FISHERY BULLETIN: VOL. 70, NO. 4 



spp. were found in the stomachs of both species 

 of tunas, but only during UN6801. Sarda sarda 

 and Scomber japonicus were also found in both 

 skipjack and yellowfin tunas, but only during 

 UN6802. 



The presence of juvenile tunas in the diet of 

 skipjack and yellowfin tunas in various parts of 

 the Atlantic Ocean has been reported by Drago- 

 vich (1969, 1970). Presence of Auxis spp. and 

 Scomber sp. in the diet of yellowfin tuna from 

 east African waters was noted by Williams 

 (1966). Suarez Caabro and Duarte Bello (1961) 

 noted juvenile blackfin tuna (Thunnus atlan- 

 ticus) (5-150 mm fork length) and skipjack tuna 

 (35-145 mm fork length) in the stomachs of 

 skipjack tuna from the Caribbean Sea. Pres- 

 ence of juvenile tunas in the diet of adult tunas 

 has been frequently observed in food studies in 

 the Pacific Ocean (Reintjes and King, 1953; 

 King and Ikehara, 1956; Alverson, 1963; Naka- 

 mura, 1965). 



COMPARISON OF FOOD OF 

 SKIPJACK AND YELLOWFIN TUNAS 



As in a previous study by Dragovich (1'970), 

 our data show a marked taxonomic similarity of 

 items in the diet of skipjack and yellowfin tunas 

 for the investigation area as a whole (Appendix 

 Tables 1-4). We also compared the taxonomic 

 composition of forage organisms at the two lo- 

 cations where skipjack and yellowfin tunas were 



Table 3. — The distribution of displacement volumes of 

 individual forage organisms collected during the cruises 

 of UN6801 and UN6802. 



Food 

 item 



Total ronge of 

 displacement 

 volumes (ml) 



Displacement 

 volumes in 90% 

 of observations 



caught together in a mixed school. For those 

 locations we performed x" tests of homogeneity 

 on the ratio of fish to total volume of food. The 

 first test indicated that the percentage of fish 

 consumed differs between the two locations 

 (X^ = 6.74; 1 df; P < 0.1) possibly reflecting 

 differences in forage-at-large composition, times 

 of day, size frequency of tuna, etc. Within-area 

 difference in percent fish between yellowfin and 

 skipjack tunas was significant in only one area 

 (X' = 62.51; Idf; P < 0.01). 



VARIATION IN FOOD AS RELATED TO 



SIZE OF TUNAS AND VOLUME OF 



STOMACH CONTENTS 



The foods of skipjack and yellowfin tunas in 

 the present study consisted principally of rel- 

 atively small organisms, based on their displace- 

 ment volumes (Table 3). The consumption of 

 organisms of comparable size in similar propor- 

 tions by skipjack and yellowfin tunas has also 

 been observed by other investigators (Reintjes 

 and King, 1953; King and Ikehara, 1956; Naka- 

 mura, 1965; Williams, 1966; Dragovich, 1970). 



To observe the differences in consumption of 

 food by volume and frequency of occurrence of 

 the three major food categories as related to size 

 of tunas, skipjack and yellowfin tunas were sep- 

 arated into 20 mm and 30 mm length intervals 

 respectively (Figure 4). Spearman's rank cor- 

 relation analysis (Steel and Torrie, 1960:409) 

 was used to see if the volumes and frequency of 

 occurrence of the two dominant forage food 

 items (fishes and crustaceans) in the diet of 

 skipjack and yellowfin tunas were correlated 

 with the size of tunas. Significant correlations 

 in the length-food data were noted between the 

 size of skipjack tuna and percentage volume of 

 fish forage (rs 0.576, 11 df,P < 0.05) and per- 

 centage of occurrence of forage fish {rs 0.565, 

 11 df, P < 0.05), suggesting that as the size 

 of tuna increased, the percentage consumption 

 of fish by volume and by frequency of occurrence 

 increased. 



It is generally recognized that the amount and 

 quality of food found in the stomach of tunas 



1094 



