Farley and Davis: Reproductive dynamics of Thunnus maccoyii 
233 
al., 1986). The mean spawning interval of reproduc- 
tively active yellowfin tuna (those whose ovaries con- 
tain advanced yolked oocytes and may or may not 
contain postovulatory follicles and contain no or less 
than 50% a atresia) was 1.14 days (Schaefer, 1996). 
Histological sections indicated that slight variations 
in the lengths of individual spawning intervals exist in 
southern bluefin tuna, which may be normal or the 
result of external stresses such as decreased food avail- 
ability or the stress of capture (Hunter et al., 1986). 
Our estimate of mean relative batch fecundity for 
southern bluefin tuna (57 oocytes per gram of body 
weight) is similar to that found in yellowfin tuna (68 
oocytes per gram of body weight) ( Schaefer, 1996) but 
less than that for black skipjack tuna (81 to 153 oo- 
cytes per gram of body weight) (Schaefer, 1987) and 
skipjack tuna (40 to 130 oocytes per gram of body 
weight) (Stequert and Ramcharrum, 1995). Hunter 
et al. (1985) reported that the minimum number of 
females needed for a reliable batch fecundity esti- 
mate in northern anchovy was 50. We, however, found 
only 21 southern bluefin tuna ovaries with unovu- 
lated hydrated oocytes from the 475 ovaries collected 
from the spawning ground. There could be several 
reasons for this. Oocytes in the hydrated stage may 
be very short-lived in southern bluefin tuna, which 
would reduce the chance of sampling females with 
hydrated ovaries. Spawning may occur during a spe- 
cific time of the day, and if sampling was not con- 
ducted just prior to this, fewer females with hydrated 
ovaries would be collected. Spawning in many tuna 
species is known to occur in the late evening or early 
morning (Hunter, et al., 1986; Schaefer, 1987; 
McPherson, 1991; Nikaido et al., 1991; Schaefer, 
1996). Because our ovaries were sampled from fe- 
males caught on Indonesian-based longlining vessels, 
which generally operate during daylight hours 
(Ishida et al., 1994), the chances of catching females 
that were about to spawn would be reduced. Further, 
if spontaneous spawning occurred while southern 
bluefin tuna were on the longlines, then fewer fe- 
males with unovulated hydrated oocytes would be 
sampled. Spontaneous spawning has been observed 
in skipjack tuna soon after capture ( Kaya et al., 1982). 
Batch fecundity in southern bluefin tuna increased 
with body length. The variation in estimates for fe- 
males of similar size may be normal because females 
were collected in different years and at different 
times in their spawning cycles. Hunter et al. (1985) 
suggested that the relation between batch fecundity 
and fish weight should be estimated annually in 
northern anchovy because batch fecundity can vary 
by a factor of 2 between years. Batch fecundity is 
also known to vary significantly during the spawn- 
ing season in many fish species (Conover, 1985). A 
decrease in batch fecundity during the spawning sea- 
son was found in Atlantic mackerel, Scomber 
scombrus (Watson et al., 1992), as individuals moved 
northwards. Batch fecundity can also peak during 
the middle of the spawning season if conditions are 
suitable, or it can remain constant if conditions are 
unpredictable (Conover, 1985). 
Spawning strategies 
We found that many southern bluefin tuna ovaries, 
collected both on and off the spawning ground, con- 
tained moderate levels of a atresia (10-50% of ad- 
vanced yolked oocytes). High levels of a atresia are 
thought to indicate a decline in the spawning rate 
(Hunter and Macewicz, 1985a) and would normally 
occur towards the end of an individual’s spawning 
season. Hunter and Macewicz ( 1980) classified north- 
ern anchovy as early postspawning if their ovaries 
contained less than 50% of advanced yolked oocytes 
in an a atretic state and postspawning if their ova- 
ries contained more that 50% a atresia. Because we 
found that ovarian atresia increased with gonad in- 
dex in the ovaries of prespawning females from the 
Southern Oceans, it appears that increased a atre- 
sia will not always mark the cessation of spawning 
in southern bluefin tuna. This ovarian atresia found 
in prespawning females could be a normal hormonal 
process that occurs during ovary maturation, as sug- 
gested by Macer ( 1974) for horse mackerel ( Traehurus 
