Torres-Orozco et al : Variation in catches of Thunnus albacares related to El Nino-Southern Oscillation events 



201 



the entrance to the Gulf of California were calculated 

 between 22°N and 23°N and 103°W to 112°W. These 

 anomalies were extracted from the Reynolds and Smith 

 (1994) monthly 1-degree SSTA climatology. This variable 

 was then compared to the catch of YFT. We did not exam- 

 ine SSTAs south of 21'N because they fall within the 

 Mexican Warm Pool (Trasvina et al., 1999), where persis- 

 tently high SSTs (above 27°C all year in the warm pool 

 region) mask the propagation of warm and cold signals 

 from the Equator. The 10-year time series (1990 to 1999) 

 was "detrended" and filtered to eliminate periods shorter 

 than three months, as described in Godin (1972). 



SSTAs in the NIN03 (Fig. 4A) showed cold and warm 

 conditions associated with five identifiable events. These 

 events were the following: 1) warm El Nifio events in 

 1991-92 (strong warm, SW), 2) in 1994-95 (moderate 

 warm, MW), and 3) in 1997-98 (strong warm, SW); 

 4) moderately cold La Nifia events that took place in 

 1995-96 and 5) in 1998-99. Strong warm events are 

 marked with a black line located at each respective 

 maximum SSTA (Fig. 4). The 1997-98 was the stron- 

 gest warm episode in the 1990-99 decade. 



The 1991-92 and 1997-98 SSTAs in El Nino region 3 

 were higher, by more than 1°C, than the SSTAs in the 

 study area (Fig. 4, A and B). Within the 10-year period, 

 near-normal conditions occurred from 1993 to 1996. The 

 coldest and most persistent event of this decade took 

 place in the study area from July 1998 to December 

 1999, i.e., during a moderate La Nina event. 



Significant interannual variability of YFT catches 

 was observed during the 10-year period in the study 

 area (Fig. 4C). This graph shows the time series of 

 monthly values of YFT catch anomalies related to the 

 average for the 10-year period. Five exceptionally high 

 catch events in years 1992, 1993, 1996, 1998, and 1999 

 were observed. These years account for 70'7f of the total 

 capture during the 10-year period. 



Several qualitative and quantitative features were 

 apparent from these results: 



1 Catch peaks in 1992 and 1998 occurred 3 (r=0.73) 

 and 4 (r=0.64) months, respectively, after the onset of 

 an El Niiio event at the equator (time span between 

 thin and thick lines) (Fig. 4, A and C). 



2 High catches of YFT at the entrance of the Gulf of 

 California in 1992 and 1998 occurred 2 (/—0.71) and 3 

 (r=0.73) months, respectively, after the SST anomaly 

 signal reached the study area (time span between 

 dashed and thin lines) (Fig. 4, B and C). 



3 Higher catches of YFT occurred during the spring 

 following an El Nifio winter. These were observed in 

 the spring of 1992 and 1998 (thin lines; Fig. 4C). 



4 Peaks of YFT catch were also observed in 1993 and 

 1999. These occurred one year after the El Niiio 

 event. The 1993 peak took place in nearly normal 

 SST conditions, whereas the 1999 peak occurred 

 during negative SST anomalies (La Nifia event. Fig. 

 4, B and C). These peaks of YFT catch were higher 

 than the ones recorded in the previous year during 

 the El Nino event 1991-92 and during 1997-98. 



1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 

 Year 



Figure 5 



Annual catches (in tons) of yellowfin tuna (Thunnus 

 albacares) in latitudinal bands within the area of study 

 for the period 1990-99. 



5 The 1996 peak seemed not to be related to the vari- 

 ability of SST anomalies alone. 



6 There was a delay from one to two months between 

 the mature phase of the El Niiio event at the equator 

 and the presence of high SSTAs in the study area 

 (time span between thick and dashed lines in Fig. 

 4, A and B). 



Annual YFT catches of stratified latitudinal bands 

 within the study area are shown in Figure 5. A progres- 

 sive increase in YFT catches from southern to northern 

 latitudes was observed in years 1993 and 1999. This 

 progressive increase may be indicative of a northward 

 movement of the resource. This phenomenon has been 

 observed in the western tropical Pacific Ocean during 

 La Nifia events where YFT have dominated the longline 

 catch (Lu et al., 2001). 



Discussion 



Catch affected by the onset El Nino 



Cross-correlation between the onset of El Nifio at the 

 equator in 1991-92 and 1997-98 with catch peaks of 

 YFT in the study area showed a delay of three and 

 four months respectively. Catch peaks of YFT at the 

 entrance of the Gulf of California in 1992 and 1998 

 occurred two to three months after the SSTA signal 

 reached the study area. These results are similar to 

 those found by Lehodey' in the western and central 

 regions of the Pacific Ocean, where rising (deepen- 

 ing) of the mixed-layer depth related to El Nifio (La 

 Nifia) was associated with an increase in the pole-and- 

 line and purse-seine CPUE of YFT and where there 



