YOSHIDA: EARLY LIFE HISTORY OF SKIPJACK TUNA 



of the specimens used to calculate the regres- 

 sions. 



Table 1. — Regressions describing the relations between 

 the standard length and lengths of the vertebral column, 

 precaudal vertebrae, caudal vertebrae, Ist-lOth vertebrae, 

 and 21st-30th vertebrae of juvenile skipjack tuna [I = 

 standard length (cm) , L = length of vertebral fragments 

 (cm)]. 



DISTRIBUTION AND ABUNDANCE 



Seasonal and areal coverage was spotty, but 

 sampling was extensive enough to permit mean- 

 ingful analysis for the present study. In the 

 following sections I will discuss the distribution 

 and the seasonal and annual apparent abundance 

 of juvenile skipjack tuna in the South Pacific 

 and near Hawaii. 



AREAL DISTRIBUTION 



Commercial longline boats engaged in collect- 

 ing billfish stomachs in the South Pacific ranged 

 over a wide area, and juvenile skipjack tuna, as 

 indicated by their presence in billfish stomachs, 

 also were widespread (Figure 1). Around 

 Hawaii fishing was restricted to a relatively 

 small area, and so a more limited picture was 

 obtained of the areal distribution of juvenile 

 skipjack tuna. 



Skipjack tuna larvae are widely distributed 

 in the Pacific Ocean (Matsumoto, 1966; Ueya- 

 nagi, 1969). Ueyanagi (1969) reports that lar- 

 vae were taken across the entire South Pacific 

 between the equator and lat 10° S. Also, west 

 of long 140° W larvae were taken as far south 

 as lat 32° S. My study shows that the distribu- 

 tion of juvenile skipjack tuna is similar to the 

 distribution of the larvae. In the North Pacific, 

 skipjack tuna larvae have been found around 

 Hawaii and across the entire Pacific between the 

 equator and lat 20° N. In the western North 

 Pacific, between long 160° W and the Asian 



continent, larvae have been taken almost as far 

 north as lat 35° N (Ueyanagi, 1969). 



My study indicates that gaps in the distribu- 

 tion of juvenile skipjack tuna in the Pacific re- 

 flect a lack of sampling. Higgins (1967) has a 

 somewhat similar viewpoint. It is likely that 

 the juveniles are as widely distributed as larval 

 skipjack tuna in the North Pacific. 



The distribution of juvenile skipjack tuna in 

 the South Pacific by quarters of the year (all 

 years combined) is shown in Figure 2. This is 

 only the apparent distribution, however, because 

 it reflects the operations of Samoa-based vessels. 

 These vessels primarily seek albacore, and there- 

 fore they fish the areas where albacore catch 

 rates tend to be high. In the first half of the 

 year, vessels generally operate north of lat 20° S, 

 and in June or July they move as far south as 

 lat 30° S before heading north again (Otsu and 

 Sumida, 1968). Samples were available mostly 

 from north of lat 20° S, and juvenile skipjack 

 tuna were found throughout the sampling range. 

 In the third and fourth quarters, samples were 

 available from a wider area, and again juvenile 

 skipjack tuna were taken from almost the entire 

 area sampled. Although seasonal coverage was 

 incomplete throughout the total area, synoptic 

 sampling would probably produce juveniles in 

 all seasons and throughout the total area. 



SEASONAL APPARENT ABUNDANCE 

 Hawaii 



Apparent abundance is expressed here as 

 number of skipjack tuna per 100 billfish stom- 

 achs. The apparent abundance of juveniles 

 around Hawaii, all years combined, is shown in 

 Figure 3. The juveniles were more numerous 

 during July, August, and September. A peak 

 in abundance usually occurred in August. These 

 observations confirm Matsumoto's (1966) con- 

 clusion that skipjack tuna in the Hawaiian 

 Islands spawned during the summer. He showed 

 that the abundance of larval skipjack tuna 

 peaked in July. 



The apparent abundance of juveniles in 1963, 

 1964, and 1965 oflfers interesting contrasts. For 

 example, in August 1963 the apparent abundance 

 peaked sharply to 100 juveniles per 100 bill- 



547 



