Yellowfin Tuna 



The yellowfin tuna is a tropical 

 species with seasonal poleward extensions 

 in range. The pattern of distribution as 

 shown in figure 9 is based, among other 

 things, on information from the greatly ex- 

 panded Japanese longline fishery which 

 developed in the last few years, among 

 others. The figure does not indicate sea- 

 sonal difference in the occurrence of the 

 fish so that relationships between the dis- 

 tribution of the fish and various seasonal 

 oceanographic changes are obscured. The 

 poleward extension in range coincident with 

 summer warming is one example. 



The Japanese fishery for yellowfin 

 tuna in the western part of the Pacific 

 extends far eastward toward Central Amer- 

 ica, and Ocean. In the eastern Pacific an 

 American fishery extends along the west 

 coasts of the Americas. Catches in the 

 tropical Atlantic from Brazil to Africa 

 represent a recent expansion of the 

 Japanese longline fishery into that Ocean. 

 A recently -developed and growing live-bair 

 fishery for yellowfin exists off the west 

 coast of Africa. 



Skipjack Tuna 



Figure 10 illustrates the distribu- 

 tion of skipjack stocks in the world. This 

 species seems to have a somewhat more pole- 

 ward range from the tropics than does the 

 yellowfin. It seems to be somewhat more 

 capable of penetrating areas of summer warm- 

 ing. Of the various species shown in the 

 series of figures, the distribution of this 

 one is probably the least accurate because 

 skipjack is taken in substantial quantities 

 only by live-bait boats and some purse 

 seiners close to the west coast of Central 

 and South America. The catches made by 

 longline gear are so small and so scattered 

 that they constitute evidence of the occur- 

 rence of the fish at a locality, nothing 

 more. The live-bait fisheries are con- 

 strained by their need for bait to confine 

 fishing operations where bait supplies are 

 available} hence the figure really shows the 

 areas where live-bait resources and skipjack 

 populations coincide. 



The purse-seine fisheries for this 

 and other species seem to be confined to 

 those areas which possess a relatively 



shallow mixed layer, a feature characteris- 

 tic of the eastern sides of the various 

 oceans. There is an absence of skipjack 

 catches from a large area in the tropical 

 western Pacific north of the Equator. This 

 absence of tuna shows up also in the dis- 

 tributional patterns for other species, 

 indicating that the current gyral which 

 occurs here is probably an area of low pro- 

 ductivity that will not support a large 

 population of tuna. 



Albacore 



Albacore have a world-wide distribu- 

 tion (fig. 11). There are general simi- 

 larities between this and the pattern of 

 distribution for other species of tuna 

 estimated from Japanese longline catches 



YIILLOWl li\ 



mr to- w w 



w xr <r w 



Figure 9. — Yellowfin tuna distribution. 



le'K-worw'W'irwtar'jf 



jr tc *>' 



Figure 10. — Skipjack tuna distribution. 



