FOOD OF YELLOWFIN TUNA 



105 



] CRUSTACEA 



SQUID 





I 



1300-OVER 



FORK LENGTH IN MM 



0-43 43-94 94-OVER 



BODY WEIGHT IN POUNDS 



Figure 9. — Variation in food composition with body size 

 for yellowfin captured at the surface within 10 miles of 

 land. N = number of stomachs. 



1° N. latitude; and (4) an area of poor catch, 

 from the Equator to 5° S. latitude. Fishing was 

 done at stations 1 and 3, but no yellowfin were 

 captured. The number of yellowfin captured in 

 each of these areas, the number of stomachs 

 examined, and the average volume of food in 

 each stomach are shown in figure 10. 



North of the Equator the yellowfin catch varied 

 generally with zooplankton volumes, but south of 

 the Equator the catch dropped off markedly while 

 the zooplankton population persisted at a rela- 

 tively high level. The few yellowfin captured 

 south of the Equator were well fed, however, as 

 indicated by the relatively high volume of food 

 found in their stomachs. 



Previous cruises of the Hugh M. Smith have 

 demonstrated the existence of a "rich zone" in 

 the region of the Equator that has been consider- 

 ably higher in chemical nutrients and zooplankton 

 than waters to the north or south (Cromwell 

 1951, King and Demond 1953). On most cross- 

 ings of the equatorial region, the rich zone was 

 found between the Equator and 4° to 5° N. lati- 

 tude. On occasion, however, perhaps because of 

 certain peculiar conditions of winds and currents, 

 the rich zone was found to be displaced three or 

 four degrees to the southward with the general 

 pattern of relative zooplankton distribution remain- 

 ing as in figure 10. It may be that for the greater 

 part of the year the region of greatest productivity 

 conforms more nearly to the distribution of the 

 yellowfin catch of cruise 11 than to the distribu- 

 tion of zooplankton abundance. The latter 

 may be more transitory in position than the fish 

 population. 



DISCUSSION 



The 1,097 yellowfin upon which this study is 

 based represent slightly more than 20 percent of 

 all yellowfin captured during experimental and 

 exploratory fishing in 1950 and 1951 by vessels of 

 the Pacific Oceanic Fishery Investigations. The 

 fish were captured by three standard commercial 

 fishing methods: longline or flagline fishing, pole- 

 and-linc fishing with live bait, and surface trolling 

 with artificial lures. The yellowfin caught on 

 longlines averaged approximately 125 pounds in 

 weight and were captured well below the surface 

 both near and away from land ; the fish caught by 

 pole-and-line were taken near shore at the surface 

 and averaged less than 30 pounds; and the fish 

 caught by trolling were all taken at the surface, 

 most of them near shore (less than 2 percent were 

 taken more than 10 miles from land), and averaged 

 approximately 30 pounds. 



There are two possible explanations for this 

 marked variation in results obtained by the dif- 

 ferent fishing methods. First, the fishing methods 



