258 
Fishery Bulletin 111(3) 
Nov Dec Jan Feb Mar Apr May Jun Jul 
0 - 1 "-t— 1 — L - H — L -r J — — l t j — L - H — L -r J — l t j — A— 1 
Nov Dec Jan Feb Mar Apr May Jun Jul 
I I IP 
■■ DP 
I I SCP 
RP 
Figure 3 
Proportion of female Yellowfin Tuna ( Thunnus alba- 
cares ) in different ovarian developmental phases by 
month and size: (A) >100 cm in fork length and (B) 
<100 cm in fork length. The phases of ovarian devel- 
opmental shown in this graph are the immature phase 
(IP), developing phase (DP), spawning-capable phase 
(SCP), and regenerating phase (RP). 
method to estimate the length at which all specimens 
achieve maturity. In our recent work, L 50 was estimated 
at 75 cm FL when the maturity threshold was defined 
as ovaries in the CA stage. This length is significantly 
smaller than the L 50 reported for all previous studies in 
the Indian Ocean (Table 5). In all of those studies, the 
L 50 was estimated on the basis of macroscopic methods 
that defined the maturity threshold by the presence of 
advanced vitellogenic oocytes in the ovaries. Besides the 
natural variability in the length at maturity (Itano 2 ), 
the main reasons for the difference in L 50 between our 
study and previous studies are the use of different meth- 
ods and oocyte stages to establish the maturity thresh- 
old for estimating L 50 . Standardization of the selected 
maturity index and accurate estimation are required 
to enable direct comparisons between estimates and to 
avoid biases. 
The cortical alveolar stage is the earliest sign of 
oocyte maturation (Brown-Peterson et al., 2011). Fe- 
males in this developmental stage normally continue 
through vitellogenesis and spawn in the upcoming sea- 
son (Wright, 2007). On the basis of the annual repro- 
ductive cycle and batch spawning behavior of Yellowfin 
Tuna, following the recommendation made by Lowerre- 
Barbieri et al. (2011), we suggest that Yellowfin Tuna 
females with ovaries in the cortical alveolar stage 
should be included in maturity estimates. Estimation 
of L 50 through establishment of maturity in vitello- 
genic oocytes (Schaefer, 1998; Itano 2 ; Zhu et ah, 2008) 
has the disadvantage that L 50 will be overestimated 
because maturing individuals (i.e., females with corti- 
cal alveolar stage oocytes) are categorized as immature 
(Lowerre-Barbieri et ah, 2011). There is no informa- 
tion on how much individual growth occurs during the 
time lag between CA and vitellogenesis. Such informa- 
tion may improve estimation of L 50 because growth be- 
tween those stages may partly explain the difference in 
length at first maturity obtained with different oocyte 
maturation thresholds. 
Reproductive cycle 
On the basis of the histological evaluation of ovaries 
and the assessment of the seasonal variation in the 
GSI, 2 main reproductive periods were identified in 
our study. The first period identified occurred from No- 
vember to February, and the second period occurred in 
June, with lower reproductive activity than the first 
period. Similar results were obtained by Stequert et ah 
(2001), who related the spawning activity of Yellowfin 
Tuna with the monsoons. They identified that spawn- 
ing activity was higher in the north monsoon (from 
November to March) than in the south monsoon (from 
June to August) — probably a result of the decrease in 
sea-surface temperature during the south monsoon pe- 
riod. Other authors identified only a single reproduc- 
tive period: from January to June (Zhu et ah, 2008), 
from January to March (Stequert and Marsac, 1989), 
and from November to April (Nootmorn et al. 4 ). The 
seasonal peaks in spawning described in this study were 
4 Nootmorn, P., A. Yakoh, and K. Kawises. 2005. Reproduc- 
tive biology of yellowfin tuna in the eastern Indian Ocean. 
Indian Ocean Tuna Commission (IOTC), Working Party Trop- 
ical Tuna ( WPTT), IOTC-WPTT-14, 378-385 p. IOTC Secre- 
tariat, P.O. Box 1011, Victoria, Seychelles. 
