Figure 1. 



--Diagrammatic view of a basket 

 of POFI longllne gear. 



Individual baskets of 5 to 

 10 hours. The line was set 

 slack in order to peirmlt It 

 to fish at various depths, 

 and for this reason 40 or 50 

 baskets extended for only 

 9-10 miles. 



DESIGN OP EXPERIMENTS 

 AND TREATMENT OF DATA 



Early In POFI 's 

 longllne fishing program 

 experiments were planned to 

 compare bait species, bait- 

 ing methods, and methods of 

 bait preservation. The scope 

 of the experimentation fea- 

 sible on any one cruise was 

 limited, e.g., to a conpari- 

 son of sardine and herriiog, and it was soon found that the differences in catches 

 were related mostly to soaking time and environmental conditions. Occasionally, 

 too, it was necessary to use bait in poor condition. Therefore the experiments 

 were designed to control statistically the effect of soaking time by alternating 

 the varlates (e.g. species or method of hooking) by baskets or groups of baskets. 

 The environmental conditions were not controllable, but an attempt has been made 

 to determine their effects in the analyses presented in this paper. 



No single criterion is entirely satisfactory for evaluating these 

 experiments. Of paramount concern is the effect on the catch of tuna, but in many 

 experiments so few were caught that only gross differences could be expected to 

 be significant. This has necessitated considerable reliance on the performance 

 of the bait as judged by its retention on the hook. 



During the course of the experimental fishing, detailed data were 

 recorded as the line was hauled. Included in the information obtained was the 

 soaking time of each basket and an exact record of the fate of each baited hook, 

 i.e., the catch if any, presence or absence of bait, dropper lines broken, and 

 fish lost during hauling. 



Prior to suialysls, several adjustments were made on the data as recorded 

 in the field. Shark catches, particularly catches of the white-tipped shark, 

 Carcharhinus longimanus {Poey)£/, tend to obscure the effects of other factors on 

 the bait, sluce msmy of them are captured at or near the surface during hauling. 

 For this reason the white-tipped and the similar silky shark have been coxinted as 

 "bait on". The great blue shark, Prionace Rlauca ^Linnaeus), the third species 

 of shark frequently caught on the longllne, has been included as part of the catch 

 as many of them have been brought alongside the vessel dead or nearly dead, indi- 

 cating capture during the soaking period, and also we have not seen them at the 

 surface in tropical waters. The other species of sharks, only occasionally encoun- 

 tered, have also been Included as part of the catch, since little is known of 

 their habits. 



A few additional corrections were made in order to make the recorded 

 catch represent more closely the number of fish taking the baited hooks diiring 

 the soaking period. One of these was to include broken branch lines as part of 

 the catch in those analyses involving total catch. This would have been unneces- 

 sary if the number of broken lines were a linear function of the magnitude of the 

 total catch, however, the niamber of broken lines differed among stations and among 

 cruises, not because of differences in the abundance of fish, but because of the 



%J The catches of the silky shark ( Carcharhinus sp.) have been combined with the 

 white-tipped shark catch because Improper identification on earlier cruises 

 does not permit separation. The results of more recent cruises in which 

 identification was more certain show that the silky shark comprised only 23 

 percent of a total of 523 Identified white-tipped and silky sharks. 



