Brown-Peterson et al.: Reproductive biology of Rcichycentron canodum 
25 
Table 8 
Monthly mean relative batch fecundity expressed as 
number of eggs/g ovary-free body weight for cobia in the 
southern United States. Batch fecundity was determined 
from oocytes >700 pm in neutral buffered formalin (NBF) 
and from histological sections of oocytes undergoing final 
oocyte maturation (FOM). All means were not statistically 
different (AN OVA, P>0.05). 
Month 
NBF 
FOM 
n 
Relative 
fecundity 
±1SE 
n 
Relative 
fecundity 
±1SE 
April 
i 
46.6 
0 
— 
May 
25 
51.2 ±12.2 
8 
24.6 ±4.9 
June 
2 
115.7 ±102.2 
3 
21.9 ±12.9 
July 
5 
44.0 ±9.2 
6 
40.2 ±12.9 
August 
2 
29.8 ±25.2 
5 
24.6 ±14.9 
September 
4 
57.7 ±27.5 
3 
33.2 ±13.0 
Overall 
39 
53.1 ±9.4 
25 
29.1 ±4.8 
maturity stages, particularly in multiple spawning fish. 
The GSI profile for both male and female cobia from the 
SEUS is similar to that described by Smith (1995) for co- 
bia from North Carolina. On the other hand, GSI peaks for 
cobia from the northern Gulf of Mexico vary among stud- 
ies. Biesiot et al. (1994) reported that female GSI values 
peaked in April, Lotz et al. (1996) found peak female GSI 
values in May, and Thompson et al. 2 reported peak female 
GSI values in June. Our data for females mirror those pre- 
sented by Lotz et al. (1996); however, we suspect annual 
differences. 
Our study was not designed to determine the size or age 
at first maturity for cobia. However, our limited data sug- 
gest that both sexes of cobia may achieve sexual maturity 
at a smaller size than that reported by Lotz et al. (1996) 
for the north-central Gulf of Mexico. This apparent differ- 
ence in the size at sexual maturity could be partially ex- 
plained by regional differences; most small female cobia in 
the present study ( <85 cm FL) were captured in EGOM, 
whereas Lotz et al. (1996) sampled in NCGOM. 
Male cobia are probably capable of spawning through- 
out the year because of the presence of sperm in the tes- 
tis. A more detailed analysis of the histological pattern 
and spermatogenesis of male cobia will be discussed sepa- 
rately (Brown-Peterson et al. 4 ). A similar longer reproduc- 
tive season for male fish has been reported for other spe- 
cies with protracted spawning seasons including common 
snook ( Centropomus undecimalis, Grier and Taylor, 1998; 
4 Brown-Peterson, N. J., H. J. Grier and R. M. Overstreet. 2000. 
Manuscript in preparation. Reproductive classes in male cobia 
(Rcichycentron canadum ) defined by changes in the germinal 
epithelium. Abstract and presentation at the 80 th annual meet- 
ing of the American Society of Ichthyologists and Herpetolo- 
gists, June 2000, La Paz, B.C.S., Mexico. 
Taylor et al., 1998), spotted seatrout ( Cynoscion nebulosus, 
Brown-Peterson et al., 1988), red drum ( Sciaenops ocella- 
tus, Grier et al., 1987), and blue tilapia ( Oreochromis au- 
reus, Grier and Abraham, 1983). 
Cobia have a protracted spawning season, yet a portion 
of the females in the population may spawn during April- 
June only. Other females remain in spawning condition 
throughout September in the Gulf of Mexico. Lotz et al. 
(1996) and Biesiot et al. (1994) reported a similar occur- 
rence in cobia from the NCGOM, and a high percentage of 
female cobia off Louisiana are spent and regressed by Ju- 
ly (Thompson 5 ). A comparable phenomenon, i.e. asynchro- 
nous cessation of spawning, was reported in the weakfish 
(C. regalis ) in Chesapeake Bay (Lowerre-Barbieri et al., 
1996). It is difficult to explain early cessation of spawning 
by some female cobia. Spent and regressed fish in July and 
August had a broad length distribution (85 tol28 cm FL), 
suggesting that multiple age classes in the fishery have an 
abbreviated reproductive season. Perhaps some females 
delay ovarian maturation and spawn between July and 
September; this theory would account for the small per- 
centage of females in the early- and mid-developing classes 
in May and June. 
Differences in the amount of oocyte atresia during the 
spawning season between cobia from the SEUS and the 
Gulf of Mexico suggest differential spawning success dur- 
ing the early portion of the reproductive season. High per- 
centages of alpha and beta atresia in cobia in the late-de- 
veloping class from SEUS during March, April, and May 
suggest that many oocytes do not reach final maturation 
and that the atresia may be related to variable or unfa- 
vorable environmental conditions during spring (March- 
May) in the region. Hay and Brett ( 1988) showed a similar 
occurrence for Pacific herring ( Clupea harengus pallasi) 
of increased atresia at the beginning of the reproductive 
season — condition they attributed to environmental fac- 
tors rather than the female’s physiological ability. Low- 
erre-Barbieri et al. (1996) used similar reasoning to ex- 
plain the increased percentage of alpha and beta atresia 
present in the ovaries of weakfish captured during the be- 
ginning of the spawning season in Chesapeake Bay. Co- 
bia from NCGOM also showed high percentages of atresia 
during March, the beginning of the reproductive season 
but prior to the initiation of spawning in that area. The 
lower percentage of females in the late-developing class 
with atretic oocytes in the Gulf of Mexico during April and 
May suggests a relatively high spawning success during 
the early portion of the spawning season in the Gulf of 
Mexico, and it also may suggest more stable environmen- 
tal conditions in this region during late spring. 
In the absence of hydrated oocytes in any cobia that we 
or Lotz et al. (1996) examined, we used three different 
methods to estimate fecundity of cobia containing large 
(>700 pm) oocytes. The wide range of results found among 
methods highlights the variations to be expected when es- 
timating batch fecundity of a multiple spawning species. 
5 Thompson, B. A. 1999. Personal commun. Coastal Fisheries 
Institute, LSU Center for Coastal, Energy and Environmental 
Resources, Baton Rouge, LA 70803. 
