THE SKIPJACK TUNA RESOURCE 



Published studies by the Hawaii Area staff have shown 

 that the skipjack tuna resource of the central Pacific Ocean 

 is capable of sustaining huge harvests over and above pres- 

 ent catches. The potential is of the order of hundreds of 

 thousands of tons. A unit of 100,000 tons of skipjack tuna 

 has a value of $25 million to the fishermen. In 1967, the 

 exvessel value of the U.S. shrimp catch was a record $103 

 million. Two 100,000-ton units of skipjack tuna would thus 

 equal half the present value of that American shrimp catch. 

 Added to the 1967 U.S. tuna catch, these units would bring 

 the tuna total to $94.5 million. These figures are cited 

 to suggest the magnitude of the payoff that would result 

 from expanded skipjack tuna production in the central 

 Pacific. 



The Behavior of Tunas 



Man has hunted several species of the swift and valuable 

 tunas for thousands of years, but he has accumulated 

 remarkably little reliable knowledge of the behavior of 

 these fish at sea. Only within recent decades has there been 

 a concerted effort to describe precisely how tunas behave 

 in the open ocean. 



In 1967 biologist Eugene L. Nakamura summarized the 

 literature on field observations of the tunas for an inter- 

 national conference of fishery experts in Bergen, Norway. 

 Nakamura found that the conditions of fishing have set 

 sharp limits on man's knowledge of the tunas. The most 

 widely used methods of catching the fish require that the 

 tunas be hungry; consequently a substantial part of the 

 body of observations concerns feeding behavior. 



Sometimes tunas appear to seek out a single species of 

 small fish as their prey; at other times they do not. Near 



Hawaii, for example, tunas sometimes appear to prefer the 

 fishes that live at a depth of 1,000 feet or more, rather 

 than the surface-living fishes or baitfishes thrown in the 

 water. 



A hungry fish may bite poorly or well. One scientist 

 has found that between the extremes of starvation and 

 satiation, the less food the fish have in their stomachs, the 

 less likely they are to bite well. But others have found 

 that fish with empty stomachs bite very well. 



Skipjack tuna, the most plentifully caught of the species 

 in the Pacific Ocean, display vertical bars on their sides 

 during feeding (fig. 7). Scientists have said that the catch 

 will be good when the fish exhibit these bars. 



Tunas travel in schools. The size of these schools can 

 vary tremendously, from a few fish to hundreds of thou- 

 sands. In 1958, near San Benito Island, off the west coast 

 of Baja California, purse seiners took 4.000 tons of bluefin 

 tuna from a single school. This is about 80 per cent as 

 much fish as is caught by the entire Hawaiian skipjack 

 tuna fleet in a whole year. 



Almost all species of tunas have been reported in mixed 

 schools of two or more species, Nakamura says. But ac- 

 cording to research conducted by Heeny S. H. Yuen at the 

 Laboratory in Honolulu, these "mi.xed" schools probably 



FIGURE 7. Skipjack tuno disploy pronounced »ertical bars on their 

 flanks when they ore feeding. These bors interrupt the normally 

 dork longitudinal stripes that charocteriie the skipjack tuna. These 

 fish were photogrophed from a viewing port of the Laboratory rc- 

 seorch vessel CHARLES H. GILBERT. Species reloted to the skipjack 

 tuna have also displayed such bors during a courting sequence 



