Table 6. — Results of experimental fishing with threadfin shad and nehu on 

 Charles H. Gilbert cruises 105, 106, and 108. 



Table 7. — Number of skipjack tuna photographed 

 in the fishing area. 



Cruise 

 no. 



Bait 



No. of 

 schools 



No. of 

 photos 



No. of 



skipjack 



tuna 



Average no. 

 of skipjack 

 tuna per 

 photograph 



Totals 



Shad 

 Nehu 



17 

 16 



1,210 

 1,471 



4,775 

 5,556 



3.8 

 3.6 



skipjack tuna per photograph with nehu during 

 cruise 110. The mean value for all three 

 cruises was 3.8 skipjack tuna per photograph 

 with shad and 3.6 skipjack tuna per photograph 

 with nehu. 



The number of skipjack tuna counted per 

 photograph should be considered only as rough 

 indications of the concentration of skipjack in 

 the fishing area because of certain inherent 

 variables associated with the photography. 

 These included the rolling motion of the vessel, 

 skipjack tuna being obscured by bubbles, 

 changes in lighting conditions caused by clouds 

 and vessel maneuvers, and the relatively slow 

 camera shutter speed. 



Behavior of Bait 



Generally speaking, threadfin shad and nehu 

 had similar swimming behavior when used as 

 chum for skipjack tuna fishing. Some differ- 

 ences occurred in their behavior, however, and 

 were marked enough to allow an experienced 

 observer to usually determine which species 

 was being chummed. 



Upon entering the water, both species dove 

 downward to escape the skipjack. The diving 

 angle of nehu varied up to 90°, but typically 

 was 45° to 90°. They appeared to dive faster 

 than shad and exhibited marked dodging in their 

 escape attempts. If several nehu landed in the 

 water in the same vicinity, they often formed a 

 school a few feet below the surface and tried to 

 dive downward and escape en masse. The div- 

 ing angle of shad also varied up to 90°, but was 

 usually about 45° to 65°, generally not as steep 

 as that of nehu. They did not appear to swim 

 as fast as nehu, nor dodge quite as vigorously. 

 Shad also formed small schools while diving. 

 During fishing, shad were often eaten by the 

 skipjack tuna before they had a chance to get 

 much deeper than 3 to 4 m , while nehu often 

 reached depths of 6 to 8 m before being eaten. 

 This sometimes resulted in the skipjack tuna 

 swimming deeper when nehu was being 

 chummed than on those schools when the bait 

 species were alternated. 



Both baits, however, can dive to considerable 

 depths. From a sea sled we have observed 



