BLACKBURN and WILLIAMS: DISTRIBUTION AND ECOLOGY OF SKIPJACK TUNA 



becoming distributed in accordance with the 

 requirements of their larvae. There is evidence 

 that skipjack larvae occur only at sea tempera- 

 tures from 23° to 31°C and are most common at 

 about 29° to 30°C (Inter-American Tropical Tuna 

 Commission 1971). Maximum surface tempera- 

 tures in the study area in October-November 1969 

 were between 27° and 28° C and occurred from 

 about lat. 4° to 11°N (Figure 29). Thus the Coun- 

 tercurrent waters at lat. 6° to 8°N could have been 

 particularly suitable for the survival of skipjack 

 larvae, and the parent fish may have been becom- 

 ing distributed accordingly. 



We found no direct relations between skipjack 

 and mixed layer depth, dissolved oxygen, surface 

 currents, chlorophyll, or zooplankton although 

 some of these properties and features should have 

 indirect effects on skipjack through their effects 

 on temperature and forage. Some of them could 

 also have direct effects upon larval or juvenile 

 skipjack. Significant correlations between skip- 

 jack and zooplankton were not found (Tables 11, 

 12). Significant correlations between large skip- 

 jack and forage are also significant between skip- 

 jack and the arithmetic product of forage and 

 zooplankton, but not between skipjack and 

 zooplankton alone. 



Although this paper has contributed to our 

 knowledge of the distribution and relative abun- 

 dance of skipjack in the offshore eastern tropical 

 Pacific, where little information was previously 

 available, the prospects for commercial fishing 

 remain unknown. Our simple experimental fishing 

 procedures served to identify zones of maximum 

 occurrence of skipjack, but commercial trials will 

 be needed to show if those zones can be exploited 

 profitably. Ideally there should be trials by live- 

 bait boats as well as purse seiners, in view of the 

 fact that live-bait fishing gave good results on an 

 experimental scale during the 1969 cruise. Our da- 

 ta and interpretations should be useful as a guide 

 to those who make these tests. 



ACKNOWLEDGMENTS 



We are grateful to associates in the Scripps 

 Tuna Oceanography Research Program, the 

 Inter-American Tropical Tuna Commission, and 

 the Southwest Fisheries Center Honolulu and La 

 Jolla Laboratories, National Marine Fisheries Ser- 

 vice, NOAA, who participated in the cruises and 

 processing of the data. Additionally, we thank the 

 Director of the Honolulu Laboratory, National 



Marine Fisheries Service, for supplying the 

 original data collected on cruise 116 of the RV 

 Charles Gilbert. Thanks are also due to T. S. Hida, 

 J. D. Isaacs, R. W. Owen, and P. E. Smith who 

 reviewed the manuscript. 



The work was part of the Scripps Tuna 

 Oceanography Research (STOR) Program of the 

 Institute of Marine Resources, University of 

 California. It was supported by the National 

 Marine Fisheries Service under Contracts 

 14-17-0007-989, 14-17-0001-2311, and N208-0047-72 

 (N) with the Institute of Marine Resources. 



LITERATURE CITED 



Anonymous. 



1973. Further experiments on lethal oxygen levels in skip- 

 jack tuna confirm earlier data. U.S. Dep. Commer., Natl. 

 Mar. Fish. Serv., Southwest Fish. Cent, Tuna Newsl. 12:7. 

 Blackburn, M. 



1965. Oceanography and the ecology of tunas. Oceanogr. 

 Mar. Biol., Annu. Rev. 3:299-322. 



1968. Micronekton of the eastern tropical Pacific Ocean: 

 Family composition, distribution, abundance, and rela- 

 tions to tuna. U.S. Fish Wildl. Serv., Fish. Bull. 67:71-115. 



1969. Conditions related to upwelling which determine dis- 

 tribution of tropical tunas off western Baja Califor- 

 nia. U.S. Fish Wildl. Serv., Fish. Bull. 68:147-176. 



1970. Collection and processing of data: Micronekton. In C. 

 M. Love (editor), EASTROPAC atlas. Vol. 4. Biological 

 and nutrient chemistry data from principal participating 

 ships, first and second monitor cruises, April-July 1967, p. 

 10- n. U.S. Dep. Commer., Natl. Mar. Fish. Serv., Circ. 

 330. 



Blackburn, M., and R. M. Laurs. 



1972. Distribution of forage of skipjack tuna (Euthynnus 



pelamis) in the eastern tropical Pacific. U.S. Dep. 



Commer., NOAA Tech. Rep. NMFS SSRF-649, 16 p. 

 Blackburn, M., R. M. Laurs, R. W. Owen, and B. Zeitzschel. 



1970. Seasonal and areal changes in standing stocks of phy- 

 toplankton, zooplankton and micronekton in the eastern 

 tropical Pacific. Mar. Biol. (Berl.) 7:14-31. 



Cromwell, T. 



1958. Thermocline topography, horizontal currents and 

 "ridging" in the Eastern Tropical Pacific. [In Engl, and 

 Span.] Inter-Am. Trop. Tuna Comm., Bull. 3:133-164. 



Gulland, J. A. (editor). 



1971. The fish resources of the ocean. Fishing News (Books) 

 Ltd., West Byfleet, Engl., 255 p. 



Hida, T. S. 



1970. Surface tuna schools located and fished in equatorial 

 eastern Pacific. Commer. Fish. Rev. 32(4):34-37. 

 HiGGINS, B. E. 



1966. Sizes of albacore and bigeye, yellowfin, and skipjack 

 tunas in the major fisheries of the Pacific Ocean. In T. A. 

 Manar (editor), Proc. Governor's Conf. Cent. Pac. Fish. 

 Resour., State of Hawaii, p. 169-195. 



Inter-American Tropical Tuna Commission. 



1966. Annual report of the Inter-American Tropical Tuna 

 Commission for 1965, 106 p. [In Engl, and Span.] 



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