 km/ 
hr). Volumes filtered through the plankton nets ranged 
from 120 to 1156 m 3 . Surface water temperatures in 
both estuaries ranged from 20.1°C to 30.0°C. Salinities 
ranged from 31.6 to 34.3 psu in PRS and from 28.4 
to 32.7 psu in SHS; these values are within tolerable 
ranges for larval, juvenile, and adult cobia (Hassler and 
Rainville, 1975; Shaffer and Nakamura, 1989; Denson 
et al., 2003). The water columns in both PRS and SHS 
were well mixed, with little variation of surface and 
bottom temperatures and salinities (+0.8°C and 0.8 psu, 
respectively). 
On the basis of size and morphological characteris- 
tics, 926 eggs were identified as cobia (562 early stage 
and 364 late stage [Tables 3 and 4; Fig. 5]). Late-stage 
eggs occurred in samples collected at all stations in 
PRS (59 eggs). At a single station (PA08) ~15 km in- 
shore, 27 early-stage eggs from PRS were collected at 
1930 hr on 5 June 2008 (Table 4). The majority of the 
eggs identified as cobia were from SHS, with 535 early- 
stage eggs and 305 late-stage eggs collected among all 
stations. Of the early-stage eggs found in SHS, 496 
came from a single sample collected at 1900 hr on 3 
June 2008 at a station (SA03) -9.7 km inshore (Table 
4). Late-stage eggs were collected between 1230 and 
1945 hr in both estuaries on 8 sampling days. Egg 
concentrations ranged from 0.14 to 62.51 eggs/100 m 3 
(Table 4). 
Egg diameters ranged from 1116 to 1393 pm 
(mean=1292 pm). The diameters of 73 intact oil droplets 
ranged from 275 to 420 pm (mean=357 pm). The mean 
diameters of eggs and oil droplets were similar to the 
diameters observed for hatchery-reared cobia eggs and 
to diameters reported elsewhere (Table 3) (Ditty and 
Shaw, 1992; Ditty, 2006). Measurements of early- (48 
eggs) and late-stage (25 eggs) eggs were combined for 
the ANCOVA. Because of egg shrinkage, only measure- 
ments from preserved hatchery-reared eggs were used 
for the ANCOVA. The mean diameter of oil droplets for 
field-collected eggs (357 pm) was lower than the mean 
for hatchery-reared cobia eggs (380 pm); however, the 
