COMPARATIVE STUDY OF FOOD OF BIGEYE AND YELLOWFIN TUNA 



IN THE CENTRAL PACIFIC 



By JOSEPH E. King and Isaac I. Ikehara, Fishery Research Biologists 



The predominant species of tuna captured on 

 longline-fisliing surveys of the Fish and Wildlife 

 Service's Pacific Oceanic Fishery Investigations 

 (POFI) are the yellowfin, Neothunnus macropterus 

 (Temminck and Schlegel), and the bigeye, Para- 

 thun?}us sibi (Temminck and Schlegel), with a 

 catch ratio of about 5 to 1 in favor of the yellow- 

 fin. These are large tanas, the yellowfin oc* 

 casionally reaching a weight of 200 pounds and 

 the bigeye a weight of 300 pounds in the tropical 

 Pacific. The two species have a marked super- 

 ficial resemblance in general body shape and 

 coloration and arc not always differentiated in 

 the commercial catch. 



Murphy and Shomura (1953a, 1953b), in dis- 

 cussing results of experimental longline fishing 

 conducted by POFI, point out interesting differ- 

 ences in the distribution of these two species. In 

 the tropical Pacific, the bigeye have been taken 

 in greatest numbers north of latitude 5° N. The 

 best catches of yellowfin, on the other hand, have 

 been made in the general region of the Equator, 

 sometimes to the north when the area is under 

 the influence of southeast tradewinds, and some- 

 times to the south when the northeast trades are 

 dominant. Tliis shift in abundance that appears 

 to be related to changes in the prevailing winds 

 can now be explained, at least partially, from our 

 knowledge of the ocean currents and their effect 

 on the basic food supply (Cromwell 1953).' Al- 

 though the peaks in abundance do not correspond 

 exactly, tlie general area of high yellowfin catch 

 is also the area of greatest zooplankton abund- 

 ance (King 1954). The horizontal distribution of 

 the bigeye, however, does not seem to conform to 

 the general pattern that the most fish are found 

 where food is most abundant. 



There is also some evidence of difference in the 

 vertical distribution of yellowfin and bigeye. 

 While the results are rather variable, there have 



' .\Iso a manuscript by O. E. Sette: Nourishment of central Pacific 

 stocks of tuna by the equatorial current system (Proceedings of the 8th 

 Pacific Science Congress). 



been indications on certain POFI cruises to the 

 equatorial area that the best catches of bigeye 

 came from greater depths than those of the 

 yellowfin (Murphy and Shomura 1953b). In Ha- 

 waiian waters the bigeye occurs in greatest num- 

 bers during the winter months from Octol)er to 

 May, whereas the yellowfin is most abundant from 

 May to September (Otsu 1954). Brock (1949) 

 points out that the Hawaiian longline fishermen 

 try to increase the catch of bigeye after the yellow- 

 fin season by lengthening the hook lines in order 

 to fish deeper. Also, unlike the yellowfin, the 

 bigeye — at least the adults — are rarely taken by 

 surface-fishing methods. Nakamura (1949) states 

 that the bigeye is thought to occur at the deepest 

 levels of any of the tunas. It appears that the 

 bigeye prefers somewhat colder water than does 

 the yellowfin, or perhaps the two species have 

 different feeding habits or food preferences which 

 influence their distribution. 



The purposes of this study are to describe the 

 food of bigeye tuna in the central Pacific, to com- 

 pare the foods of bigeye and yellowfin tuna - 

 captured at about the same time and place, to 

 determine whether differences occur which are 

 associated with the horizontal and vertical distri- 

 bution of these fish, and to obtain information on 

 food preferences of each fish which maj^ be useful 

 to the commercial fishery. The experimental 

 fishing carried out by POFI has provided collec- 

 tions of bigeye and yellowfin stomachs which are 

 essentially alike in respect of time and area and 

 which were obtained with standardized fishing 

 methods. Therefore, we believe the resulting 

 data should provide reliable comparisons of the 

 food of these fish because these several variables 

 have been controlled. 



There is an extensive literature, reviewed pre- 

 viously by Reintjes and King (1953), dealing with 

 the food of yellowfin, whereas there are only a very 



' The food of yellowfln was previously described by Reintjes and King 

 (1953). 



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