232 
Fishery Bulletin 96(2), 1 998 
200 400 600 800 1000 1200 
Oocyte diameter (pm) 
Figure 8 
Size-frequency distribution of oocytes representing the advanced stages of matu- 
ration, by 50-pm intervals, in southern bluefin tuna, Thunnus maccoyii, ovaries. 
Only oocytes >100 pm were measured. 
termined. However, the prevalence 
of females in different phases of 
spawning activity (nonspawning, 
spawning, or postspawning) on the 
spawning ground should be in di- 
rect proportion to the duration of 
that phase, if females in each phase 
are equally catchable. The ratio of 
nonspa wners : spawners :postspawn- 
ers for females caught on the spawn- 
ing ground was estimated to be 
0.30:0.69:0.01 or 0.44:1:0.01 if 
spawners are set as 1. In other 
words, spawning females were 2.3 
times more prevalent than non- 
spawning females and 108 times 
more prevalent than postspawning 
females on the spawning ground. 
This finding indicates that the du- 
ration of the nonspawning phase is 
less than half, and the postspawning 
phase one onehundredth, of the du- 
ration of the spawning phase. 
Spawning frequency 
and fecundity 
The potential annual fecundity of 
southern bluefin tuna is indeter- 
minate (not fixed prior to spawn- 
ing) because unyolked oocytes are 
continually matured and spawned 
during the season. Annual fecun- 
dity is indeterminate in many 
tuna species such as black skipjack 
(Schaefer, 1987), skipjack tuna 
(Hunter et al., 1986), and yellow- 
fin tuna (Schaefer, 1996). The fe- 
cundity of southern bluefin tuna 
was estimated by Kikawa (1964b) 
as the number of advanced yolked 
oocytes in the ovary, and by Thoro- 
good (1986) as the number of oo- 
cytes >300 pm in diameter in the ovary. These esti- 
mations of fecundity were not potential annual fe- 
cundity because southern bluefin tuna are capable 
of continuously maturing and spawning oocytes from 
a pool of unyolked oocytes (<300 pm). 
Southern bluefin tuna can spawn many times dur- 
ing a season. The ovaries of 25% of the females col- 
lected from the spawning ground contained evidence 
that they had all recently spawned (postovulatory 
follicles) and were about to do so again (migratory 
nucleus or hydrated oocytes). We assume that 
postovulatory follicles persist in the ovaries of south- 
ern bluefin tuna for about 24 hours as has been found 
in other tropical spawning tunas (Hunter et al., 1986; 
McPherson, 1988; Nikaido et al., 1991; Schaefer, 
1996). The average interval between spawning in 
southern bluefin tuna, estimated from the propor- 
tion of ovaries containing postovulatory follicles, was 
1.1 days for females in “prime spawning condition.” 
Similar spawning rates have been reported in other 
tuna species that spawn in tropical waters: 1.54 and 
1.27 days for yellowfin tuna (McPherson, 1991; 
Schaefer, 1996); 1.1 days for bigeye tuna (Nikaido et 
al, 1991) and 1.18 days for skipjack tuna (Hunter et 
