Fishery Bulletin 105(1) 



period of time when each collection group of larvae could 

 have been feeding in the estimated area of occurrence 

 (Table 2, Fig. 2). Ichthyoplankton tows were not made 

 in the Panama Bight during 1997. 



Mean standardized plankton volumes were variable 

 and significantly different (ANOVA, P<0.001) among 

 collection-group periods (Fig. 7). The mean plankton 

 volume (±SE) in 1990 (157.3 ±13.53 mL) when the fast- 

 est larval growth rate occurred was greater and differ- 

 ent from all other sampling or collection periods (SNK 

 test), and ranged from 106.5 to 310.4 mL under 10 m- 

 of sea surface (Table 3, Fig. 7). The mean plankton 

 volumes during June-July 1991 (82.3 ±3.46 mL) and 

 during August-September 1991 (62.8 ±5.86 mL), when 

 the slowest growth rate occurred, were similar and less 

 than those for all other periods (SNK test); volumes 

 ranged from 43.7 to 102.4 mL under 10 m- of sea sur- 

 face (Table 3, Fig. 7). 



Mean plankton volumes, expressed as the amount fil- 

 tered per volume of water sampled within the first three 

 depth strata, were also significantly different (ANOVA, 

 P<0.001) among collection group periods. Means were 

 similar between the 1990 and 1992 groups and be- 

 tween the two 1991 groups (SNK test). Volumes ranged 

 from 0.199 to 0.559 mL/m^ during 1990 and 1992 and 

 from 0.075 to 0.235 mL/m'^ during the two 1991 periods 

 (Table 3). 



Plankton volumes included relatively large numbers of 

 fish larvae (predominantly preflexion stages) during the 

 least (August-September 1991) and most (June 1990) 



o 

 O 



rapid growth periods, and mean values were not signifi- 

 cantly different (^test for unequal variances, P>0.20). 

 Numbers of larvae under 10 m^ of sea surface ranged 

 from 686.8 to 4786.1 and from 934.5 to 2685.6 in 1990 

 and 1991, respectively. Few scombrid larvae occurred in 

 the ichthyoplankton samples for each of the two years, 

 but were greatest during the 1991 period. The num- 

 ber of scombrid larvae under 10 m'^ ranged from to 

 2.7 and from to 12.7 in 1990 and 1991, respectively. 

 Thunnus larvae (preflexion stage) were collected only in 

 August 1991, and the numbers ranged from 0.5 to 5.5 

 larvae under 10 m- of sea surface. 



Environmental effects 



Mean SSTs were significantly different among all col- 

 lection group periods (ANOVA, P<0.0001). SSTs were 

 similar between the local sampling area (Fig. 1) and the 

 estimated region of each collection group (Fig. 2) in that 

 they were significantly lower for the August-September 

 1991 period (group III) and higher for the July-August 

 1997 period (group V) (ANOVA, P<0.0001, SNK test; 

 Table 2, Fig. 7). 



The mean wind stress was significantly lower for 

 the 1997 collection-group period (group V) when com- 

 pared with all other group periods (ANOVA, P<0.0001, 

 SNK test; Fig. 7). The monthly means within each 

 1- by 1.5-degree area were similar and ranged from 

 0.084 to 0.517 dynes/cm- for group-collection peri- 

 ods I-IV, and for the 1997 period (group V), they 

 ranged from 0.027 to 0.462 

 dynes/cm-. Wind velocities cal- 

 culated from the wind stress 

 values were low to moderate, 

 ranging from 1.59 to 3.94 m/s 

 (modes of 2.45 and 2.60 m/s) 

 for groups I-IV and from 0.90 

 to 3.72 m/s (modes of 1.2 and 

 1.7 m/s) for group V. 



Higher salinity values, rang- 

 ing from 33 to 34 psu, occurred 

 during July-August 1997 

 (group V) during an ENSO 

 event, and during the 1990-92 

 collection-group periods (I-IV) 

 values ranged from 29 to 32 

 psu, when both the fastest and 

 slowest growth rates of yellow- 

 fin tuna larvae occurred. 



6 7 9 11 13 15 17 19 21 



Age (d) 



Figure 5 



Exponential relationships between sagittal otolith diameter and estimated age 

 in days oflarval yellowfin tuna {Thunnus albacares) collected during June 1990, 

 September 1991, and August 1997. Data are also presented for yellowfin tuna 

 larvae collected during July 1991 and June-July 1992, but growth models were 

 not fitted to the limited range of data. 



Discussion 



This study describes the first 

 in situ growth rates for yel- 

 lowfin tuna larvae occurring 

 in the Pacific Ocean. Previ- 

 ous efforts to age and describe 

 growth of yellowfin tuna during 

 the early stages of develop- 



