Syamsuddin et at: Effects of El Nino- Southern Oscillation on catches of Thunnus obesus in the eastern Indian Ocean 
183 
Table 2 
Results from general additive models (GAMs) derived from catch rates of Big- 
eye Tuna (Thunnus obesus) in the eastern Indian Ocean off Java in 1997- 
2000 as a function of the oceanographic parameters (Af=2843 samples). The 
best model was selected on the basis of the significance of predictor terms, 
reduction of Akaike’s information criterion (AIC), and increase in cumulative 
deviance explained (CDE). SST=sea-surface temperature; SSHA=sea-surface- 
height anomaly; chl-a=chlorophyll-a concentrations estimated from SeaWiFS 
level-3 images (http://oceancolor.gsfc.nasa.gov). 
Model 
Variable 
P value 
AIC 
CDE (%) 
SST 
SST 
<2.00 x 
10 -16*** 
2117.04 
6.48 
SSHA 
SSHA 
<2.00 x 
10-16*** 
2153.88 
4.95 
Chl-a 
Chl-a 
<2.00 x 
IQ-16*** 
2219.38 
2.04 
SSHA 
+ 
SSHA 
<2.00 x 
10 -16*** 
2110.55 
7.18 
chl-a 
Chl-a 
<2.00 x 
p)-16*** 
SST 
+ 
SST 
<2.00 x 
IQ-16*** 
2065.54 
9.20 
chl-a 
Chl-a 
<2.00 x 
10-16*** 
SSHA 
+ 
SSHA 
<2.00 x 
10 -16*** 
1973.96 
13.30 
SST 
SST 
<2.00 x 
10-16*** 
SSHA 
+ 
SSHA 
<2.00 x 
10 -16*** 
1912.79 
16.30 
SST 
+ 
SST 
<2.00 x 
10 -16*** 
chl-a 
Chl-a 
<2.00 x 
10 -ie*** 
***indicates statistical significance at the 0.001 level. 
suspected shift in the distribution of Bigeye Tuna 
away from the fishing grounds caused the drop in the 
HR from 0.41 in September 1997 to 0.20 in October 
1997. 
During La Nina in 1999, spatial predictions indi- 
cated Bigeye Tuna catches with lower probabilities of 
20-40% occurring in the offshore area of the western 
part of the EIO off Java. In March 1999, the spatial 
prediction of Bigeye Tuna catch (20-30%) was located 
around 12-16°S, 104-115°E, but actual Bigeye Tuna 
fishing locations were at 13-15° S, 106-107° E; 12— 
15°S, 109-112°E; and 12-13°S, 114-115°E (Fig. 70. 
The spatial prediction of Bigeye Tuna catch (20-40%) 
moved to the west at 11-16° S and 104-1 11°E in April 
1999 (Fig. 7D), but the actual Bigeye Tuna catch areas 
were located at 13-14°S and 11-15°S in the longitude 
range of 108-1 18°E. 
Discussion 
Catch rates of Bigeye Tuna varied over a range of time 
scales and apparently in relation to environmental 
changes. Changes in oceanographic conditions during 
ENSO events resulted in perceivable variations in Big- 
eye Tuna catches, with an average HR of 0.67 during 
the 1997-98 El Nino event. The 1999-2000 La Nina 
event, with an average HR of 0.44, was less favorable 
for catches. 
The spatial patterns of the first and second EOF 
modes for SSHA, SST, and chlorophyll-a concentra- 
tion gave typical negative SSHA, low SST, and high 
chlorophyll-a concentration along the southern coast of 
the Indonesian archipelago, and changes in these pat- 
terns could be exposed by the temporal mode as inter- 
annual variation related to the forcing of the 1997-98 
El Nino event and upwelling evidence. Those typical 
spatial patterns are consistent with the oceanograph- 
ic conditions during September-November 1997 (Fig. 
3, A— C). The first and second modes of chlorophyll-a 
showed the characteristics of an upwelling pattern, 
with chlorophyll-a concentrations higher along the 
southern coast of Java than in other areas. Although 
it normally ends in October, upwelling was observed 
into November during the southeast monsoon in the El 
Nino event in 1997. 
Our results are consistent with the findings of 
Sprintall et al. (1999) and Ffield et al. (2000), who 
reported that the ITF brings colder and warmer wa- 
ters to the Indian Ocean during El Nino and La Nina 
events, respectively, and the local, alongshore winds 
south of Java are favorable for upwelling through De- 
cember. They also confirmed that the upwelling signal 
could account for the reduced downwelling signal from 
the November Kelvin waves. In November 1997, Kelvin 
waves were not generated in the region and this condi- 
tion caused the persistence of colder water along the 
southern coast of Java. 
Chlorophyll-a concentrations contributed the high- 
est energy variance, indicating that chlorophyll-a con- 
centration was the main indicator of the forcing mecha- 
nisms responsible for the 1997-98 El Nino event. Our 
