FOOD OF YELLOWFIN TUNA 



103 



pelagic amphipods and shrimps. Squid and 

 fish were staple foods in all environments, although 

 the former was consistently higher in average 

 volume in the larger yellowfin. 



TROLL POLE -AND- LINE LONGLINE 



CAUGHT CAUGHT CAUGHT 



Figure 7. — Differences in the composition of the food of 

 yellowfin (1,000-1,299 mm. fork length) with method 

 of capture, for two general localities. The troll- and 

 pole-and-line-caught fish were captured within 10 miles 

 of land, the longline-caught fish within 25 miles of land. 

 N= number of stomachs. 



As in the Crustacea, there were differences in 

 the kinds of fish found in the food of yellowfin 

 from different environments. Fish families com- 

 monly occurring in stomachs of the near-shore 

 yellowfin were Balistidae, Acanthuridae, and 

 Carangidae. For the offshore yellowfin, repre- 

 sentatives of Bramidae, Exocoetidae, and Gempy- 

 lidae were more prevalent in the food. The food 

 of 11 troll-caught yellowfin captured more than 10 

 miles from land consisted of 57 percent fish, 42 

 percent squid, and about 1 percent Crustacea, 

 the average volumes per stomach being, 

 respectively, 32.3, 23.8, and 0.6 cc. 



From the beginning of the study it was obvious 

 that not only the volume of stomach contents 

 but also the composition of the food varied with 

 the size of the yellowfin. While the same foods 

 were eaten by both large and small tuna, they were 

 not consumed in the same proportions. Since 

 large, medium, and small yellowfin were taken by 

 surface fishing from the same coastal areas, we 

 assume that they had equal opportunity to feed 

 on a common source of food organisms. The 

 differences in stomach contents may be due either 

 to a change in food preference or to the ability to 

 catch and swallow certain organisms as the tuna 

 increases in size. Figure 9 shows the variation 

 in the three major food categories with increase in 

 size of yellowfin. The most marked differences are 

 the low percentage of Crustacea and the high 

 percentage of fish in the large surface-caught 

 yellowfin, and the apparently greater importance 

 of squid in the food of small and medium-sized 

 yellowfin. As all but 2 percent of the surface- 

 caught fish were taken less than 10 miles from land, 

 the variation in the size of the yellowfin tuna 

 captured appears to be the greatest variable. 

 Table 6 summarizes the stomach contents of 775 

 of the 781 fish taken by surface fishing. The data 

 arranged in arbitrary size categories show that the 

 stomachs of the smaller fish contained more crusta- 

 cean elements and fewer fish than did those of the 

 larger specimens. Squid are a relatively unim- 

 portant fraction in all sizes of fish caught near 

 land at the surface. On the other hand, longline- 

 caught (subsurface) yellowfin tuna were mostly 

 larger fish and showed little variation in size; 

 however, the locality of capture, in reference to 

 distance from land, did vary greatly. Table 7 

 summarizes the stomach contents of 311 fish 

 taken well below the surface by longline fishing. 



