DISTRIBUTION AND ECOLOGY OF SKIPJACK TUNA, 



KATSUWONUS PEL AMIS, IN AN OFFSHORE AREA OF THE 



EASTERN TROPICAL PACIFIC OCEAN 



Maurice Blackburn' and Francis Williams^ 



ABSTRACT 



Distributions of skipjacit tuna, Katsuwonus pelamis, were studied in the offshore eastern tropical 

 Pacific between lat. 15°N and 5°S, long. 115° and 125°W, during two cruises in 1970 and 1971. Another 

 cruise was made there with different methods in 1969. All cruises were between October and April. 

 Various environmental properties were measured. 



Catches of skipjack included fish smaller and larger than those generally taken near the American 

 coast. This is consistent with previous hypotheses that mature adults and their larvae generally occur 

 far offshore, whereas adolescents are generally coastal, in the eastern Pacific. The juveniles arriving 

 near the coast and the older fish leaving it evidently cross the studied area on migrations from and to 

 the spawning regions. 



In 1970 and 1971 skipjack > 45 cm were most abundant in the equatorial upwelling and at the 

 northern boundary of the North Equatorial Countercurrent, and scarce in the Countercurrent. 

 Correlation coefllicients between skipjack > 45 cm and skipjack forage in 1970 were positive and 

 significant by the usual criteria, but the significance may in part be disputable because many other 

 correlations involving skipjack were nonsignificant. The apparent significance was lost when juvenile 

 skipjack ( < 45 cm) were included with the larger ones. Juveniles may have different relations to 

 environment. The 1971 data were scanty and yielded no significant correlations between skipjack and 

 forage. 



On the 1969 cruise forage was not studied. Skipjack were abundant in the Countercurrent, but at a 

 prespawning stage, whereas postspawners predominated on the other cruises. Other studies suggest 

 that skipjack larvae require relatively high temperatures, which occurred only in the Countercurrent on 

 the 1969 cruise. Skipjack may be distributed according to the environmental requirements of their 

 larvae when spawning and according to their own feeding requirements when not spawning. 



Williams (1971) described plans for a series of 

 cruises in two offshore areas of the eastern 

 tropical Pacific Ocean. This report deals with 

 results of two cruises made in 1970 and 1971 in one 

 of the areas, bounded by lat. 15°N-5°S and long. 

 115°-125°W (Figure 1). The cruises were initiated 

 by the National Marine Fisheries Service, South- 

 west Fisheries Center, and the Scripps Tuna 

 Oceanography Research (STOR) Program, Insti- 

 tute of Marine Resources, University of Califor- 

 nia. They were designed to investigate on a 

 seasonal basis the occurrence and relative abun- 

 dance of skipjack tuna, Katsuwonus pelamis, in 

 relation to environmental conditions. Coverage of 



'Scripps Institution of Oceanography, Institute of Marine 

 Resources, University of California, San Diego, P.O. Box 1529, La 

 Jolla, CA 92037. 



^Scripps Institution of Oceanography, Institute of Marine 

 Resources, University of California, San Diego, P.O. Box 1529, La 

 Jolla, CA 92037; present address: Rosenstiel School of Marine and 

 Atmospheric Science, University of Miami, 10 Rickenbacker 

 Causeway, Miami, FL 33149. 



Figure l.-Area of eastern tropical Pacific Ocean under inves- 

 tigation. 



Manuscript accepted June 1974. 



FISHERY BULLETIN: VOL. 73, NO. 2, 1975. 



382 



