Brown-Peterson et al.: Reproductive biology of Rachycentron canadum 
27 
species just mentioned. The exact location of cobia spawn- 
ing is unknown; early surveys have suggested spawning 
immediately outside the mouth of Chesapeake Bay (Jo- 
seph et ah, 1964), whereas later data from the southeast- 
ern United States have indicated that cobia spawn off- 
shore of North Carolina (Hassler and Rainville, 1975) and 
South Carolina (Shaffer and Nakamura, 1989). Egg col- 
lections in Crystal Bay, FL (Ditty and Shaw, 1992), imply 
that spawning occurs nearshore in the Gulf of Mexico, al- 
though other egg and larval evidence (Shaffer and Naka- 
mura, 1989; Ditty and Shaw, 1992) suggest spawning oc- 
curs in the Gulf of Mexico on the shelf 50-90 km from 
shore. Further evidence for offshore spawning was the col- 
lection of small larvae (3. 8-6. 8 mm) 50-90 km off the coast 
of Texas (Finucane et al. 3 ). Because most of our samples 
were captured no more than 40 km from shore, cobia in 
immediate pre- or postspawning condition may not occur 
in those locations. In addition, our hook-and-line method 
of capture may be biased against cobia in immediate pre- 
or postspawning condition owing to changes in feeding be- 
havior at these stages. 
Although spawning frequencies of cobia from the three 
study areas were not significantly different, the apparent 
lower spawning frequency of cobia in the WGOM may be 
biologically relevant. Cobia from SEUS and NCGOM were 
estimated to be capable of spawning up to 36 times during 
the six-month spawning season, whereas fish from WGOM 
were estimated as capable of spawning 15 to 20 times 
during the spawning season. Hydrologic features of the ar- 
eas may explain the differences. The southeastern United 
States and the north-central Gulf of Mexico have substan- 
tial inputs of freshwater from major river systems which 
may result in high productivity in those areas (Livingston 
et al., 1997) and hence abundant food sources. In contrast, 
there is little freshwater input along the western Gulf of 
Mexico. 
Another possible explanation for the differences in 
spawning frequencies may be that many cobia in the Gulf 
of Mexico spawn in a single location that is closer to the 
north-central region than to the western region. The lack 
of 12-h POFs and the predominance of 48-h POFs seen 
in the ovaries of cobia from the western Gulf of Mexico 
may be due to the longer distance that western cobia must 
travel from the spawning grounds. This hypothesis as- 
sumes that cobia in the Gulf of Mexico do not have distinct 
breeding areas or subpopulations — a hypothesis support- 
ed by Hrincevich’s (1993) work on the molecular genetics 
of cobia. Although Hrincevich ( 1993) did find a high degree 
of heterogeneity in cobia mtDNA, this heterogeneity did 
not support the hypothesis that discrete stocks of cobia ex- 
ist in the northern Gulf of Mexico. The genetic data, in 
combination with data from tagging studies in the north- 
ern Gulf of Mexico (Franks et al. 6 ), suggest that cobia in- 
6 Franks, J. S., J. T. McBee, and M. T. Allen. 1992. Studies on 
the seasonal movements and migratory patterns of the cobia, 
Rachycentron canadum, in Mississippi marine waters and adja- 
cent Gulf waters. Interim Contract Rep. to Miss. Dep. Wildl., 
Fish, and Parks/Bur. Mar. Res, and U.S. Fish and Wildl. Serv, 
Atlanta, GA 30303. [Available from Gulf Coast Res. Lab., 
Ocean Springs, MS 36566-7000.] 
termix not only within the Gulf of Mexico but also along 
the southeastern Atlantic coast of the United States. Thus, 
the overall similarities in the reproductive biology of cobia 
throughout the southern United States are not surprising. 
The information provided in our study on batch fecundity 
and spawning frequency of cobia should aid effective man- 
agement of cobia stocks, as well as underscore areas where 
additional research is needed. 
Acknowledgments 
We thank the Mote Marine Laboratory (MML) sampling 
team (Teresa DeBruler, Carole Neidig, Marion Hersey, 
Diana Skapura, Roger DeBruler, and Sasha Koulish [MML 
staff] and Jonnie Walker, Rob Roberts and Don and Toma 
Marshall [MML volunteers] ), the Gulf Coast Research Lab- 
oratory (GCRL) sampling team (Don Barnes, Casey Nich- 
olson, Nate Jordan, Jody Peterson, Jason Steckler, Nicola 
Garber, and Melanie Griggs), Scott Holt (The University of 
Texas Marine Science Institute), and Hal Osborne (Texas 
Department of Parks and Wildlife) for assistance with col- 
lecting cobia samples. We also thank the many anglers, 
charter boat captains, tournament directors, and student 
interns who assisted our sampling efforts. Histological 
support for this project was provided by Marie Wright, Kim 
Lamey, Mary Tussey, and Tershara Matthews (GCRL); 
Leslie Christmas (GCRL) furnished the fecundity counts. 
We thank Susan Carranza (GCRL) for photographic devel- 
opment and drawing of Figure 1. Mark Peterson (GCRL) 
assisted with statistical analysis and reviewed the manu- 
script. Joseph W. Smith (National Marine Fisheries Ser- 
vice, Beaufort, NC) was the technical monitor for this 
project. This project was funded by the National Oceanic 
and Atmospheric Administration, National Marine Fish- 
eries Service (grant numbers NA57FF0294, NA86FL476, 
and NA96FL0358). 
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