Foods of Yellowfin and Blackfin Tuna 49 



brates. Eleven of the 28 food categories occurred in the stomachs of 

 both species, and 6 of the 10 most important categories to blackfin also 

 ranked in the top 10 for yellowfin. The obvious conclusion is that both 

 species have similar diets when they occur together off the coast of 

 North Carolina. Statistically, however, the correlation coefficients were 

 all nonsignificant at the 0.05 (0.344; 29 dO level. The correlation coeffi- 

 cients were: Spearman, 0.2273; Kendall, 0.1451; and Pearson, 0.2273. 



Comparative Diets by Predator Size 



Differences in stomach contents by fish size may of course be 

 attributable merely to the availability of food in the environment, but 

 they may also be attributable either to a change in food preference, or 

 to the ability of the predator to capture and swallow certain organisms 

 as it increases in size. Our objectives of comparing diets by tuna size 

 were to determine if near-surface feeding was related to tuna size and to 

 ascertain if basic changes in the diets occurred as the fish grew larger. 



Different studies throughout the world's oceans generally suggest 

 that as tunas grow larger, their diets change. Reintjes and King (1953) 

 reported that the overall high occurrence of crab larvae, stomatopod 

 larvae, squid, and juvenile fishes indicates a preference by Pacific yel- 

 lowfin tuna for small food items. These authors further explained that 

 small tuna feed predominantly on crustacean larvae; medium-size fish 

 feed on fish, crustacean larvae, and squid; and large yellowfin mainly 

 consume fish and squid. These findings were substantiated by Nakam- 

 ura (1965) and Batts (1972) for skipjack tuna whose diets reflected a 

 decline in crustaceans and a subsequent higher percentage of fish, as 

 tuna size increased. 



To accomplish our evaluations we first grouped the fish into size 

 classes (mm FL) (Tables 4, 5). Next, selected food groups — fish, adult 

 fish, juvenile fish, invertebrates, squid, larval crustaceans and plants — 

 were established to demonstrate food size (i.e., adult fish vs. larval crus- 



Table 4. Selected food items consumed by different sized yellowfin tuna, 

 expressed as percent frequencies of occurrence. 



Fish size (mm FL) 

 Contents 501-700 701-900 901-1100 >1100 



Fish 



Adult fish 



Juvenile fish 



Invertebrates 



Squid 



Larval crustaceans 



Plants 



77.8 



81.8 



75.0 



73.8 



5.5 



10.9 



15.0 



9.5 



16.7 



12.7 



36.4 



11.9 



77.8 



89.1 



76.3 



85.7 



44.4 



34.5 



56.3 



64.3 



38.9 



70.9 



35.0 



35.7 



55.5 



32.7 



30.0 



14.3 



