Foods of Yellowfin and Blackfin Tuna 51 



Table 6. Frequencies of food volumes by species of tuna. 





Yellowfi 



n tuna 



Blackfin tuna 



Volume range (ml) 



Number 



Percent 



Number 



Percent 



O.I- 10.0 



64 



32.6 



46 



51.7 



10.1- 50.0 



67 



34.2 



26 



29.2 



50.1-100.0 



24 



12.2 



10 



11.2 



100.1-150.0 



15 



7.7 



3 



3.4 



150.1-200.0 



9 



4.6 



3 



3.4 



200.1-250.0 



6 



3.1 



- 





250.1-300.0 



3 



1.5 



1 



1.1 



300.1-350.0 



3 



1.5 



- 



- 



350.1-400.0 



1 



0.5 



- 



- 



400.1-450.0 



1 



0.5 



- 



- 



450.1-500.0 



1 



0.5 







500.1-550.0 



- 



- 



- 



- 



550.1-600.0 



- 



- 



- 



- 



600.1-650.0 



- 



- 



- 



- 



650.1-700.0 



1 



0.5 



- 



- 



700.1-750.0 



1 



0.5 

 99.9 



89 



- 



Totals 196 



100.0 



mately 33% of the yellowfin had food volumes exceeding 50 ml, a pro- 

 portion similar to that of the 29% found by Reintjes and King (1953). 

 By comparison, Dragovich (1970) noted volumes of less than 20 ml for 

 85% of the yellowfin from the Atlantic. We found that only 19% of the 

 blackfin, a much smaller species, had contents over 50 ml. 



To determine the relationship of volume to fish body weight, we 

 first derived the following equation for converting volume in ml to 

 volume in grams: 



Volg = -1.4009 + 1.0846 (Volj^i), N=25, r=0.999. 

 Comparisons were then made between estimates of stomach contents 

 and the body weights of some of the tunas selected at random. Percent- 

 ages of food weight to fish weight varied from trace (<0.002) to 2.02 

 for yellowfin, and from 0.02 to 3.20 for blackfin tuna. Qnly 10% of the 

 yellowfin had contents exceeding 1% offish body weight, whereas 20% 

 of the blackfin tuna had contents exceeding this percentage. Usually our 

 observations were well below 1%, as were those of Dragovich (1970). 



In summary, yellowfin and blackfin tuna appear to be fast, aggres- 

 sive predators capable of capturing swift, relatively large prey. On the 

 other hand, they use their gill apparatus to strain small, near-surface 

 items from the water. During feeding, non-food materials (inorganic as 

 well as organic) are ingested, probably incidental to normal prey. The 

 variability of specific food organisms within the major categories (fish, 

 cephalopods, and crustaceans) in the diets suggests that tunas are non- 

 selective feeders. This is undoubtedly a factor in their wide geographic 



