THE EARLY LIFE HISTORY OF SKIPJACK TUNA, Katsuwonus pelamis, 



IN THE PACIFIC OCEAN 



Howard 0. YosHroA' 



ABSTRACT 



This study investigates the early life history of skipjack tuna, including the distribution, abundance, age, 

 and growth. The study is based on 1,742 juvenile skipjack tuna that were found in the stomachs of 

 6,867 billfishes caught in Hawaiian waters and in the South Pacific by commercial longline boats. The 

 smallest juvenile taken near Hawaii was 5.9 cm in standard length, and in the South Pacific 1.6 cm 

 in standard length. Regressions describing the relations between the standard length and lengths of 

 (1) the vertebral column, (2) the precaudal vertebrae, (3) caudal vertebrae, (4) the Ist-lOth verte- 

 brae, and (5) the 21st-30th vertebrae of juvenile skipjack tuna were determined. The regressions pro- 

 vided estimates of the standard length of fragmentary specimens. Juvenile skipjack tuna were widely 

 distributed between lat 5° and 32° S, and long 137° W and the 180th meridian. North of the equator, 

 the commercial longline boats fished close to the main Hawaiian Islands, and thus only a limited pic- 

 ture was obtained of the areal distribution of juvenile skipjack tuna. Juvenile skipjack tuna were 

 found in almost all months in Hawaiian waters. They were most numerous in July and August. In 

 the South Pacific, juveniles were also found in almost all months between lat 5° and 20° S. Peaks 

 in the apparent abundance were evident in April and October in the area north of lat 10° S. Juvenile 

 skipjack tuna appeared to be more numerous in the South Pacific than around Hawaii. 



Length-frequency distributions of juvenile skipjack tuna from Hawaii showed well-defined modes, 

 which progressed with time. The growth of the juveniles was estimated by using the modal lengths 

 determined from the monthly length-frequency distributions. Skipjack tuna between 9 and 40 cm 

 around Hawaii are estimated to grow 2.0 cm per month. One-year-old fish are estimated to be 31 

 cm in standard length. 



In 1968, 70,746 metric tons of skipjack tuna, 

 Katsmcotnis pelamis, were landed in the eastern 

 Pacific ( Inter- American Tropical Tuna Commis- 

 sion, 1970) and 109,018 metric tons were landed 

 in Japan (Japan. Fisheries Agency, Research 

 Division, 1970). Because of its commercial im- 

 portance much knowledge has been accumulated 

 on the biology of the skipjack tuna. Information 

 on early life history, however, is incomplete. 

 Matsumoto (1958) described skipjack tuna lar- 

 vae and their temporal and spatial distribution 

 in the central Pacific. Ueyanagi (1969) report- 

 ed on the distribution of larval skipjack tuna 

 in the Pacific Ocean between 1960 and 1967, 

 and Higgins (1967) summarized the distribu- 

 tional records of juvenile skipjack tuna in the 

 Pacific. 



' National Marine Fisheries Service, Hawaii Area 

 Fishery Research Center, Honolulu, Hawaii 96812. 



The present study is based on immature skip- 

 jack tuna between 1.6 and 40 cm taken from the 

 stomachs of billfishes near Hawaii and in the 

 South Pacific. Included are observations on geo- 

 graphical and seasonal distribution, length-fre- 

 quency distributions, and age and growth rates. 



MATERIALS AND METHODS 



The stomachs of 6,867 billfishes" were exam- 

 ined in this study. Those examined included 

 4,118 striped marlin, Tetrapturus audax; 1,606 

 blue marlin, Makaira nigricans; 383 shortbill 

 spearfish, T. angustirostri^; 216 sailfish, Istio- 

 phoms platypterus; 196 swordfish, Xiphias 

 gladius; 171 black marlin, M. indica; and 177 

 billfishes that were not identified to species. 



Manuscript accepted February 1971. 



FISHERY BULLETIN: VOL. 69, NO. 3, 1971. 



' The term billfishes as used in this paper includes 

 swordfish. 



545 



