354 
Fishery Bulletin 118(4) 
Annual index of abundance 
2005 2010 2015 
Figure 4 
Standardized index of abundance for adult female blue crab (Call- 
inectes sapidus) in the northwestern Gulf of Mexico during the study 
period 2000-2019. The gray shaded area indicates the 95% confi- 
dence interval. The horizontal dotted line indicates a value of zero, 
representing the average annual predicted catch per unit of effort 
across the full study period. 
- - Estuarine 
— Offshore 
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2010 
Figure 5 
Estimated total abundance (in millions) of adult female blue crab 
(Callinectes sapidus) in offshore (solid line) and estuarine (dashed 
line) areas of the northwestern Gulf of Mexico west of the Missis- 
sippi River during June and July in 2006-2018. 
On average across both seasons, 91.7% (SE 0.01) of eggs 
were fertilized and embryos were developing normally, 
with 91.6% (SE 0.01) of embryos developing normally in 
the summer and 93.0% (SE 0.05) of embryos developing 
normally in the fall. Eggs that were not developing nor- 
mally were either unfertilized or fertilized but had ceased 
development in an earlier developmental stage than the 
majority of the egg mass. Average egg size was 243.6 pm 
(SE 0.5) along the long axis and 236.1 pm (SE 0.5) along 
the perpendicular axis. Average egg volume was 
0.0073 mm? (SE 0.0001). There was a significant 
relationship between egg volume and embryonic 
developmental stage (F=29.76, P<0.0001), with a 
general increasing trend as the embryos develop 
further (Fig. 7). There was no significant relation- 
ship between egg volume and maternal CW 
(F=2.371, P=0.1286) and no significant interac- 
tion between maternal CW and embryonic devel- 
opmental stage (F=2.363, P=0.1023). 
The parasitic barnacle O. muelleri was com- 
monly found on the gills of blue crab, with prev- 
alence of 66.1% in summer and prevalence of 
100.0% in fall. The nemertean worm C. carci- 
nophila that is a predator of crab eggs was less 
common, with prevalence of 6.4% in summer and 
prevalence of 0.0% in fall. 
Discussion 
Results of this study indicate that the offshore 
waters of the northwestern Gulf of Mexico are 
an important spawning habitat for blue crab. 
Mature female blue crab are routinely caught 
in offshore waters of the northwestern Gulf of 
Mexico each year in the SEAMAP groundfish 
trawl survey, as far as 200 km from shore (Fig. 3). 
Our results support the substantial numbers of 
spawning blue crab in offshore areas of the Gulf 
of Mexico documented in previous studies (Gelpi 
et al., 2009, 2013). 
The CPUE of adult female blue crab in offshore 
waters was greater in the summer than in the 
fall during most years (Fig. 3). This difference in 
CPUE between summer and fall may be indica- 
tive of a postspawning return to inshore habitats, 
as proposed by Tankersley et al. (1998). Given 
that findings from more recent studies indicate 
that migration back to estuaries is unlikely 
(Hench et al., 2004; Darnell et al., 2012), this 
pattern more likely is indicative of an offshore 
population decline from natural mortality due 
to predation and the high energy expenditures 
of migration and reproduction. It is important to 
note that the SEAMAP-Gulf of Mexico groundfish 
surveys are limited to summer (primarily June— 
July) and fall (primarily October-November). 
Blue crab recruitment in the northwestern Gulf 
of Mexico generally peaks from August to October 
(Rabalais et al., 1995; Perry et al. 2003), which corresponds 
with the observed peak in adult female densities during 
the summer SEAMAP trawl survey. Given that earlier 
recruitment peaks are observed in some areas (Rabalais 
et al., 1995; Sullivan and Neigel, 2017), late spring may 
also be an important time for offshore spawning in this 
region (data are not available to test this hypothesis). 
Throughout the study period, the highest densities of 
adult female blue crab were present in the waters off 
