Fishery Bulletin 105(1) 



(lATTC; IATTC2; Lauth and Olson, 1996; OwenS) and 

 experiments with captured scombrid larvae (from 1986 

 to 1997) at the Achotines Laboratory of the Inter-Ameri- 

 can Tropical Tuna Commission (lATTC) (Olson and 

 Scholey, 1990; Margulies, 1993; Scholey, 1993; Wexler, 

 1993) have provided an opportunity to explore factors 

 controlling prerecruit growth and survival of scom- 

 brids. These small- and fine-scale studies may provide 

 some understanding of the recruitment variability of 

 yellowfin tuna in the Panama Bight, considering that 

 yellowfin tuna exhibit limited, small-scale movements 

 within the EPO (Schaefer, 1991; Wild, 1994) and that 

 processes important to recruitment probably occur at 

 small scales (Fortier and Leggett, 1985). 



The Panama Bight is characterized by distinct sea- 

 sonal and interannual variations in atmospheric and 

 oceanic conditions (Wooster, 1959; Smayda, 1963, 1966; 

 Forsbergh, 1963, 1969). The climatological and physical 

 oceanographic properties that occur within the Pana- 

 ma Bight are determined by the north-south seasonal 

 movement of the northeast trade winds of the Atlantic 

 Ocean, the equatorial calm belt (i.e., the doldrums), 

 the southeast trade winds of the Pacific Ocean, and 

 the convergence of these trade wind systems within 

 the doldrums (i.e., the intertropical convergence zone, 

 ITCZ) (Smayda, 1966). From January through April, 

 the ITCZ is displaced to the south and strong north- 

 erly trade winds create a dry season and produce local 

 upwelling. From about May through December, the 



ITCZ is displaced to the north and the Panama Bight 

 is dominated by southeast trade winds and a rainy 

 season characterized by reduced upwelling, higher sea- 

 surface temperatures (SSTs), lower ocean salinities, 

 and a deeper thermocline and mixed layer (Lauth and 

 Olson, 1996). The growth and subsequent survival of 

 yellowfin tuna larvae that occur during the reduced 

 upwelling season may be regulated more by the spatial 

 patchiness of prey organisms coincident with lower 

 plankton volumes (Owen, 1989). ENSO events could 

 further affect the seasonal availability of nutrients and 

 food organisms during this period (Barber and Chavez, 

 1986; Dessier and Donguy, 1987; Fiedler, 1992; Chavez 

 et al., 1999). A mild ENSO event occurred during our 

 sampling periods in 1991-92 (Barber et al., 1996) and a 

 strong event occurred in late 1997 (Chavez et al., 1999; 

 Strutton and Chavez, 2000; Glynn et al., 2001). 



The objectives of this study were 1) to identify the 

 species of Thunnus sampled in the northwestern Pan- 

 ama Bight by molecular analysis, 2) to determine ages 

 and compare the size-at-age data of yellowfin tuna lar- 

 vae collected during the periods of reduced upwelling of 

 1990, 1991, 1992, and 1997, and 3) to explore relation- 

 ships between the temporal variation in growth rates 

 and measured levels of plankton and physical processes 

 in the Panama Bight. 



Materials and methods 



' lATTC (Inter-American Tropical Tuna Commission). 

 1992. Annual report of the Inter-American Tropical Tuna 

 Commission 1990, 261 p. lATTC, 8604 La Jolla Shores 

 Drive, La Jolla, CA 92037. 



2 lATTC (Inter-American Tropical Tuna Commission). 

 1992. Annual report of the Inter-American Tropical Tuna 

 Commission 1991, 271 p. lATTC, 8604 La Jolla Shores 

 Drive, La Jolla, CA 92037. 



^ Owen, R. W. 1997. Oceanographic atlas of habitats of 

 larval tunas in the Pacific Ocean off the Azuero Peninsula, 

 Panama, 32 p. Inter-American Tropical Tuna Commission 

 Data Report 9. lATTC, 8604 La Jolla Shores Drive, La Jolla, 

 CA 92037. 



Larval fish collections 



Fish larvae were collected in the northwestern Panama 

 Bight (Fig. 1) during the seasons of reduced upwelling 

 in June 1990, July and September 1991, June and July 

 1992, and August 1997 (Table 1). Most of the larvae 

 were collected with a dipnet just below the ocean surface 

 after they were attracted with an underwater light at 

 night (night-lighting, NL) (Olson and Scholey, 1990) near 

 Frailes del Sur in the vicinity of the 100- and 200-meter 

 isobaths. Larvae were also collected in this area in July 

 1991 by a light trap (LT) (design described in Thorrold, 



