290 
Fishery Bulletin 115(3) 
(Table IB). By mid-September, some males were in 
the spawning capable reproductive phase, and 72% of 
males were spawning capable by early October (Table 
IB). Spawning capable males dominated collections 
from October through March. Males in the regressing 
and regenerating phases were first seen in early March, 
and by mid-April all males had ceased spawning (Table 
IB). During the early portion of the spawning season 
(October and November), most males were found in the 
early GE (Fig. 2C) subphase of the spawning-capable 
reproductive phase, indicating active spermatogenesis 
throughout the testis. Active, although somewhat re- 
duced, spermatogenesis continued during January 
and February (mid GE subphase). At the end of the 
spawning season in March, all males were in the late 
GE subphase (Fig. 2D) and reduced spermatogenesis, 
but abundant spermatozoa, were still present in the 
tubules. Histological analysis showed that most males 
were spawning capable during the same 5.5-month pe- 
riod when females were spawning capable. 
Age-based differences in gonadal recrudescence 
were evident in both females and males at the begin- 
ning of the reproductive season. Although there was 
no significant difference in the distribution of repro- 
ductive phases between age-1 and age-2 females in 
September (P= 0.088), no age-1 females were in the 
developing phase (Fig. 4). There was a significant dif- 
ference between age-1 and age-2 females in October 
(P=0.047, Fig. 4), with a much higher percentage of 
age-1 fish in the immature, early developing, and de- 
veloping phases compared with age-2 females. There 
was no difference in distribution across all 3 ages, or 
between ages-2 and -3 (P=0.165 and 0.875, respective- 
ly; Fig. 4) in October. Therefore, younger female Gulf 
menhaden appeared to have a delayed beginning of 
the reproductive season. 
Age-1 and age-2 males did not exhibit a significant 
difference (P=0.514) in the distribution of reproduc- 
tive phases (Suppl. Fig. 1) (online only) in August at 
the beginning of gonadal recrudescence. However, in 
September a significant difference (P=0.Q17) in phase 
distribution was seen (Suppl. Fig. 1). A high percent- 
age of age-1 males were in the immature and early 
developing phases and no spawning capable individu- 
als were observed, whereas immature age-2 males were 
not present in September, a high percentage of age-2 
males were present in the developing phase, and some 
age-2 males were spawning capable (Suppl. Fig. 1). 
Although there was no significant difference in repro- 
ductive phase distribution between ages-1, -2, and -3 
in October (P=0.056), the percentage of males in the 
developing phase decreased with increasing age, and 
all age-3 fish captured were spawning capable (Suppl. 
Fig. 1). Therefore, younger male Gulf menhaden also 
exhibited a delayed beginning of the reproductive sea- 
son when compared to older males. 
Spawning frequency Spawning frequency varied de- 
pending on the method used (Table 2). The OM method 
provided a more consistent estimation of spawning fre- 
10 - 
• Female 
a Male A 
8 - 
6 - 
i 
S I . 
co 4 ' 
0 
2 - 
4 
0 - 
* * 
• ^ ® oo 
Month 
Figure 3 
Mean bimonthly gonadosomatic index (GSI) values of 
male and female Gulf menhaden ( Brevoortia patronus) 
collected from the northern Gulf of Mexico in 2014 
through 2016. Error bars indicate standard errors of 
the means. 
quency than the POF method (Table 2) with the excep- 
tion of the early October sample, when Gulf menhaden 
were just beginning to spawn. Spawning frequency 
estimates from February and March should be viewed 
with caution because of low sample sizes. Combining 
fish from all months, spawning frequency ranged from 
every 2.1 to 4.3 days, depending on the method used. 
Assuming a 5.5-month spawning season, these spawn- 
ing frequency estimates suggest that an individual fe- 
male Gulf menhaden has the potential to spawn 38-79 
times during the reproductive season. Smaller, age-1 
females likely have fewer potential spawns than older, 
larger females during the reproductive season (35-71 
times) because of the delay in reaching the actively 
spawning subphase in early October (Fig. 4). 
Fecundity Samples were analyzed from 16 actively 
spawning Gulf menhaden from late October through 
March. Fish ranged in size from 165 to 203 mm FL 
and were primarily age 2. Batch fecundity ranged from 
2829 to 59,423 eggs per individual, with an overall 
mean BF of 15,367 eggs (standard error [SE] 3260; 
median: 11,577). The RBF ranged from 31.1-328.3 
eggs/g ovary-free body weight with an overall mean of 
107.8 eggs/g ovary-free body weight (SE 17.1; median: 
101.7). Batch fecundity was significantly (F 115 =5.629, 
P=Q.Q33), positively related to FL, although this rela- 
tionship explained only 28.7% of the variation in BF 
(Fig. 5). Batch fecundity was also significantly positive- 
ly related to weight (F 1 15 =5.278, P=0.038, coefficient of 
determination [r 2 ]=0.274). Log 10 transformation of the 
data resulted in the best predictive fit for BF and FL; 
the predictive equation was: 
log BF = 4.695(logFL) - 6.547. 
( 6 ) 
