PRE-EXPLOITATION ABUNDANCE OF TUNAS IN THE EQUATORIAL 



CENTRAL PACIFIC 



Garth I. Murphy^ and Richard S. Shomura^ 



ABSTRACT 



The tuna resources and the environment in the equatorial central Pacific were investi- 

 gated from 1950 to 1954. This report summarizes the data from these investigations 

 and examines the distribution and relative abundance of tunas in relation to the en- 

 vironment. 



The circulation of the waters of the equatorial region is described. The generally 

 accepted view of a zonally oriented flow pattern of the South Equatorial Current between 

 the equator and the Countercurrent is modified, and a model is proposed which regards 

 the circulation as a series of homologous northwestward flowing cells. The evidence 

 supporting this type of flow pattern is presented and discussed. 



The variation in abundance of deep-swimming yellowfin tuna in space and in time is 

 analyzed in relation to various environmental features. The hypothesis developed is that 

 the change in abundance of yellowfin tuna is a function of the variation in the wind-driven 

 ocean circulation and the attendant changes in the development of forage organisms. 



A study of the distribution and abundance of surface tunas (skipjack tuna and small 

 yellowfin tuna) revealed that more schools are sighted near land than in the open ocean. 

 In the open ocean more tuna schools were observed in the countercurrent than in the zone 

 from the equator to lat 5°N, where deep-swimming yellowfin tuna are most abundant. 

 Rather than respond to the general level of standing crop of forage, the surface tunas 

 appear to respond to secondary factors that make it possible to forage more effectively. 

 The distribution of "fronts" is such a secondary factor influencing the distribution of 

 surface schools. 



In 1949, before the establishment of the Bu- 

 reau of Commercial Fisheries Biological Lab- 

 oratory, Honolulu,'' the tuna resources of the 

 equatorial central Pacific were little known and 

 even less well understood. In the eastern trop- 

 ical Pacific the Americans had a highly devel- 

 oped fishery for surface schools of yellowfin and 

 skipjack tunas, and in the western Pacific the 

 Japanese had an active longline fishery for the 

 larger subsurface tunas. In the central Pacific, 

 however, information on the tuna resources was 



^ Department of Oceanography, University of Hawaii, 

 Honolulu, HI 96822. 



" National Marine Fisheries Service, Tiburon Fisher- 

 ies Laboratory, Tiburon, CA 94920. 



" Then called the Pacific Oceanic Fishery Investiga- 

 tions. The facility is presently known as the National 

 Marine Fisheries Service, Southwest Fisheries Center, 

 Honolulu Laboratory, 



limited to the experiences of a few commercial 

 operators who had prospected in the area.* 



From 1950 to 1953 the Bureau of Commercial 

 Fisheries surveyed this unknown area, roughly 

 the region between long 120 °W and 180°, and be- 

 tween lat 15°N and 10°S. Studies undertaken 

 during this period involved the distribution of 

 tunas, the productivity of the waters, and the 

 circulation of the ocean. The results of the sur- 

 veys of the tunas form the substance of this re- 

 port. Although the other investigations are de- 

 tailed elsewhere, they are brought to bear where- 

 ever they help to explain the distribution of the 

 tunas. Studies in the equatorial central Pacific 



Manuscript accepted March 1972. 

 FISHERY BULLETIN: VOL. 70, 



NO. 3, 1972. 



* It should be noted that today (1972) the oceanwide 

 longline fishery is fully developed (see, for example, 

 Suda and Schaefer, 1965). The results reported herein 

 are representative of "virgin stock" conditions, and it 

 is highly unlikely that a similar assessment can ever 

 be repeated. 



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