288 
Fishery Bulletin 115(3) 
150 
200 
1 
150 
Fork length (mm) 
200 
Figure 1 
Logistic regression of maturity at size for (A) female and (B) male 
Gulf menhaden ( Brevoortia patronus ) collected from the northern 
Gulf of Mexico during 2014 through 2016. Solid black lines indi- 
cate size at 50% maturity. The gray line is the mean prediction, 
and the dotted lines are the 95% confidence intervals. 
Overall, the sex ratio of Gulf menhaden 
was not significantly different from 1:1. Fe- 
males dominated the collections in February, 
June, September, October, and for all months 
combined, and significantly more females than 
males were sampled in September (P=0.019). 
Relatively equal sex ratios suggested that fish 
of both sexes are equally susceptible to fishing 
pressure and there is no difference in mortal- 
ity associated with gender. 
Size at sexual maturity was found to be vir- 
tually identical for males and females. Length - 
at-50%-maturity was calculated as 137.2 mm 
FL and 140.0 mm FL for females (Fig. 1A) and 
males (Fig. IB), respectively. Fish this size 
typically recruit into the commercial fishery in 
the late summer and early fall and were con- 
sidered to be late age-1. 
Histological analysis There was poor agree- 
ment between macroscopic and histological 
assessment of reproductive phase for female 
Gulf menhaden (% 2 318 =198.2, PcO.OOl). How- 
ever, the Yates corrected chi-square test for 
comparison of histological and macroscopic 
agreement in reproductive assessments of 
the spawning capable and actively spawning 
phases showed there was not a significant dif- 
ference in misclassification between these 2 
phases (Yates x 2 72=3.44, P=0.064). There was 
also poor agreement between macroscopic and 
histological assessment of reproductive phase 
for male Gulf menhaden (x 2 275=137.Q, PcO.001). 
However, agreement between macroscopic and 
histological assessments of reproductive phase 
improved when Gulf menhaden were classified 
as either reproductively active or inactive. For 
females, 99% of fish macroscopically assessed as repro- 
ductively inactive remained in that classification after 
histological inspection and there was an 82% agreement 
between macroscopic and histological assessment for re- 
productively active females. Similarly, males showed a 
98% agreement for reproductively active fish and a 93% 
agreement for reproductively inactive fish when macro- 
scopic and histological classifications were compared. 
Gulf menhaden ovarian tissue showed asynchronous 
oocyte development and evidence of batch spawning. 
Females in the spawning capable phase had oocytes in 
all stages of development, and there was evidence of 
POFs in some ovaries in early October (Fig. 2A). Fe- 
males in the actively spawning subphase (Fig. 2B) had 
oocytes undergoing OM or hydrated oocytes, as well as 
vitellogenic oocytes in various stages of development 
(Fig. 2B). The immature phase was histologically dis- 
tinguished from the regenerating phase by the smaller 
size of the primary growth oocytes, lack of space among 
oocytes, and the large amount of interstitial tissue 
present in the immature ovary. 
The testis of Gulf menhaden is an anastomosing 
tubular type, characterized by highly branched germi- 
nal compartments that do not terminate at the testis 
periphery. Testicular tissue showed differences in the 
number of spermatocysts along the tubules as the re- 
productive season progressed. Active spermatogenesis 
was evident in males in the early germinal epithelium 
(GE) subphase of the spawning capable phase (Fig. 
2C), and spermatogonia, spermatocytes and spermatids 
present within cysts completely lined the tubules; ad- 
ditionally, tubules were filled with spermatozoa. In the 
late GE subphase, typical of the end of the reproductive 
period, the tubules were still swollen with spermato- 
zoa, but little active spermatogenesis was taking place, 
and few spermatocysts were seen lining the tubules 
(Fig. 2D). The regenerating phase was histologically 
distinguished from the immature phase in males by 
the presence of some residual spermatozoa (Sz) within 
the tubules. Testicular tissue from males collected in 
September and October showed microspoordian infec- 
tion, although this was not a common occurrence. 
Spawning seasonality The GSI values increased in early 
October for both males and females, and reached peak 
values for females by the second half of October (Fig. 3). 
