are distinct schools of different species of tunas drawn to- 

 gether by a common stimulus, such as food. 



In the eastern Pacific Ocean, site of the largest U.S. tuna 

 fishery, skipjack tuna and yellowfin tuna are often caught 

 together in purse seines. Scientists there have found that 

 the excitable skipjack tuna calmed down when they were 

 placed in a baitwell with the less erratic yellowfin tuna. 



Schools of tunas usually consist of fish of the same 

 size. The reason probably is that swimming speed varies 

 with size and therefore that fish tend to school with 

 other fish of the same size, which swim at the same rate. 

 Thus even if two or more species are present, they will 

 be approximately the same size. 



A Japanese scientist has ob.served that schools of skip- 

 jack tuna that bite well maintain an orderly formation, 

 "like marching troops" ; those that bite poorly are "dis- 

 orderly." 



Some tunas school at night. Schooling is thought to be 

 a function of sight; Nakamura thinks that at night the fish 

 can see well enough to school by the light of the moon or 

 the light shed by luminescent organisms. Some fishermen 

 locate schools by watching for the luminescence of plank- 

 tonic organisms disturbed by the fish. 



In the eastern Pacific, purse seining for tuna has been 

 most successful when a shallow upper mixed layer of the 

 sea has been underlain by a pronounced thermocline. That 

 is, the temperature drops off sharply within a few dozen 

 feet. This sharp gradient is widely believed to deter tuna 

 from sounding (diving) and escaping the nets. It may not, 

 however, be temperature alone that causes the fish to avoid 

 the thermocline. The water there is often turbid and some- 

 times the layer just below the thermocline has perilously 

 little oxygen. 



Like many other fishes, tunas appear to seek out floating 

 objects. Some scientists think they use them as "land- 

 marks" in a largely featureless sea. Recently, other scien- 



tists have said that the chief function of the floating objects 

 appears to be to offer shelter. In any event, tunas are often 

 found near logs, driftwood, floating vessels, even dead 

 whales. Japanese scientists say that schools may wander 

 as far as 7 or 8 miles from such an object and then return. 

 If this pelagic homing does occur, Nakamura says, it im- 

 plies that the tunas have some sort of navigational system. 



The animal association of the tunas that is most prof- 

 itable to man is that with birds, although in some fisheries 

 the association with porpoises is more important. In the 

 central Pacific, and in some other areas, fishermen depend 

 almost wholly on sighting bird flocks to locate tuna, Na- 

 kamura says. "They even rely on the behavior of the 

 birds to determine certain characteristics of the schools . . . 

 The number and spread of the birds is an indication of 

 school size. If the birds dive and circle fast and erratically, 

 the fish are small. If the birds are seen diving into the 

 water, the tunas have driven their prey to the surface and 

 are feeding actively. If the birds scatter or sit on the sea 

 surface, the fish have sounded." 



The tunas are among the swiftest of the fish. Their 

 measured speeds have been as great as 25 m. a second 

 (56 miles per hour). 



Nakamura, Chief of the Behavior and Physiolog>' Pro- 

 gram at the Laboratory in Honolulu, is interested in the 

 behavior of tunas both at sea and in the laboratory. His 

 group is concentrating on two aspects of the behavior of 

 the fish : their reaction to different species of live bait 

 and their subsurface distribution in the sea. 



The Hawaiian Live-Bait Fishery 



What the Hawaiians call "nehu" (Stolephorits purpuretis) 

 is a silver sliver of a fish that is one of the anchovies, 

 much like the kind that come salted and packed in cans 

 (fig. 8). The nehu is sporadically plentiful in Hawaii's 

 dozen or so shallow bays, but probably is not abundant 



10 



