STOMACH CONTENTS OF FISHES 

 INGESTED BY SKIPJACK AND 

 YELLOWFIN TUNAS 



Stomach contents of fishes ingested by skip- 

 jack and yellowfin tunas were examined to 

 learn more about the food chain of yellowfin 

 and skipjack tuna and the trophic dependence 

 of ingested fishes. The results of this examina- 

 tion, as in the study by Nakamura (1965), 

 further confirmed the dependence of forage 

 fishes upon macrozooplankton. The stomach 

 contents of fishes ingested by both species of 

 tunas consisted primarily of crustaceans and 

 fish remains, but some of the larger fishes con- 

 tained remains of squid and juvenile fishes. A 

 Scomber japonicus from the stomach of a 

 yellowfin tuna collected off West Africa held 

 three juvenile Sardinella rouxi (50-60 mm. fork 

 length). 



Because crustaceans were better preserved 

 than fishes, I could distinguish broad taxonomic 

 categories. Although I made no quantitative 

 estimates, copepods appeared to be the domi- 

 nant single group in the diet of most of the 

 ingested fishes. As many as 60 to 100 were 

 found in a single stomach. Among identifiable 

 copepods from the western Atlantic, Cen- 

 tropages typicus, Temora turbinata, and other 

 calanoids were most numerous; Scolecithrix 

 danae, Megacalanua princeps, Scolecithricella 

 sp. and other calanoids were most numerous in 

 samples taken off the coast of Africa. Other 

 identifiable crustaceans were: amphipods, 

 euphausiids, shrimp, Lucifer sp., larval stoma- 

 topods, megalopal stages of Raninidae, and 

 megalopal and zoeal stages of Brachyura. 



VARIATION IN FOOD IN RELATION TO 

 DISTANCE FROM LAND, TIME OF 

 DAY, AND SIZE OF TUNA 



Stomach contents of tunas collected at all 

 locations were classified according to the dis- 

 tance from the nearest land. The arbitrary 

 scale (0-24 nautical miles or 0-43.2 km., 25-49 

 nautical miles or 45.0-88.2 km., 50-74 nautical 

 miles or 90.0-133.2 km., 100-149 nautical 

 miles or 180.0-268.2 km., 150-200 nautical 

 miles or 270.0-360.8 km., 201-300 nautical 

 miles or 361.8-540.0 km., 301-400 nautical 



miles or 541.8-720.0 km. and 401 nautical miles 

 or 721.8 km. plus) used in this study to delin- 

 eate distances from land is the same as that 

 used by King and Ikehara (1956). The mean 

 volumes of 12 taxa most frequently identified 

 as tuna forage from the stomachs of skipjack 

 and yellowfin tuna were plotted for each area 

 and distance. For most of the distances, how- 

 ever, the number of tuna stomachs was too 

 small to warrant any conclusions. 



Data were insufficient for analyses of 

 monthly or seasonal variations, and the trend 

 of feeding throughout the day could be con- 

 sidered only for skipjack tuna from areas 2 

 and 3. Average volumes of stomach contents 

 were calculated for each hour of the day. The 

 percentage of empty stomachs appeared high- 

 est, and the mean volume of food in stomachs 

 lowest, near midday. A chi-square test of 

 heterogeneity (Snedecor and Cochran, 1967: 

 248) indicated that the obsei-ved differences 

 in percentage of empty stomachs near midday 

 was significant (P<0.005) in areas 2 and 3. 



The diurnal fluctuation in feeding was sim- 

 ilar to that observed by other investigators 

 (Talbot and Penrith, 1963; Nakamura, 1965; 

 and Sokolov, 1967). Nakamura (1965) sug- 

 gested that the diurnal feeding pattern of skip- 

 jack tuna was based on availability of food 

 (movement of zooplankton and forage orga- 

 nisms, during the midday period of maximum 

 illumination, to depths beyond those occupied 

 by the surface-dwelling skipjack tuna) or to 

 satiation of tuna after the morning feeding 

 period. 



The relation between the sizes of skipjack 

 and yellowfin tunas and the taxonomic composi- 

 tion of food was studied in terms of frequency 

 of occurrence and percentage volume of the 

 three principal food categories (fishes, mol- 

 lusks, and crustaceans). The occurrence of 

 crustaceans tended to decline as the size of 

 skipjack tunas increased ; the frequency of oc- 

 currence of fish increased with the size of tunas 

 of both species. I observed no relation between 

 the size of tunas and the consumption of 

 cephalopods and other mollusks. 



Spearman's rank correlation analysis (Steel 

 and Torrie, 1960: 409) was used to determine 

 whether the frequency of occurrence of the two 



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U.S. FISH AND WILDLIFE SERVICE 



