466 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



westward-flowing South Equatorial Current 

 (SEC) from approximately 5° K, to 11/0° N.; (2) 

 a zone of upwelling in the SEC from li/^° N. to 

 11/2° S. ; (3) the SEC from 11/0° S. to 5° S., an area 

 with a deep thermocline; and (4) the SEC from 

 5° S. to 16° S., the southern limit of sampling and 

 a region of shoaling thermocline towards the 

 south. This representation of the SEC does not 

 take into account Reid's (1959) report of a weak 

 easterly countercurrent near 10° S. 



"Values for zooplankton and trawl-caught orga- 

 nisms (small nekton for the most part) were high- 

 est near and just north of the Equator, where the 

 abundance of albacore was lowest. The assump- 

 tion has often been made that, all other things 

 being equal, areas with the higher concentrations 

 of zooplankton and small nekton should support 

 the higher concentrations of large carnivores, such 

 as albacore. The fact that this assumption is not 

 supported by the data shown in figure 7 probably 

 means that environmental factors other than the 

 abundance of available food, such as water tem- 

 perature, exert a strong influence on albacore 

 distribution. 



Whether or not the albacore captured in the 

 areas of high zooplankton and nekton abundance 

 were utilizing available forage to best advantage 

 is not clear from figvire 7, since the total range of 

 stomacli volumes was only 0.4—0.7 cc./lb. of body 

 weight. The lowest stomach volumes occurred 

 in the latitudes of best albacore catches, a situa- 

 tion similar to that found for yellowfin tuna in the 

 equatorial area (King and Ikehara, 1956). One 

 factor which adds to the uncertainty is that the 

 trawling ujion which the values shown in section 

 B of figure 7 are based was done at night and many 

 of the animals captured are not utilized by tunas 

 as forage (King and Iversen, 1962). However, 

 King and Iversen also found a high positive cor- 

 relation between trawl catches and zooplankton 

 abundance, and the assumption that trawling re- 

 sults are a valid estimate of potential tuna forage 

 should not be dismissed, since some animals that 

 make up a considerable portion of the trawl 

 catches (e.g. myctophids) are actively pursued by 

 animals which make up a large fraction of albacore 

 food, such as squid. 



The utilization of forage was further investi- 

 gated by examining the occurrence and amounts 

 of squid, fish, and Crustacea in the diet of equa- 

 torial albacore. The results are shown in figures 

 8 and 9. 



I00| ,50, ^ ^ 1 



(62) SQUID  ^B ^M 



jl III 



IUU| -(18)- 1 



'Jl III 



80 

 ^ 60 



UJ 



^ 40 

 20 

 



CRUSTACEA 



)~ ■!■ (^ (18) H - 



fi I i li 



I6°S.-5°S. 5°S.-I°30'S. l°30'S.-l°3ON. I°30'N,-5»N. 



LATITUDE 



Figure 8. — Percent occurrence of major food Items in the 

 diet of equatorial kmgline-caught alliacore, 120° W.- 

 180°. (Numbers in parentheses indicate sample sizes.) 



Squid occurred more often and in larger 

 amounts than fish in the stomachs of albacore 

 taken from li/4° S. to 5° N. This is contrary to 

 the findings of King and Ikehara (1956), who 

 found that fish formed overall a larger portion 

 of yellowfin and bigeye food in equatorial waters 

 than did squid. 



There were also differences in the fish consumed 

 by longline-caught albacore in the equatorial Pa- 

 cific when compared to longline-caught albacore 

 from the temperate North Pacific (table 3). It is 

 puzzling tliat no bramids or gempylids were found 

 in the stomachs of longline-caught albacore from 

 the temperate North Pacific, since species of both 

 families occur in this area. It may be that bramids 

 and gempylids are distributed close to the surface 

 in the north and in deeper water in the equatorial 

 area. In such a case they would not be as available 

 to the albacore fished with longlines in the north 

 as they would be to the albacore exploited by the 

 same gear in equatorial waters. A possibly anal- 

 ogous tropical submergence or deepening of 



