197 



Abstract — Fishery catch data on yel- 

 lowfin tuna {Thunnus albacares) were 

 examined to study the effects of El 

 Nino events between 1990 and 1999 

 for an area in the northeastern tropi- 

 cal Pacific (18-24°N, 112-104'W). The 

 data were extracted from a database 

 of logbook records from the Mexican 

 tuna purse-seine fleet. Latitudinal 

 distribution of the catches increased 

 from south to north for the 10-year 

 period. Highest catches and effort 

 were concentrated between 22°N 

 and 23°N. This area accumulated 

 48% of the total catch over the 10- 

 year period. It was strongly correlated 

 with El Nino-Southern Oscillation 

 (ENSO) events. At least two periods 

 of exceptionally high catches occurred 

 following El Niiio events in 1991 and 

 1997. Peaks of catches were triggered 

 by the arrival of positive anomalies of 

 sea surface temperature (SST) to the 

 area. A delay of two to four months 

 was observed between the occur- 

 rence of maximum SST anomalies 

 at the equator and peaks of catch. 

 Prior to these two events, negative 

 SST anomalies were the dominant 

 feature in the study area and catch 

 was extremely low. This trend of nega- 

 tive SST anomalies with low catches 

 followed by positive SST anomalies 

 and high catches may be attributed 

 to northward yellowfin tuna migra- 

 tion patterns driven by El Nifio forc- 

 ing, a result that contrasts with the 

 known behavior of decreasing relative 

 abundance of these tuna after El Nino 

 events in the eastern Pacific. How- 

 ever, this decrease in relative abun- 

 dance may be the result of a local or 

 subregional effect. 



Variation in yellowfin tuna iThunnus albacares) 

 catches related to El Niiio-Southern Oscillation 

 events at the entrance to the Gulf of California 



Ernesto Torres-Orozco 



Universidad de Colima, Facultad de Ciencias Marinas 

 Km 20, Carretera Manzanillo-Barra de Navidad 

 C P 28860 

 Manzanillo, Colima, Mexico 



Arturo Muhlia-Melo 



Centre de Investigaciones Biologicas del Noroeste, S.C. 



Mar Bermeio No. 195 



Col Playa Palo de Santa Rita 



Apdo, Postal 128 



La Paz, BCS 23090, Mexico 



Email address (for A Mulilia Melo, contact author) amuhlia04(a'cibnormx 



Armando Trasvina 



Oceanogralia Fisica, CICESE en BCS 

 Miraflores 334 e/Mulege y La Paz 

 Fracc. Bella Vista 

 La Paz, BCS 23050, Mexico 



Sofia Ortega-Garcia 



Centre Interdiscipiinarie de Ciencias Marinas 

 La Paz, BCS, Mexico, 23000, Becano COFAA 



Manuscript submitted 3 December 2003 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 9 August 2005 by the Scientific Editor. 

 Fish. Bull. 104:197-203 (2006). 



The entrance to the Gulf of Califor- 

 nia (from 18°N to 24°N and 104°W to 

 112°W) is located in the convergence 

 zone of the North Pacific Gyre, where 

 the California Current separates from 

 the coast to feed the North Equato- 

 rial Current. It has a complex hydro- 

 graphic structure due to the confluence 

 of different water masses (Roden and 

 Groves, 1959; Roden, 1972; Alva- 

 rez-Borrego and Schwartzlose, 1979; 

 Bray and Robles, 1991; Torres-Orozco, 

 1993 ). This region is highly responsive 

 to the El Niiio phenomenon. Its main 

 response is characterized by positive 

 sea level anomalies, warming of the 

 upper layer, and a general alteration 

 of water current patterns (Baumgart- 

 ner and Christensen, 1985; Robles and 

 Marinone, 1987; Torres-Orozco, 1993; 

 Lavin et al., 1997; Ortega-Garcia et 

 al, 1999; Trasvifia et al, 1999; Castro 

 et al., 2000; Bernal et al., 2001). 



High abundance of yellowfin tuna 

 (Thunnus albacares; YFT) is reported 

 in the study area (Allen and Punsly, 

 1984; Castro-Ortiz and Quiiiones- 



Velasquez, 1987; Muhlia-Melo, 1993; 

 Ortega-Garcia, 1998). However, stud- 

 ies about the YFT interaction with the 

 physical environment of the Mexican 

 Pacific are lacking. Blackburn (1965, 

 1969) considers that the abundance of 

 the YFT correlates with sea surface 

 temperature (SST) in the range of 

 20°C to 30°C, but it can also be pres- 

 ent in regions having temperatures 

 between 18°C and 31°C. Blackburn 

 (1969) considered 30°C as an optimal 

 estimate of the maximum tempera- 

 ture of occurrence of YFT. Similarly, 

 Ortega-Garcia (1998) reported that 

 YFT are distributed in regions where 

 SST ranges from 17°C to 31°C and 

 that they are frequently observed in 

 waters close to 28°C. Other studies 

 such as that of Laevastu and Rosa 

 (1963) and Castro-Ortiz and Quirio- 

 nes-Velasquez (1987) have indicated 

 that YFT are found at concentrations 

 favorable to commercial fisheries in 

 regions where the SST ranges from 

 20°C to 28°C. In summary, YFT 

 catches are historically reported to 



