Farley and Davis: Reproductive dynamics of Thunnus maccoyii 
227 
3 Nonspawning (mature): Ovary contains advanced 
yolked oocytes but no evidence of spawning ac- 
tivity (migratory nucleus or hydrated oocytes or 
postovulatory follicles). Less than 100% of ad- 
vanced yolked oocytes are in the a stage of atre- 
sia. If >50% of advanced yolked oocytes are atretic, 
early yolked oocytes are considered nonatretic. 
4 Postspawning: Ovaries contain either: 1) >50% 
of both early and advanced yolked oocytes in the 
a stage of atresia; 2) 100% of advanced yolked 
oocytes in the a stage of atresia; or 3) no yolked 
oocytes are present but oocytes in the P stage of 
atresia are, and residual hydrated oocytes may 
or may not be present. 
Fecundity 
To find out if the annual fecundity of southern blue- 
fin tuna could be determined before spawning be- 
gan, we measured the distribution of oocyte sizes 
within ovaries of four females at various stages of 
maturity. Ovarian subsamples containing at least 
1,000 oocytes were removed from each ovary, teased 
apart, and each oocyte (greater than 100 pm in di- 
ameter) was measured in a random orientation to 
the nearest pm under a stereomicroscope. 
We estimated batch fecundity (the number of hy- 
drated oocytes released per spawning) by the gravi- 
metric method (Hunter et al., 1985) for 21 females 
with unovulated hydrated oocytes (Fig. 2F). A split- 
plot analysis of variance (ANOVA) was used to de- 
termine the appropriate locations to subsample the 
ovary. The data were structured with 6 fish exam- 
ined as blocks, ovarian lobe (left 
or right) as main plots and 
twelve subsamples as subplot 
effects. Six subsamples were 
taken from each ovarian lobe in 
the anterior, middle, and poste- 
rior regions of both lateral sides 
(Table 1). A significant differ- 
ence between ovarian lobes was 
found in the number of hy- 
drated oocytes per gram of to- 
tal ovary weight and in the lobe 
x lateral side interaction. Con- 
sequently, a subsample of less 
than 1 g was taken from both 
sides of each ovarian lobe. Each 
subsample, consisting of a core 
from the periphery to the lu- 
men, was weighed to the near- 
est 0.01 mg and fixed in 10% 
buffered formalin. Each sub- 
sample was teased apart and 
washed through two sieves, similar to those of 
Lowerre-Barbieri and Barbieri ( 1993) to separate out 
the hydrated oocytes, which were counted under a 
stereomicroscope. The number of hydrated oocytes 
per gram of ovary was raised to the weight of both 
ovaries to give an estimate of batch fecundity for each 
of the four subsamples. 
Results 
Ovary maturation 
Ovaries obtained from fish on the spawning ground 
(northeast Indian Ocean), the staging ground (south- 
east Indian Ocean), and the feeding grounds (South- 
ern Ocean) show clear differences in development 
based on ovary weight (Fig. 3). Females less than 
140 cm showed no or minor ovary development; there- 
fore it appears that they would not spawn in the com- 
ing season. The majority of ovaries collected from the 
feeding grounds weighed less than 1 kg and had a 
gonad index (GI) of <3.2, whereas ovaries from the 
southeast Indian Ocean weighed up to 2.8 kg and 
had GI values up to 4.9. Some of the ovaries of fe- 
males caught between August and December on the 
feeding grounds showed signs of maturity, because 
their MAGO’s were in advanced yolked stage with 
diameters greater than 400 pm. Females from the 
spawning ground had larger ovaries, weighing up to 
7.4 kg. All ovaries from fish collected on the spawn- 
ing ground had a MAGO diameter greater than 
400 pm, except three ovaries with MAGO’s between 
Table 1 
Split-plot ANOVA of the effect of location of tissue sample from southern bluefin tuna, 
Thunnus maccoyii, ovaries on the number of hydrated oocytes per gram of ovary. 
Source 
df 
Sum of 
squares 
Mean 
square 
F-value 
P-value 
Blocks 
Fish 
5 
17,201,897 
3,440,380 
279.721 
<0.001 
Main plots 
Lobe 
1 
168,326 
168,326 
13.686 
0.014 
Main plot error 
5 
61,497 
12,299 
0.764 
0.580 
Subplot effect 
Lateral side 
1 
711 
711 
0.044 
0.834 
Region 
2 
34209 
17,104 
1.062 
0.353 
Lateral side x region 
2 
82,599 
41,300 
2.565 
0.087 
Lobe x lateral side 
1 
152,159 
152,159 
9.450 
0.003 
Lobe x region 
2 
12,944 
6,472 
0.402 
0.671 
Lobe x region x lateral side 
Subplot error 
2 
50 
58,207 
805,044 
29,103 
16,101 
1.808 
0.175 
