Syamsuddin et al Effects of El Nino- Southern Oscillation on catches of Thunnus obesus in the eastern Indian Ocean 
179 
s n = a smoothing function for 
each of the model covari- 
ates x n . 
The constructed GAM could then 
be used to predict the Bigeye Tuna 
catch probability with the predict, 
gam function in the mgcv package 
in R (Wood, 2006). The Bigeye Tuna 
catch probability is the relative prob- 
ability of catching one or more Bigeye 
Tuna at a location given the oceano- 
graphic conditions in the area (Teo 
and Block, 2010). The prediction maps 
were produced with the best model 
selected from a set of 7 models. The 
distribution of the GAM-predictions of 
Bigeye Tuna catch was compared with 
monthly fishery data. With GMT tools, 
we made monthly predictions of Big- 
eye Tuna catch probabilities. 
Results 
Catch rates of Bigeye Tuna 
During the El Nino event in 1997-98, 
catch rates of Bigeye Tuna peaked in 
May-July 1997 (0.87-0.94 HR) and 
were still high in February^July 1998 
(0.69-0.90 HR), followed by a continu- 
ous decline in the later months of 1998 
(Fig. 2A). During the La Nina event in 
1999-2000, catch rates were reduced 
throughout the year (HR <0.67). The 
average HR was higher during the 
1997-98 El Nino event (0.67) than 
during the 1999-2000 La Nina event 
(0.44). 
The number of hooks deployed 
ranged from 14,392 to 178,109 per 
month (Fig. 2B), and more hooks were 
used during the 1997-98 El Nino 
event (147,676 hooks) than during the 
1999-2000 La Nina event (144,283 
hooks). In June 1997, 178, 109 hooks 
(maximum number) were deployed 
and in November 2000, 14,392 hooks 
(minimum number) were deployed. 
The catchability coefficient (q) varied 
with the number of hooks (Fig. 2B). 
Catchability was high in May 1997 
(1.56xl0- 7 ), June 1998 (1.47xl()- 7 ), 
and November 2000 (2.73xl0~ 7 ). 
Catch rates of Bigeye Tuna showed 
seasonal variations and were higher 
during the southeast monsoon (May- 
October; SE) than during the north- 
El Nino (>0.5°C) 
La Nina (>-0.5°C) 
-1 
-2 
Jan97 Jan98 
0.8 
0 6 
0.4 
0.2 
0.0 
I I Southeast (May-Oct) 
□ Northwest (Nov-Apr) 
1997 
1998 
1999 
2000 
Year 
Figure 2 
(A) Variability in catch rates of Bigeye Tuna ( Thunnus obesus) in the east- 
ern Indian Ocean off Java by hook rate (HR) percentage (solid line) and 
SST anomalies from the Nino 3.4 index during 1997-2000 (dashed line). 
El Nino and La Nina events were identified when the average of the Nino 
3.4 index exceeded +0.5°C for El Nino or -0.5°C for La Nina for at least 5 
consecutive months. The El Nino event is indicated by a black bar on the 
top of this graph, and the La Nina event is indicated by a gray bar on the 
top. (B) The total number of hooks deployed (gray bars) and time series 
variation of the catchabillity coefficient (solid line) during 1997-2000. (C) 
Seasonal variations in Bigeye Tuna HR in 1997-2000. The gray bar repre- 
sents the southeast monsoon ( May-October), and the white bar represents 
the northwest monsoon (November-April). 
