381 
Size distribution of southern bluefin tuna 
(Thun nus maccoyii ) by depth on 
their spawning ground 
Tim L. O. Davis 
Jessica H. Farley 
CSIRO Division of Marine Research 
PO Box 1538, 
Hobart 
Tasmania 7001, Australia 
E-mail address (for T L. O Davis): tim.davis@manne.csiro.au 
Indonesian and Japanese longline ves- 
sels catch different-size southern blue- 
fin tuna ( Thunnus maccoyii) on their 
spawning ground in the Indian Ocean 
south of Bah. The length distributions 
of southern bluefin tuna (SBT) caught 
by Japanese longline are markedly 
smaller than those caught by the Indo- 
nesians (Davis et al. 1 ; Itoh 2 ). Both 
measurement error and misidentifi- 
cation of small SBT as bigeye tuna 
(Thunnus obesus ) in the Indonesian 
catch data have been suggested as 
causes of this discrepancy (Suzuki 
and Nishida 3 ), but neither has been 
substantiated. Japanese vessels target 
bigeye tuna by using deep longline sets 
(Itoh 2 ), whereas most Indonesian ves- 
sels target yellowfin tuna (Thunnus 
a! bacares) by using shallow longline 
sets (Davis et ah, 1995). The differ- 
ence in types of longline sets raises 
the possibility that SBT on the spawn- 
ing ground are segregated by size with 
depth. 
Three types of boats operate in the 
Indonesian fishery (Davis et ah, 1995). 
Deep longline boats (generally >50 
tonnes) use multifilament mainlines 
that are set deep. Mini (<20 tonnes 
gross weight) and regular longline 
boats (20-50 tonnes) use monofilament 
mainlines and generally make shal- 
low longline sets. However, the depth 
at which the lines fish varies consid- 
erably because they carry live or fro- 
zen baits according to different phases 
of the moon, and both the number of 
hooks and their placement on the cat- 
enary between floats changes. Predic- 
tion of fishing depth based on catenary 
geometry, line length, and distance be- 
tween floats (Yoshihara, 1954) differs 
significantly from actual depth fished 
(Saito, 1973; Nishi, 1990; Boggs, 1992). 
In this fishery, the number of hooks be- 
tween floats is recorded (Davis et ah, 
1999), but this parameter alone is a 
poor indicator of the depth of fishing. 
Using hook timers, Boggs (1992) de- 
termined depth at the time of hook- 
ing. He found that bigeye catch rates 
peaked at 360-400 m and 8-10°C 
(temperature), but were still high at 
200-360 m. Bigeye tuna have a shal- 
lower distribution at night (modal 
depth of 80 m) than during the day 
(220 m) (Holland et ah, 1990). However, 
on the SBT spawning ground, longline 
setting starts at about 06:00 h and 
hauling starts at about 14:00 h (Davis 4 ); 
therefore most bigeye tuna would be 
caught during the day when they are 
deeper. 
The preferred depths of bigeye tuna 
vary regionally depending on thermo- 
cline structure, but lie within 10° and 
15°C (Hanamoto, 1986; Mohn et ah, 
1996) and where 0., > 1 mL/L (Hana- 
moto, 1986). These temperatures occur 
at 180-400 m on the SBT spawning 
ground (Yukinawa and Miyabe, 1984; 
Yukinawa and Koido, 1985; Yukinawa, 
1987). Yellowfin tuna, on the other 
hand, are found in warmer waters and 
are mainly caught at depths of 40-230 
m (Suzuki and Kume, 1982; Yang and 
Gong, 1988; Boggs, 1992). The propor- 
tion of bigeye to yellowfin tuna might 
therefore be used as a proxy for the 
depth of fishing in the Indonesian long- 
line fishery. In our study we used this 
depth proxy to investigate whether 
there is size partitioning by depth of 
SBT on the spawning ground, and what 
underlying biological processes might 
be involved. 
Methods 
We used catch data obtained from 
15,882 Indonesian longline landings 
monitored at export processing facto- 
ries at the Port of Benoa, Bali, from 
1992 to 1999 (Davis et ah, 1995; 1999). 
About 65% of the SBT in these land- 
ings were measured (fork length in 
cm). Fewer high-grade export tuna 
(30%) were measured than low-grade 
tuna (89%) because the former were 
immersed in an ice slurry immediately 
after grading, leaving little opportu- 
nity for measurement. Grading, how- 
ever, was not dependent on size. There 
was no significant difference in the 
length distributions of 102 export tuna 
and 102 low-grade tuna from 20 land- 
ings in which all tuna were measured 
(Kolmogorov-Smirnov two sample test, 
P= 0.22). 
1 Davis, T. L. O., J. H. Farley, and S. Bahar. 
1996. Catch monitoring of the fresh tuna 
caught by the Bali-based longline fishery. 
Commission for the Conservation of South- 
ern Bluefin Tuna scientific meeting, 26 
August-5 September 1996, Hobart, Aus- 
tralia, Rep. CCSBT/SC/96/6, 26 p. CSIRO 
Marine Laboratories, PO Box 1538, Hobart, 
Tasmania 7001, Australia. 
- Itoh, T. 1997. Longline survey in south- 
ern bluefin tuna spawning ground. Com- 
mission for the Conservation of Southern 
Bluefin Tuna scientific meeting, 28 July-8 
August 1997, Canberra, Australia, Rep. 
CCSBT/SC/97/12, 4 p. CSIRO Marine 
Laboratories, PO Box 1538, Hobart, Tas- 
mania 7001, Australia. 
;i Suzuki, Z., and T. Nishida. 1997. Com- 
parison of information on the catch and 
size of fish in the spawning ground of 
southern bluefin obtained from Indone- 
sian and Japanese longline fisheries. Com- 
mission for the Conservation of Southern 
Bluefin Tuna scientific meeting, 28 July-8 
August 1997, Canberra, Australia, Rep. 
CCSBT/SC/97/13, 8 p. CSIRO Marine 
Laboratories, PO Box 1538, Hobart, Tas- 
mania 7001, Australia. 
4 Davis, T. L. O. 1999. Unpubl. data. 
CSIRO Marine Laboratories, PO Box 1538. 
Hobart, Tas 7001, Australia. 
Manuscript accepted 12 September 2000. 
Fish. Bull. 99:381-386 (2001). 
