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Fishery Bulletin 96(2), 1 998 
trachurus). Atresia can also be caused by starvation 
as Scott (1962) found in maturing rainbow trout 
( Salmo gairdneri). It is not known if prespawning 
southern bluefin tuna are resorbing developing oo- 
cytes to gain the energy required for migration to 
the spawning grounds or because the ovary can only 
accommodate a certain volume of oocytes. 
Many ovaries from nonspawning southern bluefin 
tuna ovaries collected from the spawning ground con- 
tained 10-50% atresia of their advanced yolked oo- 
cytes. The precise reproductive stage of these females 
is unclear. The similarity in atretic levels between 
these females and the prespawning females from the 
southern oceans, and in their high mean GI values, 
suggests that these females may have only just ar- 
rived on the spawning ground or were in the early 
stages of their spawning cycle. If this is the case, the 
relatively large numbers of nonspawning females 
(30% of females sampled) indicates that southern 
bluefin tuna may delay the onset of spawning possi- 
bly to recover from the energetic costs of migration. 
Their presence on the spawning ground throughout 
the spawning season suggests that there is a con- 
tinual supply of new spawners onto the ground. Al- 
ternatively, southern bluefin tuna may not spawn 
continuously while on the spawning ground, but in 
pulses. The nonspawning females may be experienc- 
ing a lull in spawning activity between spawning 
episodes. The presence of many nonatretic yolked 
oocytes in their ovaries suggests that they could re- 
commence spawning in the current season. Lunar 
spawning cycles have been documented in many 
tropical spawning fishes (see reviews by Johannes, 
1978; Taylor, 1984; Robertson et al., 1990), however, 
we detected no evidence of a lunar cycle. 
Spawning in southern bluefin tuna is not synchro- 
nized for the stock as a whole. There are several lines 
of evidence to support this statement: the presence 
of prespawning females both on and off the spawn- 
ing ground throughout the spawning season; the 
absence of a peak in GI during the spawning season; 
the constant level of spawning intensity (percentage 
of females spawning) during the spawning season; 
the absence of any increase in the incidence of oo- 
cyte atresia towards the end of the spawning season; 
and the presence of postspawning females off the 
spawning ground both early and late in the spawn- 
ing season. Thus it appears that there is a turnover 
of new spawners replacing old spawners on the 
spawning ground throughout the season. A similar 
turnover of pre- and postspawning southern bluefin 
tuna has been reported on the “Oki” fishing ground 
(Fig. 1) south of the spawning ground (Kikawa, 
1964b). Nonsynchronized spawning has been found 
in other multiple spawning species such as skipjack 
tuna (Cayre and Farrugio, 1986), chub mackerel, 
Scomber japonicus (Dickerson et al., 1992), and At- 
lantic croaker, Micropogonias undulatus (Barbieri et 
al., 1994). Cayre and Farrugio (1986) reported that 
spawning in skipjack tuna in the Atlantic is synchro- 
nized within schools. Individuals in a school can 
mature rapidly and spawn batches of oocytes simul- 
taneously when conditions become favourable. It is 
unclear if southern bluefin tuna can do this, or to 
what extent the long spawning season is the result 
of individual fish or schools arriving on the spawn- 
ing ground and maturing at different times. It is also 
unclear if the spawning period is constant for all in- 
dividuals. In many species, including jack mackerel, 
Trachurus symmetricus, and chub mackerel, Scomber 
japonicus , older spawners are reported to have a 
longer spawning period than younger spawners 
(Knaggs and Parrish, 1973; MacCall et al., 1980). 
The low number of postspawning female southern 
bluefin tuna (3) found on the spawning ground sug- 
gests that as soon as individuals have finished spawn- 
ing they quickly move off the ground. The reasons 
for this departure are uncertain, but it may be due 
to decreased food availability through increased com- 
petition because many fish are gathering to spawn 
in a relatively small area, or to an inability to with- 
stand the warmer water temperatures on the spawn- 
ing ground for extended periods of time. Adult blue- 
fin tuna are unique among the tunas because they 
live predominantly in cold water (as low as 5°C) and 
only move into warmer waters to spawn (Olson, 
1980). Their ability to maintain their body tempera- 
ture above ambient water temperature, through the 
development of an increased lateral blood supply and 
heat exchangers, has enabled them to occupy higher 
latitudes than many tuna species can tolerate. This 
adaptation to cold water, however, may preclude them 
from extended stays in warm water, resulting in a rapid 
migration off the spawning ground after spawning. 
After spawning, southern bluefin tuna migrate 
south from the spawning ground into the West Wind 
Drift (Mimura, 1962) to feed and gain condition over 
the southern winter months. The minimum time for 
an individual to travel to the Southern Ocean how- 
ever, is unknown. The maximum sustained swimming 
speeds of small yellowfin and skipjack tunas are pre- 
dicted to be between 2 and 4 body lengths/s (Brill, 
1996), and these values are thought to be similar in 
other active fish species. Southern bluefin tuna are 
unlikely to travel at their maximum sustained swim- 
ming speed in a straight line from the spawning 
grounds to the southern oceans because they will be 
feeding on the way south. If a 180-cm fish travelled 
at between 1 and 2 body lengths/s, from the spawn- 
ing ground to Tasmania (6,000 km), it would take 
