FISHERY BULLETIN: VOL. 70, NO. 4 



penetration of Formalin' and placed in a labeled 

 polyethelene bag containing lO-^r Formalin. 



In the laboratory the stomachs were first clas- 

 sified into those containing food and those that 

 were empty. The stomach contents were then 

 identified to the lowest possible taxonomic units 

 which were subsequently sorted, counted, and 

 their displacement volumes measured. Length 

 measurements were taken of many forage or- 

 ganisms, particularly fishes. Bait fishes were 

 found in some of the stomachs, but they were 

 not considered as part of the regular diet of 

 skipjack and yelloA\iin tunas; therefore, stom- 

 achs which contained only bait were considered 

 empty. Stomachs that contained parasitic trem- 

 atodes were also considered empty. 



This study was no exception in regard to dif- 

 ficulties encountered in the identification of for- 

 age organisms (Dragovich, 1969) . In numerous 

 instances the identification of ingested fishes, 

 particularly juvenile tunas, was made from ver- 

 tebrae using methods employed by PotthoflP and 

 Richards (1970) . Cephalopod identification was 

 particularly difficult since many diagnostic ex- 

 ternal characters usually are the first destroyed 

 during digestion. 



The following methods of analysis were used: 

 1) the volumetric method — the individual vol- 

 ume of each taxon and the total aggregate vol- 

 ume of broad taxonomic groups, 2) frequency 

 of occurrence method — the frequency of occur- 

 rence of a food item and of broad taxonomic 

 groups, and 3) numerical method — number of 

 individuals in the same taxonomic group. 



Spearman's rank correlation test, x" test of 

 homogeneity, and paired ^-test of diff"erence be- 

 tween the means were used. A method consist- 

 ing of ranking of food organisms according to 

 their geographic distribution, relative abun- 

 dance, and biomass was also employed. 



COMPOSITION OF FOOD 



Fishes, crustaceans, and cephalopods were the 

 three principal food categories found in stom- 



' Use of trade names does not imply endorsement by 

 the National Marine Fisheries Service. 



achs of both skipjack and yellowfin tunas (Fig- 

 ure 3). Food items that do not fall into these 

 three categories consisted of mollusks other than 

 cephalopods, salps, polychaetes, and siphono- 

 phores. Other mollusks and salps were found 

 in both species of tunas; polychaetes and siphono- 

 phores were present only in stomachs of skip- 

 jack tunas. A checklist of all food items, num- 

 ber of organisms, frequency of occurrence, dis- 

 placement volumes, and length measurements of 

 some organisms are presented according to the 

 cruises in Appendix Tables 1 to 4. Fishes were 

 represented in the diet of skipjack and yellow- 

 fin tunas by 90 different taxa, crustaceans by 45, 

 and mollusks by 24. 



The percentage composition of five food cat- 

 egories in terms of number, volume, and fre- 

 quency of occurrence is shown in Figure 3. Fish 

 was the dominant food item by volume for both 

 species of tunas, except for yellowfin tuna cap- 

 tured during UN6802, when cephalopods were 

 dominant. Fish occurred most frequently in the 

 diet of both species of tunas sampled during 

 UN6801; however, crustaceans occurred most 

 often in the collections from UN6802. In the 

 diet of yellowfin tuna, fishes were numerically 

 the most important food items during both cruis- 

 es; in the diet of skipjack tuna, fishes were the 

 most important by numbers during UN6801, but 

 crustaceans were most numerous during 

 UN6802. 



The group of forage organisms classed as 

 other mollusks consisted primarily of pteropods 

 and heteropods. Salps, polychaetes, and siphono- 

 phores were the principal components of the 

 group of forage organisms classed as miscella- 

 neous — this group was not prominent by vol- 

 ume, frequency of occurrence, or by numbers. 



FISHES 



Fishes utilized as food consisted mainly of 

 postlarval and juvenile forms of pelagic and reef 

 fishes. Some adult fishes, primarily Vinciguerria 

 nimharui, were also present in the diet of both 

 species of tuna. Although fishes were repre- 

 sented by a larger number of families, only a few 

 families were important in terms of volume, 

 frequency of occurrence, and numbers. 



1090 



