256 
Fishery Bulletin 111(3) 
[SD 0.2]), January (0.52 [SD 0.3]), and February 
(0.43 [SD 0.3]), when the GSI increased slightly 
during the main spawning period of large speci- 
mens of Yellowfin Tuna. There was no increase 
in the mean GSI in June for the smaller size 
group (Fig. 4B). 
The monthly variation in HSI and K of both 
size groups showed statistically significant dif- 
ferences between months: HSI for fish <100 cm 
FL (77 154 =29.1, P<0.01) and for fish >100 cm FL 
Cffi22=26.3, P<0.01), and K for fish <100 cm FL 
(/J669=175.5, P<0.01) and for fish <100 cm FL 
(7/313=30.6, P<0.01). For females >100 cm FL, K 
values had a pattern that was opposite the one 
seen in the GSI. The K of large females was low 
from November (1.82 [SD 0.1]) to January (1.80 
[SD 0.0]) and increased from February (1.82 [SD 
0.0]) to July (1.89 [SD 0.2]) (Fig. 4A). Except for 
January, the mean HSI values showed a similar 
pattern to the one observed in the GSI. The HSI 
values of females >100 cm FL decreased from 
February (1.27 [SD 0.3]) to April (0.61 [SD 0.1]) 
and then increased slightly from May (0.70 [SD 
0.1]) to July (0.72 [SD 0.2]) (Fig. 4A). The data 
series for the HSI was shorter than the data se- 
ries for the other 2 condition indices because no 
liver samples were collected in the cannery. 
Oocyte developmental stage and condition indices 
There were significant differences in the GSI 
(T7 2 75=162.1, P<0.01), HSI (77 27 5=49.9, P<0.01), and K 
(77275=10.68, P<0.05) between ovarian developmental 
phases. The GSI showed lowest values at the prima- 
ry growth stage (0.23 [SD 0.1]) and values increased 
throughout the vitellogenesis process (1.13 [SD 0.8]) 
until the maximum values at GVM (2.17 [SD 0.7]) and 
hydration (1.98 [SD 0.8]) stages were reached. After- 
ward, the GSI showed a sharp decrease in the regen- 
erating phase (0.44 [SD 0.3]) (Fig. 5). The HSI showed 
a decreasing trend from immature phase ovaries (0.94 
[SD 0.2]) to ovaries in vitellogenesis (0.77 [SD 0.2]), 
and then it followed the pattern shown by the GSI, 
increasing from vitellogenesis to hydration stages (1.33 
[SD 0.3]). The HSI had its minimum value in the regen- 
erating phase (0.66 [SD 0.1]). The K followed the oppo- 
site trend from that of the GSI and HSI; it decreased 
from the immature phase (1.94 [SD 0.2]) throughout 
the maturation process, obtaining minimum values at 
the hydration stage (1.77 [SD 0.0]). The K increased in 
the regenerating phase (1.85 [SD 0.1]). 
Fecundity Estimation 
The estimated mean BF was 3.07 million oocytes and 
varied from 0.32 million to 6.91 million oocytes. The 
estimated mean BFrel was 74.4 oocytes per gram of go- 
nad-free weight and fluctuated between 9.2 and 180.8 
oocytes per gram of gonad-free weight. Batch fecundity 
Table 2 
Summary of logit parameters of the female maturity 
(L 5 o=length at which 50% of the population is mature) curve 
for 2 criteria used in our study of Yellowfin Tuna ( Thunnus 
albacares ) reproduction in the western Indian Ocean. The 
symbols a and p represent the coefficients of the logistic equa- 
tion, and r 2 is the coefficient of determination. In criterion 1, 
the L 50 was calculated with the assumption that females with 
ovaries at the cortical alveolar stage onward were mature. The 
L 50 in the criterion 2 was estimated with the assumption that 
females with cortical alveolar oocytes as the most advanced 
developmental stage were immature and those with advanced 
vitellogenic oocytes were mature. 
Criterion 1 
Criterion 2 
Parameters 
a 
P 
a 
P 
Estimate 
Standard error 
-8.654 
1.604 
0.113 
0.021 
-6.965 
1.246 
0.068 
0.012 
Estimates Estimates 
Number of females 423 423 
L 50 (-a/p) 74.7 cm 102.0 cm 
r 2 0.89 0.91 
The females <100 cm FL showed a considerable pre- 
dominance of immature phase ovaries from April to 
November with levels of 50% or higher (Fig. 3B). These 
percentages of immature phase ovaries decreased be- 
low 30% during December, January, and February, 
and the percentage of ovaries in the spawning-capable 
phase increased to 4.4%, 5.3%, and 6.9%, respectively, 
for each month. It is noteworthy that ovaries in the 
regenerating phase appeared during the spawning sea- 
son for females <100 cm FL, with values of 8.9% and 
10.5% in December and January, but ovaries in this 
phase for larger Yellowfin Tuna (>100 cm FL) appeared 
in April and May. 
Condition indices 
The monthly variation in the GSI of the previously 
defined 2 size groups showed statistically signifi- 
cant differences between months for fish <100 cm FL 
(77669=136.9, P<0. 01) and fish >100 cm FL (77313 = 206.4, 
P<0.01). Females >100 cm FL had a peak in mean 
GSI values that coincided with the highest proportion 
of females in the spawning-capable phase from No- 
vember to January (GSI>2.0). Afterward, mean GSI 
values decreased sharply with a minimum value in 
April of 0.26 (standard deviation [SD] of 0.0). In June, 
the mean GSI increased again to 0.98 (SD 0.5) in a 
second spawning period but with lower reproductive 
activity compared with the main spawning period 
(Fig. 4A). In contrast, females <100 cm FL had almost 
constant mean GSI values except in December (0.44 
