Zudaire et a!.: Reproductive potential of Thunnus ciibacares in the western Indian Ocean 
255 
Total length (cm) 
Figure 2 
Proportion of mature female Yellowfin Tuna ( Thunnus albacares ) in 
the western Indian Ocean at 5-cm length intervals and fitted to a 
logistic regression curve in our study of the reproduction potential 
of this species in this region. Circles represent the proportions of 
females considered mature when their ovaries were at the cortical 
alveolar and later stages; the gray solid line indicates the logistic 
regression curve for these females. Crosses represent the propor- 
tions of females considered mature when their ovaries were at the 
vitellogenic and later stages; the dark solid line indicates the logis- 
tic regression curve for these females. The horizontal, dotted line 
indicates L 50 , the length at which 50% of the female Yellowfin Tuna 
population were mature. 
whole ovaries was assumed on the basis of previous 
studies on tuna (Stequert and Ramcharrun, 1996). 
For BF analyses, 3 subsamples of 0.1 g (±0.01) were 
collected from each ovary. Each subsample was satu- 
rated with glycerin and oocytes were counted under a 
stereomicroscope (Schaefer, 1987, 1996, 1998). Batch 
fecundity was calculated as the weighted mean den- 
sity of the 3 subsamples multiplied by the total weight 
of the ovary. A threshold of 10% for the coefficient of 
variance was applied for the 3 subsamples, and when 
this threshold was surpassed, more subsamples were 
counted to reach it. Relative batch fecundity (BFrel) 
was estimated by dividing the BF by the gonad-free 
weight of the fish. The relationships between the BF 
and BFrel with the FL, weight, and condition indices 
(GSI, HSI, and K) of females were determined. The sea- 
sonal trend in fecundity was analyzed through estima- 
tion of monthly mean BF and BFrel. 
Statistical analyses 
A nonparametric Kruskal-Wallis test (H- 
test) was applied to determine differences in 
the GSI, HSI, and K among months and be- 
tween maturation stages. The relationships 
between the BF and BFrel and other bio- 
logical parameters, such as length, weight, 
and condition indices (HSI and K), were 
analyzed through application of simple cor- 
relation and regressions. Analysis of vari- 
ance (ANOVA) was applied to analyze the 
differences in the estimates of mean BF and 
mean BFrel by month during the spawning 
season. 
Results 
Length at 50% maturity 
L 50 was estimated to be 75 cm FL when 
females with ovaries at the CA stage and 
onward were considered mature. This es- 
timate increased to 102 cm FL when the 
second criterion was applied (i.e., when the 
maturity threshold was defined as the pres- 
ence of advanced vitellogenic oocytes, Fig. 
2). In both cases, the proportion of mature 
females by length provided a good fit to the 
logistic model (coefficient of determination 
[r 2 ]=0.89 and r 2 =0.91, respectively) (Table 
2 ). 
Reproductive cycle 
The analysis of the female maturation pro- 
cess throughout the year showed that 30.1% 
of the individuals sampled were in the im- 
mature phase, 44.4% were in the developing 
phase, 20.3% were in the spawning-capable 
phase, and 5.12% were in the regenerating phase (Ta- 
ble 3). Overall, 69.8% of the sampled females were in 
the mature state, 92.6% of which were reproductively 
active. 
The analysis of the maturation process was car- 
ried out for 2 size groups: individuals >100 cm FL 
and individuals <100 cm FL. The females >100 cm FL 
showed ovaries more developed and closer to spawning 
from November to February and in June than in other 
months (Fig. 3A). The period between November and 
January was especially important because more than 
90% of the females sampled were in the spawning-ca- 
pable phase. In contrast, during April and May, there 
was no spawning activity for this size group (fish >100 
cm FL), with 40% and 30% of the ovaries in the regen- 
erating phase in each month, respectively. The occur- 
rence of immature phase ovaries increased to 50% and 
38% in April and May. 
