Fishery Bulletin 105(1) 



(2001) found another restriction enzyme {Tsp 5091) that 

 was diagnostic for bigeye tuna regardless of where the 

 specimens came from. Therefore, in addition to using 

 Mse I digestion, Tsp 5091 was also used for all individu- 

 als collected in 1997. 



Back-calculated dates 



Spawning dates were back-calculated for each larva by 

 subtracting the number of otolith increments counted 

 from the date the larva was collected. An additional 

 day was also subtracted because the first increment 

 in yellowfin tuna is present at hatching approximately 

 20 hours after fertilization (senior author, personal 

 commun.) and the second increment does not form 

 until the third day after fertilization (approximately 

 two days after hatching); increments are formed daily 

 thereafter (Wexler et al., 2001). During the reduced 

 upwelling season when SSTs are warmer, first feed- 

 ing of the larvae occurs at first light, approximately 

 three days after hatching (Margulies et al., in press) 

 when, on average, three increments are present in the 

 sagittae. Therefore, three days were subtracted from 

 the estimated spawning date to estimate the time 

 period that the larvae of each collection group were 

 feeding until they were collected (Table 2). "Collec- 

 tion-group period" is defined as the time period from 

 the time of first spawning to the time when larvae 

 were sampled. 



Estimated area occupied by larval cohorts 



The yellowfin tuna larvae that were collected near the 

 Frailes Islands may be recruited locally from offshore 



areas; this conjecture is based on measurements of the 

 mean monthly fields of velocity and direction of the 

 North Equatorial Countercurrent (NECC) (up to 0,25 

 m/s) (Fiedler, 2002), southerly surface winds (up to 5 

 m/s) (Fiedler, 2002), and the location and proportion of 

 reproductively active female yellowfin tuna (Schaefer, 

 1998) that are found during June-September in the 

 Panama Bight area (Fig. 2). The earliest back-calcu- 

 lated spawning date for a larva within each collection 

 group was used to estimate the maximum amount of 

 time the larvae within that group were exposed to 

 environmental and feeding conditions (Table 2). This 

 amount of time and the maximum current speed and 

 direction during this season were used to calculate 

 maximum average distances traveled and the potential 

 area occupied by each collection group until sampled at 

 the Frailes Islands (Table 2, Fig, 2). 



Ichthyoplankton and oceanographic surveys 



During 1990-92 ichthyoplankton and oceanographic 

 sampling were conducted from a 25-ft Boston whaler 

 along the Morro Puercos (P) and Punta Mala (M) tran- 

 sects (Fig. 1) (lATTCi; IATTC';Lauth and Olson, 1996). 

 Data collected from these surveys were used to describe 

 the temporal variation of conditions within the plank- 

 tonic community that may correspond to that of larval 

 yellowfin tuna growth rates. In 1990, oblique bongo 

 tows were made from the surface to 50 m along both 

 transects with 335-f(m mesh nets (Lauth and Olson, 

 1996). As a measure of relative abundance, standard- 

 ized plankton volumes under 10 m- of sea surface were 

 calculated by following procedures of Smith and Rich- 

 ardson (1977), and the estimates for each side of the 



