Farley and Davis. Reproductive dynamics of Thunnus maccoyit 
229 
0.6 
0.5 
0.4 
0.3 
0.2 
0.1 
0 . 0 - 
1992-93 
nd nd nd 
1 1 
Figure 4 
CPUE of southern bluefin tuna, Thunnus maccoyii, (number caught 
per vessel per fishing day) by month for three consecutive spawning 
seasons. From December 1993, additional companies with higher catch 
rates were monitored causing an increase in CPUE from that date. 
Spawning frequency 
The ovaries of 68.7% of females had evidence of re- 
cent or imminent spawning activity. The absence of 
a peak in the percentage of females spawning dur- 
ing the season (Fig. 7) suggests that spawning in- 
tensity was constant. The ovaries of 120 females gave 
evidence of two spawning events, that is, they con- 
tained maturing oocytes (either migratory nucleus 
or hydrated) and postovulatory follicles. The major- 
ity of ovaries contained progressively more developed 
oocytes with progressively older postovulatory fol- 
licles. If postovulatory follicles are resorbed within 
24 hours of spawning, then southern bluefin tuna 
are capable of spawning daily. 
The fraction of females that spawned per day, mea- 
sured by the fraction of females with postovulatory fol- 
licles, was 0.62 (Table 2). This gave a weighted mean 
spawning interval of about 1.6 days. If we examine only 
those females in “prime spawning condition” 
(<10% a atresia), then the spawning frac- 
tion was 0.90 giving a weighted mean inter- 
val of 1. 1 days. This value suggests that once 
females start spawning, they spawn daily. 
Batch fecundity 
The ovaries of nonspawning females col- 
lected from the southeast Indian Ocean 
contained a large number of unyolked oo- 
cytes less than 150 pm in diameter (Fig. 
8). This number was reduced as females 
matured during the season and began 
spawning. Oocytes of all stages were 
present in the ovary of spawning females; 
therefore it appears that southern bluefin 
tuna have an asynchronous pattern of oo- 
cyte development. Unlike species with de- 
terminate annual fecundity, a gap in the 
oocyte size-frequency distribution did not 
appear between unyolked (<200 pm dia- 
meter) and early yolked (200 to 400 pm 
diameter) oocytes (Fig. 8). This finding in- 
dicates that advanced yolked oocytes are 
continually maturing during the spawn- 
ing season, from the pool of unyolked oo- 
cytes, and are spawned. The ovaries of 86% 
of females identified as spawning contained 
no or <10% a atresia, indicating that the 
majority of advanced yolked oocytes that are 
matured are spawned. Residual hydrated 
oocytes were found in the ovaries of only 5.6% 
of females collected from the spawning 
ground, which suggests that all hydrated 
oocytes are released at each spawning. 
The mean relative batch fecundity and 95% Cl’s 
estimated for 20 females of known weight was 56.5 
(±16.1) oocytes per gram of body weight. The rela- 
tion between length (cm) and batch fecundity (BF) 
was best described by the equation 
BF = (4.78242 x 10“ 17 ) x L 7 - 530 
(F= 23.9, df=l,18, P<0.001) (Fig. 9). 
However, the relation was highly variable with only 
57% of the variance explained by the regression. 
Discussion 
Spawning ground and season 
All female southern bluefin tuna sampled on the 
spawning ground were mature, and the smallest was 
