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Fishery Bulletin 96(2), I 998 
153 and 207 |im. These three female had recently 
finished spawning because their ovaries contained 
either 100% of advanced yolked oocytes in an a atretic 
state or residual hydrated oocytes but no healthy 
advanced yolked oocytes. 
Spawning Season 
Southern bluefin tuna were caught on the spawning 
ground during every month, except July. Catch per 
unit of effort (CPUE), expressed as the number of 
fish caught per vessel fishing day, showed that abun- 
dance was low from May to August (Fig. 4). In the 
1993-94 season, abundance peaked in February, and 
in the following season, abundance peaked in Octo- 
ber and February. 
Females with high GI values were caught on the 
spawning ground during all months that ovaries were 
collected. Gonad index values were variable, however, 
ranging from 1.3 to 13.1. There were no peaks or 
trends during this time. These observations suggest 
that spawning activity is constant throughout the 
season. 
Histological classification 
All ovaries examined histologically from the South- 
ern Ocean were classified as immature, nonspawning 
(mature), or postspawning. Females greater than 140 
cm, with advanced yolked oocytes, collected between 
August and December were probably prespawners 
preparing to migrate to the spawning ground. Sur- 
prisingly, 78% of these prespawning females had a 
atresia of advanced yolked oocytes. The level of atre- 
sia present was related to ovary weight or GI (Fig. 5). 
The ovaries of all females with GI’s<2 had no or <10% 
a atresia (atretic states 0 or 1), whereas 80% of fe- 
males with GI’s>2 had 10 to 50% a atresia (atretic 
state 2). This finding suggests that females begin 
resorbing yolked oocytes as their ovaries mature. 
Atresia of unyolked or early yolked oocytes was not 
observed in these females. Postspawning females 
were found in the Southern Ocean in all months that 
ovaries were collected, including both the early and 
late months of the spawning season (September to 
December). Ovaries, however, were not collected be- 
tween January and March from the Southern Ocean. 
The ovaries of all females sampled on the spawn- 
ing ground were classified as mature because they 
contained either advanced yolked oocytes or oocytes 
in either an a or P stage of atresia. Of the females, 
69.2% were classed as spawning, 30.2% as non- 
spawning, and the remaining 0.6% as postspawning 
on the basis of oocyte stage, atretic stage, and 
postovulatory follicle class present. The mean GI 
8000 n 
7000 
6000- 
5000- 
| 4000 
6 3000- 
2000 
1000 - 
o Spawning ground 
X SE Indian Ocean 
• Tas./N.Z./S.Afr. 
o- 1 
80 100 120 140 160 180 200 220 
Length (cm) 
Figure 3 
The relation between ovary weight and body length for 
southern bluefin tuna, Thunnus maccoyii, caught on the 
spawning ground in the northeast Indian Ocean, on the 
staging grounds in the southeast Indian Ocean, and on 
feeding grounds off Tasmania, New Zealand, and South 
Africa. The line represents a gonad index of 2. 
(±95% Cl) of spawning and nonspawning females was 
similar at 5.4 (0.371 and 5.6 (0.29) but GI was not 
calculated for postspawning females because the 
sample size was too small (n=3). Spawning and 
nonspawning females were found on the spawning 
ground throughout the spawning season. All 
postspawning females were collected in October. The 
majority of females identified as spawning (86.1%) 
had ovaries containing no or less than 10% a atre- 
sia. The remaining 13.9% of females identified as 
spawning contained more than 10% a atresia. Nearly 
90% of nonspawning females, however, contained 10- 
50% of advanced yolked oocytes in an a atretic state. 
We have used this criterion of <10% a atresia to group 
fish we considered to be at the height of spawning, 
which we refer to as “prime spawning condition.” 
There did not appear to be any seasonal trends in 
the proportions of each atretic state throughout the 
spawning season (Fig. 6), suggesting that spawning 
in southern bluefin tuna is not synchronized. This is 
confirmed by an absence of an increase in the inci- 
dence of postspawning females at the end of the 
spawning season. 
