230 
Fishery Bulletin 96(2), 1998 
147 cm long. Davis 2 showed that the mean length at 
which 50% of southern bluefin tuna were mature was 
around 152 cm based on oocyte diameters>400 pm, 
and 162 cm based on GFs>2. Warashina and Hisada 
( 1970) considered that southern bluefin tuna caught 
on the “Oka” grounds (Fig. 1) in the 1960’s reached 
maturity at 130 cm, although reanalysis of these data 
2 Davis, T. L. O. 1995. Size at first maturity of southern blue- 
fin tuna. Council for the conservation of southern bluefin tuna 
scientific meeting; 10-19 July 1995, Shimizu, Japan. Far Seas 
Fisheries Res. Lab., Shimizu, Japan, Rep. CCSBT/95/9, 9 p. 
3000 - 
a 
no atresia 
o 
< 10 % atresia 
• 
1 0 - 50 % atresia 
— 
Gl =2 
■g 2000- 
5 
£■ 
ns 
1000 - 
0 J 
130 
150 
170 
190 
210 
Length (cm) 
Figure 5 
Levels of atresia by length of southern bluefin tuna, Thunnus 
maccoyii, caught off the spawning ground in the northeast In- 
dian Ocean. 
showed that this was the smallest size at which 
they matured and that 50% maturity was not 
reached until 146 cm (Anonymous 3 ). Size at 
maturity appears to have increased progres- 
sively since then, being 154 cm in the period 
1985-89. The increase in length at maturity can 
be attributed, in part, to an increase in the 
growth rate of southern bluefin tuna between 
the 1960’s and 1980’s reported by Hearn, 4 be- 
cause maturity appears to be determined by age 
rather than length. 
The abundance of southern bluefin tuna on 
the spawning ground was not constant through- 
out the spawning season. Catch-per-unit-of-ef- 
fort data indicated that a peak in catches oc- 
curred in October for the 1993-94 season and 
in October and February for the 1994-95 sea- 
son. Japanese CPUE data also indicate there 
were two peaks in abundance on the spawning 
ground in the early years of the fishery; the first 
in September and October and the second in 
February and March (Davis and Farley 5 ). The 
reason for two peaks is unknown but could be 
linked to the widespread distribution of south- 
ern bluefin tuna along the feeding grounds of 
the West Wind Drift. It is possible that the cues 
for migration to the spawning grounds take 
place at different times depending on where the 
fish are prior to spawning. Fish may reach 
spawning condition earlier in some areas than in 
others. Also the time needed to migrate to the 
spawning grounds would differ between areas. 
Spawning duration 
o 
tr 
o 
Cl 
O 
CL 
1 993 1 994 1 995 
Atretic 
stages 
H=4 
a =3 
0=2 
□ =1 
□ =0 
Figure 6 
The proportion of each atretic stage by month recorded in southern bluefin 
tuna, Thunnus maccoyii , caught on the spawning ground. 
This study extends the known du- 
ration of the spawning season of 
southern bluefin tuna. Females were 
caught on the spawning ground in 
3 Anonymous. 1994b. Report of the south- 
ern bluefin tuna trilateral workshop, 17 
January-4 February 1994, CSIRO, Hobart, 
Australia, 172 p. 
4 Hearn, W. S. 1994. Models for estimat- 
ing SBT age at length during the transi- 
tion period. 13th SBT trilaterial scientific 
meeting; 19-29 April 1994, Wellington, 
New Zealand. Ministry of Agriculture 
and Fisheries, Wellington, New Zealand, 
Rep. SBFWS/94913, 16 p. 
5 Davis, T. L. O., and J. H. Farley. 1995. 
Catch monitoring of fresh tuna caught by 
the Bali-based Indonesian/Taiwanese 
longline fishery. Council for the conserva- 
tion of southern bluefin tuna scientific 
meeting; 10-19 July 1995, Shimizu, 
Japan. Far Seas Fisheries Res. Lab., 
Shimizu, Japan, Rep. CCSBT/95/2, 18 p. 
