50 Charles S. Manooch, III and Diane L. Mason 



Table 5. Selected food items consumed by different sized blackfin tuna, 

 expressed as percent frequency of occurrence. 







Fish size 



(mm FL) 





Contents 



<500 



501-700 



701-900 



901-1100 



Fish 



50.0 



57.4 



87.1 



100.0 



Adult fish 



0.0 



7.4 



25.8 



0.0 



Juvenile fish 



50.0 



16.7 



9.7 



0.0 



Invertebrates 



100.0 



90.7 



64.5 



100.0 



Squid 



50.0 



31.5 



25.8 



100.0 



Larval crustaceans 



100.0 



66.7 



38.7 



0.0 



Plants 



50.0 



14.8 



9.7 



0.0 



taceans) and materials that we believed to be consumed on or near the 

 surface (i.e., floating plants). Contents are presented as percent fre- 

 quency of occurrence (Tables 4, 5). 



Yellowfin Tuna. — Size of food items showed little change as fish size 

 increased or decreased (Table 4). The three key food categories — adult 

 fish, juvenile fish, and larval crustaceans — neither steadily increased 

 nor decreased in occurrence as tuna size increased. This finding is con- 

 trary to that of Dragovich (1970), who found that the frequency occur- 

 rence offish in stomachs of yellowfin increased with fish size. However, 

 he discovered no relationship between squid in the diet and tuna size. In 

 our study, the occurrence of floating plants decreased for the larger size 

 classes, indicating that perhaps smaller individuals fed more extensively 

 near the surface. 



Blackfin Tuna. — The size of prey items and feeding proximity to the 

 surface appeared to change with fish size. As fish size increased, large 

 food items (i.e., adult fish) generally occurred more frequently, and 

 small food items (i.e., larvel crustaceans and juvenile fish) occurred less 

 frequently (Table 5). Surface feeding, as suggested by the incidental 

 ingestion of floating plants, decreased as fish attained larger sizes. 

 Volumes of Contents Related to Species and Fish Body Weight 



Since the quantity and types of foods ingested by fishes are often 

 converted into caloric equivalents for energetics studies, we present fre- 

 quencies of the range of food volumes for the two species (Table 6). The 

 displacement volume for yellowfin averaged 67.9 ml (72.2 g), compared 

 with 28.6 ml (29.6 g) for blackfin tuna. Volumes of stomach contents of 

 yellowfin and blackfin varied from 0.1 to 745.0 ml and from 0.1 to 257.5 

 ml, respectively. The largest volumes were found in a 40 kg yellowfin 

 and an 8.8 kg blackfin. The volume range for yellowfin from the Pacific 

 was similar, 0.1 to 1,000 ml (Reintjes and King, 1953). The extremes in 

 our data were much greater than those described by Dragovich and 

 Potthoff (1972): 0.1 to 20.0 for skipjack, and 0.1 to 60.0 ml for yellow- 

 fin tunas collected off the west coast of Africa. In our study, approxi- 



