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Fishery Bulletin 104(2) 



occur at SST values ranging from 12^ to 30°C, and 

 favorable catches are reported between 17° and 25°C 

 (Lehodey^). More recently, Bautista-Cortes (1997) has 

 studied the relationships between varying vertical tem- 

 perature structure and YFT catches in the Mexican 

 Pacific to 140° longitude W, finding that the depth of the 

 23°C isotherm is related to catch and that the shallower 

 the isotherm, the larger the tuna catches. 



Castro-Ortiz and Quinones-Velasquez (1987) reported 

 reduced catches of YFT during the 1982-83 El Niiio 

 events in the northeastern Pacific, in contrast to the 

 following 1984-85 seasons. The 1982-83 El Nmo event 

 was reported to have caused the largest decrease in the 

 availability-vulnerability index (AVI=CPUE/{1-SSR); 

 where SSR is the successful-set ratio and CPUE is 

 catch per unit of effort; IATTC-). This finding is con- 

 sistent with the resource being less susceptible to fish- 

 ing in the northeastern Pacific during an intense El 

 Nino. Lehodey^ analyzed data of purse-seine and long- 

 line CPUE of skipjack {Katsuwonus pelamis), yellowfin 

 (Thunnus albcares), bigeye (Thuimus obesus), and alba- 

 core (Thunnus alalunga) tunas during El Nino and La 

 Nina events, from 1982 to 1998 for the whole tropical 

 Pacific Ocean (20°S to 20°N). He concluded and sup- 

 ports the hypothesis that ENSO affects the recruitment 

 of skipjack, yellowfin, and bigeye tunas positively in 

 the western Pacific. A direct positive effect related to 

 the vertical change in the thermal structure during El 

 Nino increased purse-seine catches of yellowfin in the 

 western Pacific. He suggested two theoretical cases 

 be considered for yellowfin and bigeye in the eastern 

 Pacific: either a positive La Niiia effect on the recruit- 

 ment and catchability, or conversely a positive El Nifio 

 effect on recruitment and catchability. His statistical 

 analysis did not identify the direct effects of ENSO 

 on catchability for yellowfin and bigeye tunas. He sug- 

 gested the need for additional analyses in the eastern 

 Pacific region and for comparisons with independent 

 results from modeling and observations to confirm the 

 preliminary results. Lu et al. (2001) analyzed catches 

 of YFT and bigeye tuna caught by the longline fleet of 

 the tropical Pacific Ocean. They concluded that high 

 hook rates for both species were mostly associated with 

 regions where SST increased during El Niiio or La Niiia 

 years. During La Niiia episodes, YFT populations ap- 

 pear to undergo a poleward displacement thus shifting 

 both north and south of the equator. During El Nifio 

 events YFT populations in the equatorial Pacific are 

 found where SST anomalies are higher, in the central 

 and eastern equatorial Pacific. 



For the region of interest, Ortega-Garcia et al. (1999) 

 studied the impact of the ENSO during the 1997-98 

 El Nino events in comparison to the 1996-97 non El 

 Niiio year within the eastern Pacific Mexican purse- 

 seine tuna fishery. They reported that during the first 

 months of the ENSO (July-December 1997) of the El 

 Niiio event, the extent of the YFT catches was higher 

 in oceanic waters than the observed catches during a 

 non El Niiio year in these areas for the same period. A 

 decline of effort was observed along the coast of Baja 

 California and inside the Gulf of California. In contrast, 

 during the second part of the ENSO El Niiio 1997-98 

 (January- June 1998), an increase of effort was ob- 

 served along the coasts of Baja California and inside 

 the Gulf of California; however, a decrease of effort was 

 observed in oceanic areas. 



The relative abundance of YFT in the eastern Pa- 

 cific is reported to diminish during El Niiio events; 

 however, as supported by the IATTC° report, the YFT 

 vertical displacement during El Niiio events generates 

 a diminishing fishing effort for the purse-seine fleet in 

 traditional catch areas. As a consequence, this has led 

 to a good recruitment and greater yield-per-recruitment. 

 Large recruitments after El Niiio events are reported 

 to occur in the eastern Paciflc. Joseph and Miller^ ana- 

 lyzed YFT catch data from the eastern Pacific for a 

 22-year period and found positive anomalies in tuna 

 recruitment after El Niiio events. Large recruitments 

 were observed in years 1971, 1974, 1978 and 1985, all 

 of which were preceded by the El Niiio events of 1969, 

 1972, 1976, and 1983, respectively. 



The objective of the present study was to examine 

 the effect of the ENSO El Nifio and La Niiia events for 

 a 10-year period (1990-99) on YFT catches in the en- 

 trance to the Gulf of California, an area of importance 

 for the industry and for the ecology of the resource in 

 the eastern Pacific. 



Materials and methods 



The classification of El Niiio intensities was taken from 

 the Climate Prediction Center (NOAA'*). The process 

 of classification takes into account re-analyzed SSTs 

 produced at the National Centers for Environmental 

 Prediction (NCEP), at the Climate Prediction Center, 

 and at the United Kingdom Meteorological Office. 



Environmental data in the form of monthly means of 

 sea surface temperature anomalies (SSTAs) from 1990 to 

 1999 were extracted from the NCEP monthly SSTA data- 



' Lehodey, P. 2000. Impacts of the El Nino Southern Oscilla- 

 tion on tuna populations and fisheries in the tropical Pacific 

 Ocean. Working Paper. RG-1, 32 p. Oceanic Fisheries 

 Programme, Noumea, New Caledonia, Secretariat of the 

 Pacific Community. http;//www.spc.org.nc/OceanFish/Html/ 

 SCTB/SCTB13/rgl.pdf [accessed on 20 February 2002]. 



- lATTC ( Inter American Tropical Tuna Commission). 1989. 

 Annual Report of the Inter-American Tropical Tuna Com- 

 mission, 1988. Annu. Rep. lATTC, 288 p. httpV/www.iattc. 

 org/PublicationsSPN.htm [accessed on 11 March 2002]. 



3 Joseph, J., and F. R. IVIiller. 1988. El Nino and the surface 

 fishery for tunas in the eastern Pacific. In Proceedings 

 of tuna fish. res. conf., p. 199-207. Japan Fish. Agency- 

 Far Seas Fish. Res. Lab. IVIuguro Gyogyo Kyogikai Gjiroku, 

 Suisancho-Enyo Suisan Kenkyusho. 



•• NOAA (National Oceanic and Atmospheric Administration). 

 2001. Cold and warm episodes by season. Website: http:// 

 www.cpc.ncep.noaa gov/products/analysis_monitoring/enso- 

 stuff/ensoyears.html [accessed on 10 January 2002). 



