106 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 6. — Surface-caught yellowfin tuna, localities combined: stomach-content analysis of 775 fish 



as employed by our vessels may be selective. 

 Surface trolling and live-bait fishing appear to be 

 more effective near land than away from land, and 

 in the central Pacific they take small and medium- 

 sized fish. Longline fishing, in contrast, is 

 effective both near and away from land, but the 

 catch is composed almost exclusively of medium- 

 sized and large fish. On the other hand, the dis- 

 tribution of the fish may actually be in accordance 

 with the catch; i. e., large fish scarce in surface 

 waters both offshore and near shore but more 

 abundant at subsurface levels ; small fish occurring 

 most abundantly in surface waters near shore, 

 uncommonly in surface waters offshore, and rarely 

 in subsurface waters, whether offshore or near 

 shore; and medium-sized fish existing through- 

 out the ranges of both the smaller and the larger 

 fish. 



As the collection of stomach material was just 

 one of several objectives of each fishing cruise, it 

 was not possible to schedule the trips so as to 

 yield samples evenly distributed in time for each 

 major area and habitat. For example, pole-and- 

 line fishing was conducted only during January 

 and July 1950 and June 1951. Longline fishing 



was done near Canton Island (Phoenix Group) in 

 July 1950, and in the general area of the Line 

 Islands in November 1950 and September 1951. 

 Surface trolling was done on all cruises but, as 

 previously stated, yielded few fish from waters 10 

 miles or more from land. It is apparent, there- 

 fore, that while our samples were rather homo- 

 geneous within themselves, they differed in time 

 and area of capture, habitat, average size, and in 

 other ways. Consequently any comparisons be- 

 tween different lots of fish must be qualified 

 with respect to these different factors. 



According to Ricker (1946), fish are usually 

 classified as bottom feeders, plankton eaters, or 

 fish eaters. From the results of this study it is 

 apparent that maturing and adult yellowfin of 

 the central Pacific feed on macrozooplankton, fish, 

 and also pelagic cephalopods. In addition to the 

 great variety in the food, it is surprising to dis- 

 cover that small organisms of 0.2 to 0.5 cc. in 

 volume are regular prey of these large, fast-moving 

 fish. 



Since crustacean larvae were so prominent in 

 the food of those small yellowfin sampled, the ques- 

 tion arises as to whether the apparent greater 



