FISHERY BULLETIN: VOL. 69, NO. 3 



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SOUTH PACIFIC 



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tUTh^ rv^-EmiTV 



"0 10 20 30 40 



STANDARD LENGTH {CM ) 



Figure 5. — Length-frequency distribution of juvenile 

 skipjack tuna. 



length) . A prominent mode was located at the 

 14-14.9 cm size class, a lesser mode at 25-25.9 cm, 

 and a relatively obscure mode was evident be- 

 tween 30 and 40 cm. The length frequencies 

 of juvenile skipjack tuna from the South Pacific 

 were similar to those from Hawaii. The smallest 

 juvenile taken in the South Pacific was 1.6 cm 

 SL. A prominent mode was evident at the 12- 

 12.9 cm length class, a lesser mode between 20 

 and 30 cm, and a relatively obscure mode was 

 apparent between 30 and 40 cm. 



The monthly length-frequency distributions of 

 the juveniles near Hawaii show well-defined 

 modes in the summer and fall ( Figure 6) . Some 

 of the length distributions have two or more 

 well-defined modes. These modes probably rep- 

 resent progeny from two or more sj^awnings 

 spaced more or less closely in time. An increase 

 in size of the juveniles, as indicated by a pro- 

 gression of the modes, was evident in the 

 monthly length distributions. In contrast, Hig- 

 gins (1970) found that modal groups of juvenile 



skipjack tuna taken in midwater trawls did not 

 show an increase in length with time. 



Because the modal lengths increased with 

 time, an attempt was made to describe the 

 growth of the juveniles. To facilitate the des- 

 ignation of modes, the length data were smoothed 

 by a moving average of three. Modes were not 

 considered in samples that had fewer than 10 

 fish, nor in length groups containing fewer than 

 5 fish. The selected modes are given in Figure 7 

 and Table 3. Using the method of least squares, 

 straight lines were fitted to groupings of modal 

 points to represent the apparent growth of the 

 juveniles. The slope of the lines for 1962, 1964- 

 65, and 1965-66 indicates that juvenile skipjack 

 tuna grow about 2.0 cm per month. The slope 

 of the line for 1963-64 indicates a growth of 1.2 

 cm ]3er month. These data suggest annual dif- 

 ferences in the growth of the juveniles. I be- 

 lieve that an increment of 2 cm per month is the 

 best estimate of the growth rate of juvenile 

 skipjack tuna within the length range covered 

 in this study. Rothschild (1967) used data from 

 tag and recapture experiments and the von Bert- 



Table 3. — Modes in the length-frequency distributions 

 of juvenile skipjack tuna from Hawaii (from Figure 6). 



Year Month Standard length (cm) 



1964 



April 



32.5 



550 



