PART 1 



THE WORLD TUNA RESOURCE AND FISHERY 



THE TUNA RESOURCE IN RELATION TO OCEANOGRAPHIC FEATURES 



by 



Vernon E. Brock 1/ 



The tuna resources of the world occupy the surface waters of the open sea, and 

 thus are known as pelagic fishery resources. More than a dozen species are included 

 in the catch, but the most important are yellowfin, skipjack, albacore, bigeye, and 

 bluefin. In weight of landings the bonito outranks several tuna species. Bonito, 

 however, is not strictly a tuna, and the fishery is relatively localized, principally 

 in waters exploited by Peru. 



The principal kinds of tuna named above are not necessarily true species, but 

 composites of several closely related forms. Bluefin tuna, particularly, include sev- 

 eral species not very closely related. The common names do, however, characterize fish 

 of similar habits, caught in general by the same fishing methods, hence suitable for 

 discussion as a unit. 



FOOD RELATIONSHIPS 



Let us consider very briefly the 

 role of the tuna in the economy of the sea. 

 Tuna are predaceous fishes living in the 

 upper layers of the ocean; even the so- 

 called sub-surface species, such as large 

 bigeye, yellowfin and albacore in the 

 Tropics, find suitable habitats in the 

 upper 100 fathoms of water, a very thin 

 layer in terms of the average oceanic 

 depths of 2000 to 3000 fathoms. The large 

 tuna - bigeye, yellowfin, and bluefin - 

 rank among the climax predators, while 

 medium-sized and smaller forms are inter- 

 mediate predators that feed on even smaller 

 forage animals and are eaten in turn by 

 larger predators. It is of interest and of 

 importance to consider this aspect of the 



1/ Area Director, Hawaii, Bureau of 



Commercial Fisheries, Honolulu, Hawaii 



food habits of tuna in some detail, even 

 if relatively little is known on the sub- 

 ject. 



As is true on land, all food chains 

 in the open sea begin with plant life, and 

 in this case with single-cell drifting 

 plants, the phytoplankton. These in turn 

 are consumed by many of the small animals 

 forming collectively the assortment of 

 creatures called zooplankton. These 

 animals are in turn fed upon by predaceous 

 members of the zooplankton and by small 

 fishes and squid. In general the predatory 

 forms are larger than the prey and there 

 are fewer of them. Each level in a food 

 chain is estimated to pass along only one- 

 tenth as much material it obtains from the 

 level below it; hence, the greater the 

 number of steps an animal is removed from 

 the primary food (phytoplankton) , the 

 smaller will be the mass of predators it 

 can support. 



