76 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 7. — Variations in average volume per stornach of the major food categories as related to the current system 

 (Boundaries of each area or division of current system are defined in text; volume is measured in cc] 



On the basis of average volume of food per pound 

 of body weight there was little difTerence between 

 the two species. 



The region of convergence has yielded the best 

 yellowfin catches but has produced consistently 

 poor bigeye catches. In this region the bigeye 

 had about 20 percent more food in their stomachs 

 than did the yellowfin, but the bigeye were also 

 larger in average body size. Again the two species 

 were almost identical with respect to average 

 volume of food per pound of body weight. 



In the CC, tlie thermocline occurs at shallow to 

 moderate depths, while in the convergent zone it 

 lies much deeper. Accompanying changes in the 

 deptli and velocity of the surface currents may 

 greatly affect the fishing depth of the longliiie. 

 In the region of sliallow thermocline it is possible 

 that, as a result of the streaming of the line caused 

 by the marked sliear between the moving surface 

 waters and the relatively quiet waters below the 

 thermocline, all hooks may be fishing at about the 



same level (Murphy and Shomura 1953b), and no 

 marked difference might be expected in the food 

 between the various hook levels. In a region of 

 deep tliermocline the longline can hang vertically 

 and lie entirely within the homogenous surface 

 layer. A marked difference in hook depth and 

 possible differences in the stomach contents of the 

 catch may then result. 



Data have been assembled in table 8 and figure 

 12 to illustrate the variations in average volume 

 per stomach for the major food categories with 

 deptli of capture of the tanas in these two ocean 

 areas. In the CC there is greater change in the 

 food of yellowfin with depth than in the convergent 

 zone; this is evidenced by a consistent increase with 

 deptli, in the CC, in the utilization of Bramidae, 

 Gempylidae, and total fish. In the bigeye the 

 only important and consistent variation shown in 

 the CC is a marked increase with deptli in the 

 amount of Crustacea eaten and a decrease in the 

 importance of Gempylidae, as contrasted with 



