nehu swim as deep as 18 m. They were still 

 heading downward when lost to view. On one 

 occasion scuba divers followed two shad and 

 one Tilapia mossambica down to 23 m before 

 breaking off the pursuit. 



One difference between the two baits is their 

 visibility. Shad are deeper bodied than nehu and 

 have a greater portion of their sides covered 

 with silvery scales. The result is a larger sil- 

 very reflecting surface. To the underwater 

 observer, shad is more visible than nehu. Shad 

 scales are highly deciduous and create a pat- 

 tern of small silvery points of reflected light 

 in the water. On one occasion, observers on 

 the sea sled 23 m astern of the Gilbert were 

 able to determine when a switch in bait from 

 nehu to shad occurred just by watching for the 

 silvery glints in the water from the shad scales. 



Both shad and nehu have been observed to 

 return to the surface when pursued by skipjack 

 and to flee to the side of the vessel and swim 

 along beside it. Whether this occurs more fre- 

 quently when using shad or nehu is not known. 

 Both shad and nehu are avidly eaten by skip- 

 jack; we have found skipjack stomachs gorged 

 with shad, nehu, and both. 



DISCUSSION 

 Bait Characteristics 



The basic characteristics necessary for a 

 live bait to be effective in the Hawaiian skip- 

 jack tuna fishery are the ability to: (1) lure 

 skipjack to the stern of the fishing vessel, (2) 

 concentrate the skipjack at the stern in suffi- 

 cient numbers, and (3) produce a good catch 

 rate. If the bait can be easily obtained, is 

 hardy, and is readily handled, its effectiveness 

 should increase. 



Threadfin shad satisfy the three basic char- 

 acteristics, and are also hardy and readily 

 handled. During Gilbert cruises 109 and 110, 

 shad were more successful than nehu in luring 

 skipjack tuna to the stern of the vessel, since 

 50.0% of the schools chummed with shad were 

 successfully fished and 35.9% of the schools 

 chummed with nehu were fished. On cruises 

 105, 106, and 108. nehu were more successful 

 than shad; 55.6% of the schools initially 

 chummed with nehu were successfully fished 

 and 12.0% of the schools initially chummed 

 with shad were successfully fished. With the 

 exception of shad on cruises 105, 106, and 108, 

 these percentages are roughly comparable to 

 those reported by Royce and Otsu (1955), who 



found that only 43% of Hawaii skipjack tuna 

 schools are successfully fished, and Yuen 

 (1959), who reported a 48% response to nehu. 

 The low percentage response to shad on cruises 

 105, 106, and 108 was due to the necessity of 

 initially chumming shad on successive schools 

 in order to conserve low supplies of nehu. Had 

 nehu been chummed on alternate schools, the 

 percent response for both baits probably would 

 have been similar. 



The ability of shad to produce catches com- 

 parable to nehu is shown in the summaries 

 given in Tables 2 and 5. On cruises 109 and 

 110, shad produced a catch rate of 7.5 skipjack 

 tuna per minute, slightly less than the nehu 

 catch rate of 8.2 skipjack tuna per minute. On 

 cruises 105, 106, and 108, the catch rate aver- 

 aged 9.1 skipjack tuna per minute with shad, 

 slightly better than that of 7.7 skipjack tuna per 

 minute with nehu. 



The major differences between shad and nehu 

 during cruises 109 and 110 were in the total 

 weight of skipjack tuna caught and in the dura- 

 tion of fishing. The higher weight of skipjack 

 tuna caught with nehu on cruises 109 and 110 

 was due to large skipjack tuna being heavily 

 fished from several schools when nehu were 

 used as bait, such as station 52, cruise 109, and 

 station 6, cruise 110. Since the bait species 

 were switched on each school, and there is no 

 prior way of knowing what size skipjack tuna 

 are in successive schools, it appears the dif- 

 ference is due to chance. The lower duration 

 of fishing with shad (10.8 min) is due to the 

 effects of two schools (cruise 109, stations 7 

 and 35) when only one skipjack tuna was caught 

 from each. If data from these two schools are 

 removed, the mean duration of fishing with shad 

 for both cruises becomes 12.4 min, compared 

 with nehu at 15.4 min. 



The amount of bait necessary to produce a 

 good skipjack tuna catch is another important 

 factor in developing an alternate live bait. Here 

 shad also appear promising. During cruises 

 109 and 110, an average of 4.0 buckets of shad 

 and 4.8 buckets of nehu were used on each 

 school successfully fished. These amounts 

 were calculated from the chummer's estimate 

 of how much bait had been used, the estimate 

 being made just after fishing ended. 



In considering bait requirements, the effects 

 of mortality must also be taken into account. 

 Brock and Uchida (1968) found that nehu usually 

 averaged about 25% mortality a day after being 

 placed in the baitwells. I have no exactly com- 



