Kemberling and Darnell: Distribution, abundance, and reproductive output of spawning female Callinectes sapidus 347 
for larval survival. Spawning female blue crab are often 
found in large numbers at or near the mouths of estuaries 
(e.g., Rittschof et al., 2010; Anderson et al., 2017); these 
areas are typically assumed to be the primary spawning 
grounds. The blue crab is a highly fecund species, with 
females producing multiple clutches of several million lar- 
vae at a time (Hines et al., 2003). Each brood of larvae 
is released into the water column as zoeae that disperse 
through currents for 30-50 d (Costlow and Bookhout, 
1959) before their return and recruitment to coastal habi- 
tat as megalopae soon after (Rabalais et al., 1995; Ogburn 
et al., 2009). 
Mature females can travel great distances (>200 km; 
Aguilar et al., 2005) during their spawning migration, 
and migration rates can exceed 8 km/d (Carr et al., 2004). 
Female blue crab continue migrating throughout their 
reproductive period (Hench et al., 2004) and are, there- 
fore, likely moving out of estuaries and continuing their 
migration in offshore waters. Adults of this species typ- 
ically are considered estuarine, but large numbers of 
spawning female blue crab have been found in waters off 
both the Atlantic coast (Rittschof et al., 2010; Ogburn and 
Habegger, 2015) and Gulf coast (Gelpi et al., 2009, 2013) 
of the United States. Although observed densities of blue 
crab are typically lower offshore than in estuaries, evi- 
dence from the Atlantic coast of the southeastern United 
States indicates that this often-overlooked offshore popu- 
lation may represent a substantial fraction of the regional 
spawning stock (Ogburn and Habegger, 2015). In the 
Gulf of Mexico, Gelpi et al. (2009, 2013) found large num- 
bers of spawning females residing on Ship, Trinity, and 
Tiger Shoals and surrounding waters, 20-40 km off the 
coast of Louisiana. All developmental stages of eggs were 
observed, and 81% of females had full ovaries at the time 
of capture, indicating that they would continue to spawn 
at least one additional clutch of eggs during the season 
(Gelpi et al., 2009). Furthermore, spawning female blue 
crab are regularly captured in trawl surveys conducted 
by the Southeast Area Monitoring and Assessment Pro- 
gram (SEAMAP) in the Gulf of Mexico, as far as 200 km 
offshore. 
Given the large extent of relatively shallow offshore 
habitat in the Gulf of Mexico and with offshore habitat 
proving to be of larger importance to the spawning stock 
of blue crab in the Atlantic Ocean than previously thought 
(Ogburn and Habegger, 2015), we sought to identify the 
contribution of this offshore habitat to the spawning stock 
of blue crab in the northwestern Gulf of Mexico. The goal 
of this study was to conduct an investigation of spawning 
female blue crab in offshore waters of the northwestern 
Gulf of Mexico. We examined temporal and spatial varia- 
tion in catch of blue crab in fishery-independent SEAMAP 
groundfish trawl surveys over a 20-year period (2000- 
2019) and examined the reproductive output and overall 
condition of females collected in offshore waters during 
the SEAMAP groundfish trawl surveys in 2017. We also 
compared results for spawning females found offshore 
in our study with previous findings for blue crab found 
Spawning in inshore areas. 
Materials and methods 
Survey design and methods 
The SEAMAP involves the cooperation of state, federal, 
and university partners for the collection of fishery- 
independent data. The SEAMAP-Gulf of Mexico ground- 
fish trawl survey is conducted twice each year during 
summer and fall at distances from shore up to 370 km (200 
nautical miles) and at depths up to 109.7 m (60 fathoms). 
The summer survey typically occurs in June and July but 
can start as early as May and end as late as August. The 
fall survey typically occurs during October and November 
but can start as early as September and end as late as 
December. Sampling is done by using a stratified random 
design and a 12.2-m (40-ft) trawl net with stretched mesh 
of 40.3-41.9 mm. Environmental conditions, including 
salinity and temperature, were measured either at 3 posi- 
tions in the water column, the surface, mid-water, and bot- 
tom, or through a profile of the full water column at each 
sampling location. 
Prior to fall 2008, sampling was stratified by depth 
(with 23 depth strata, including 1-fathom strata for 5-20 
fathoms, a 2-fathom stratum of 20—22 fathoms, a 3-fathom 
stratum of 22—25 fathoms, 5-fathom strata for 25-50 fath- 
oms, and a single stratum of 25-50 fathoms), by geographic 
area (based on groupings of 2-3 shrimp statistical zones 
[SSZ], which are defined by the National Marine Fisheries 
Service), and by time of day (day or night) (Nichols”). Trawl 
tows were conducted perpendicular to the depth contours 
and covered the entire depth stratum at each sampling 
location. Tow time ranged from 10 to 55 min; if sufficient 
depth coverage was not possible in a single 55-min tow, 
multiple consecutive tows were conducted. Sampling was 
limited to SSZ 11-21. A survey design change occurred 
between summer and fall 2008. Under the new survey 
design, selection of sampling locations was stratified by 
individual SSZ (Fig. 1), and sampling extended into the 
eastern Gulf of Mexico. The stratifications for time of day 
and for depth were dropped. In 2014, depth stratification 
resumed with 2 depth strata: 5-20 fathoms and 20-60 
fathoms (Pollack et al.*). The number of randomly selected 
sampling locations (stations) within each stratum that 
combined geographic area and depth was proportional to 
the geographic area of that stratum. The trawl was towed 
for 30 min at each sampling location. 
For all tows, abundance of each species in the catch was 
recorded. In the case of an extremely large catch (typi- 
cally >22.7 kg), the catch was subsampled and total abun- 
dance of each species was extrapolated. For blue crab, 
carapace widths (CWs), measured as the distance between 
the tips of the lateral spines, of up to 20 individuals 
2 Nichols, S. 2004. Derivation of red snapper time series from 
SEAMAP and groundfish trawl] surveys. Southeast Data Assess. 
Rev. SEDAR7-DW-01, 26 p. [Available from website.] 
3 Pollack, A. G., D. S. Hanisko, and G. W. Ingram Jr. 2016. 
Wenchman abundance indices from SEAMAP groundfish sur- 
veys in the northern Gulf of Mexico. Southeast Data Assess. Rev. 
SEDAR49-DW-19, 27 p. [Available from website.] 
