COMPARATIVE STUDY OF FOOD OF SKIPJACK AND YELLOWFIN 

 TUNAS OFF THE COAST OF WEST AFRICA' 



Alexander Dragovich and Thomas Potthofp^ 



ABSTRACT 



Stomach contents of 711 skipjack tuna (Katsmvonus pelamis) and 132 yellowfin tuna 

 {Thunnus albacares) captured in 1968 by live bait and trolling off the coast of West 

 Africa were examined. A marked taxonomic similarity was noted between the organisms 

 in the diets of the two tunas. Frequency of occurrence, displacement volume, and num- 

 bers of each food item identified are presented for each species of tuna. Fishes, mollusks, 

 and crustaceans were the principal foods with fishes generally dominant. The most 

 prominent fish families were Acanthuridae, Carangidae, Dactylopteridae, Gempylidae, 

 Gonostomatidae, Lutjanidae, MuUidae, Priacanthidae, Scombridae, Serranidae, and 

 Trichiuridae; mollusks were chiefly cephalopods (squids), and crustaceans consisted 

 mostly of macrozooplankton. Juvenile tunas were present in the diet of both species 

 of tunas. 



Estimates of the size of forage organisms were primarily based on displacement vol- 

 umes. In the majority of observations, food organisms displaced less than 1.0 ml and 

 the displacement volumes of stomach contents varied for skipjack tuna from 0.1 to 20.0 ml 

 and for yellowfin tuna from 0.1 to 60.0 ml. 



Spearman's rank correlation analysis was used to test for a relation between the food 

 type (in volume and frequency of occurrence) and the lengths of skipjack and yellowfin 

 tunas. Significant correlations were noted between the size of skipjack tuna and both 

 the volume and the frequency of occurrence of forage fish. 



A comparison between the findings of our study and that of other food studies off 

 the coast of West Africa showed greater taxonomic similarity in tuna forage when the 

 studies were made in the same general area and that only several types of food were of 

 primary importance in each given area. Seasonal changes in taxonomic composition 

 of forage organisms were also discussed. 



The method used to evaluate food organisms consisted of ranking the organisms ac- 

 cording to their dispersal indices, abundance indices, and biomass contribution. Stomato- 

 pods, the amphipod Phrosina semiluyiata, Teuthoidea, Carangidae, Serranidae, and 

 megalopal stages were most important constituents of food throughout the investigation 

 area. 



The principal surface tuna fishery in the trop- 

 ical Atlantic Ocean is located off the coast of 

 West Africa (Jones, 1969). One of the major 

 tasks of the Southeast Fisheries Center, Miami 

 Laboratory, has been the study of the biology 

 and ecology of tunas and tunalike fishes in the 



' Contribution No. 218, National Marine Fisheries 

 Service, Southeast Fisheries Center, Miami Laboratory, 

 Miami, Fla. 



^ National Marine Fisheries Service, Southeast Fish- 

 eries Center, Miami Laboratory, 75 Virginia Beach 

 Drive, Miami, FL 33149. 



Manuscript accepted February 1972. 



FISHERY BULLETIN: VOL. 70, NO. 4, 1972. 



tropical Atlantic Ocean. In view of the recog- 

 nized importance of food as an ecological factor 

 in the life history of tunas, one project of this 

 investigation consisted of a study of the food 

 and feeding habits of skipjack (Katsmvonus 

 pelamis) and yellowfin {Thunmis albacares) 

 tunas — the two predominant species in com- 

 mercial catches in those waters. 



We describe and compare the food of skipjack 

 and yellowfin tunas and discuss the relative im- 

 portance of different forage organisms. We 

 compare our findings with those of other investi- 

 gators working in the same general area. This 



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