yellowfin were taken in somewhat greater numbers on the deeper hooks, 

 while albacore and bigeye were taken in markedly greater numbers on 

 the deeper hooks. 



As we were not able to determine reliably the absolute depth of 

 the line during 1953, the distribution of the catches must be considered 

 in terms of relative depth (see Murphy and Shomura 1955 for a sum- 

 mary of the measurements of absolute depth). We have separated the 

 catches by relative depth by dividing the line into three levels, each 

 occupied by two hooks that fish at the same level if the sag of the line 

 forms a symmetrical curve. The records for the individual cruises in 

 1953 show that yellowfin were usually, but not always, taken in greater 

 numbers on the deepest or on the two deeper levels (table 3). The 

 catch of bigeye and albacore was always poor on the shallowest hooks, 

 and usually best on the deepest hooks (table 3). 



The difference in catch with depth is probably the result of at 

 least three factors. In the Pacific, albacore and bigeye are taken in 

 substantial numbers farther north and in cooler water than yellowfin 

 and thus in the tropics these two species might be expected to be taken 

 in greater numbers on hooks fishing the deeper, cooler strata. Secondly, 

 King and Ikehara (MS) found slightly more food in the stomachs of yellow- 

 fin and bigeye taken on deeper hooks, suggesting that food is more avail- 

 able in the deeper levels fished by the gear. A complication is the 

 tendency for more baits to remain on the deeper hooks through the fish- 

 ing period (Shomura 1955). This latter cannot be the only cause of 

 increased catch with depth, however, for the difference in bait retention 

 is not as great as the difference in catch, and differences in bait effec- 

 tiveness with depth can hardly explain the variation in catch with depth 

 among the three species. The conclusion is that temperature preference 

 and abundance of food are probably the most important factors associated 

 with variation in the vertical distribution of deep-swimming tunas in the 

 equatorial Pacific. 



SIZE COMPOSITION OF THE CATCHES OF TUNA 



Yellowfin 



The size composition of longline-caught yellowfin is in part a 

 function of the nearness of land. This is shown when the 1953 catches 

 are grouped into those made within 80 miles of land and those made 

 farther from land (fig. 7). This separation, the same as that made 

 when considering abundance in an earlier section, clearly points up 



14 



