FOOD OF BIGEYE AND YELLOWFIN TUNA 



73 



V7?i SQUID 



I TOTAL FISH 



olOO 



S 

 o 

 (- 



f 75 

 a. 



UJ 



a. 

 o 50 





 150 



5 100 



< 



z 

 o 



YELLOWFIN 



(71) 



(128) 



(107) 



(123) 



^ 1 I ^ i 



25 



BIGEYE 



(25) 



(46) 



i 



i 



(41) 



(54) 



I 



I 



-JUL AUG- SEP 



SEASON 



OCT -NOV 



Figure -KT — Variation in the major foods as related to the 

 general longitude of capture of the tunas. Number of 

 stomachs is shown in parentheses. 



of the major food items shown in figure 10. The 

 chief simihirity between the two species lies in the 

 lower volume of total fish in the food of tunas 

 captured in the region of 140°-150° W. longitude. 

 The utilization of squid, Bramidae, Gempylidae, 

 and TluHinidae does not vary in any regular 

 pattern for the two species. A majority of the 

 Thunnidae appearing in the food of yellowfin 

 captured in the area of 120°-130° W. were Auxis 

 ^Aojo/y/, which was not prominent in the food in the 

 more western regions and which in the l)igeye was 

 represented by only one specimen, also from the 

 120°-13n° \V. region. 



P^or both bigeye and yellowhn, the largest 

 specimens were captured in the eastern region 

 (120-1:50° W.) and the smallest in the western 

 region (155° W-180°). When the variation in 

 volume of stomach contents is considered in terms 

 of imit volume per iniit of body weight, we find 



388734 O- 56 3 



only slight regional diflferences for the yellowfin 

 but a rather large variation for the bigeye (table 

 ()). \w the bigeye, specimens from the western 

 region contained 1.5 cc. of food per pound of body 

 weight, as compared with 0.6 cc. for specimens from 

 the central region and 1.0 cc. for specimens from 

 the eastern region. These three values closely 

 parallel the corresponding average volumes of 

 total fish per stomach (115.3, 55.4, and 95.8 cc). 



Variation with the Current System 



The general pattern of the Pacific equatorial- 

 current system has been described by Sverdrup 

 and associates (1942, pp. 708-712). In brief, the 

 major surface currents of this region are the North 

 and South Equatorial Currents flowing toward the 

 west, with the eastward-flowing Equatorial Coun- 

 tercurrent sandwiched in between. Although the 

 width of the Countercurrent (CC) may vary with 

 longitude and season, its southern and northern 

 boundaries are ordinarily near latitudes 5° X. 

 and 10° X. in the Central Pacific. The South 

 Eqiuitorial Current (SEC) is therefore on both 

 sides of the Equator, while the North Equatorial 

 Current (NEC) is confined entirely to the Xorthern 

 Hemisphere. 



The prevailing east to southeast tradewinds, 

 together with the Coriolis force resulting from the 

 earth's rotation, induce a divergence of the surface 

 waters at the Equator that is accompanied by up- 

 welling. Under certain conditions, described by 

 Cromwell (1953) a convergence may be formed, 

 between the Equator and the southern boundary 

 of the CC, which, we hypothesize, may tend to 

 concentrate plankton and, consec|uently. the tinia 

 forage organisnis. 



Over the range of latitude sampled (17° X".- 

 14° S.), there are therefore certain natural sub- 

 divisions of the environment that may be estab- 

 lished on the basis of the features mentioned above. 

 These may be defined as follows: (1) The XEC 

 from the northern limit of our sampling (17° X.) 

 to the northern boundary of the CC; (2) the CC, 

 with its boundaries determined at the time of each 

 crossing from vertical temperature sections;' (3) a 

 zone of convergence in the SEC extending — accoril- 

 ing to our definition — from the southern boundary 

 of the CC to latitude 11^° X.; (4) a zone of diver- 

 gence or upwelling in the SEC along the Equator 

 from latitude 1J^° X. to latitude \)<i° S.; and (5) 



• Provided in the roports of Murphy and Shomura (1953a, 1953b, 1955). 



