below the maximum capacity (7 percent of 

 body weight) shown by Magnuson. Thus, my 

 figure of 20 ml. probably does not represent 

 the average daily ration for either species, par- 

 ticularly because the volumes were as high as 

 149 ml. in skipjack tuna and 499 ml. in yellow- 

 fin tuna. The fact that less than 20 ml. of food 

 was found in most stomachs I examined (both 

 species) also may suggest that (1) food is not 

 always available and (2) the rates of digestion 

 are rapid. 



COMPARISON OF FOOD OF SKIPJACK 

 TUNA AND YELLOWFIN TUNA 



When data from all areas are combined, the 

 large number of forage taxa common to skip- 

 jack and yellowfin tunas indicates a marked 

 similarity in the diets of the two species. When 

 the taxonomic composition of the food in stom- 

 achs of the two species was compared at loca- 

 tions where they were caught simultaneously, 

 however, less than half of the total taxa at 23 

 of 25 stations were common to both species — 

 suggesting that feeding is selective. Crusta- 

 ceans made up the greatest number of taxa 

 common to both species, and fishes the least 

 (fig. 5). 



Consideration of feeding selectivity of skip- 

 jack and yellowfin tunas required that the data 

 be examined quantitatively. The percentage 

 volume for each food category (fish, mollusks, 



and crustaceans) and for each station was 

 transfonned by Y = arc-sin Vx, where x is the 

 fraction of the volume and y the transformed 

 variate upon which the Spearman's coefficient 

 of rank correlation was applied to see if the 

 volume of fish among food constituents varied 

 directly or inversely between the skipjack and 

 yellowfin tunas. I found that the rank correla- 

 tion coefficient of 0.241 was not significant. A 

 comparison (paired i-test) of the mean per- 

 centages of the three main food categories (fish, 

 crustaceans, and mollusks) at the same stations 

 gave evidence that the mean percentage of 

 crustaceans in the stomachs was greater in 

 skipjack tuna than in yellowfin tuna (P<0.01). 

 Findings by other investigators concerning 

 selectivity in feeding by different species of 

 tunas varied. Among fish from the same loca- 

 tions in the Pacific, Iversen (1962) found that 

 the food of albacore was more similar to that 

 of yellowfin tuna than to that of bigeye tuna — 

 even though bigeye tuna and albacore inhabit 

 deeper water than yellowfin tuna. Some selec- 

 tivity in feeding might be inferred from Iver- 

 sen's study. King and Ikehara (1956) studied 

 the food of bigeye and yellowfin tunas in the 

 central Pacific and concluded that their forage 

 was, in general, remarkably similar. They 

 stated that when the two species live in the 

 same area, food habits are alike, and that food 

 selectivity is probably attributable to a move- 

 ment by the tunas to different feeding areas. 



MOLLUSKS 



CRUSTACEANS 



FISHES 



TOTALS 



SJ YF BOTH 



SJ 



YF 



BOTH 



SJ 



YF 



BOTH 



SJ+YF BOTH 



1:1 



Number of taxa - ii.ii | iiii| 



5 10 



Figure 5.— Number of different forage taxa from the stomachs of SJ (skipjack tuna) and YF (yellowfin tuna), 



collected simultaneously at the same stations. 



454 



U.S. FISH AND WILDLIFE SERVICE 



