THE FOOD OF SKIPJACK AND YELLOWFIN TUNAS 

 IN THE ATLANTIC OCEAN' 



BY ALEXANDER DRAGOVICH, FISHERY BIOLOGIST 

 BUREAU OF COMMERCIAL FISHERIES TROPICAL ATLANTIC BIOLOGICAL LABORATORY 



MIAMI, FLA. 33149 



ABSTRACT 



Samples were examined from the stomachs of 1,060 

 skipjack tuna and 611 yellowfin tuna captured by live 

 bait, longline, trolling, and purse seine in the eastern 

 and western tropical and subtropical Atlantic Ocean in 

 1965 and 1966. Fish, mollusks, and crustaceans were the 

 principal foods of both species. The major families of 

 fish represented were Scombridae, Carangidae, Serrani- 

 dae, Balistidae, Gempylidae, and Tetraodontidae; mol- 

 lusks consisted chiefly of cephalopods (squids); and 

 crustaceans were principally larval macrozooplankton. 

 Frequency of occurrence and displacement volume are 

 given for all taxons of food organisms identified from the 

 stomachs of each species. 



Fish was the predominant forage volumetrically 

 throughout the Atlantic Ocean and numerically in the 

 western Atlantic; crustaceans were predominant nu- 

 merically in tunas caught in the eastern Atlantic. In 

 general, crustaceans predominated in the stomachs of 

 the smaller tunas and fish in the stomachs of the larger 

 ones. When the mean percentages of the three principal 

 sources of tuna food — fish, mollusks, and crustaceans — 

 were compared (for tunas caught concurrently), the 

 evidence was that skipjack tuna ate more crustaceans 

 than did yellowfin tuna. Juvenile tunas were eaten by 

 both skipjack and yellowfin tunas. 



This report on the food and feeding habits of 

 skipjack and yellowfin tunas in the Atlantic 

 Ocean is part of an investigation of the ecology 

 of Atlantic tunas carried on at TABL (Tropical 

 Atlantic Biological Laboratory) of BCF (Bu- 

 reau of Commercial Fisheries). A thorough 

 knowledge of food and feeding habits of tuna 

 is important to an understanding of the biology, 

 abundance, and distribution of tuna species 

 (Reintjes and King, 1953; King and Ikehara, 

 1956; Iversen, 1962; and Nakamura, 1969). 



Atlantic tuna fisheries are sustained in great 

 part by five species considered of woi'ldwide 

 importance: yellowfin tuna (Thunnus alha- 

 cares), skipjack tuna {Katsntconus pelumis), 

 albacore (T. alalunfja), bigeye tuna (T. obesus), 

 and bluefin tuna (T. thynnus). 



The literature on the food and feeding habits 

 of tunas in the Atlantic Ocean was reviewed by 

 Dragovich (1969). Many publications are avail- 

 able on the feeding habits of the various species 

 of Atlantic tunas, but most of them are based 

 on casual observations involving small samples. 

 Some papers consist of mere taxonomic listings 



1 Contrilnition No. lOTj, Muir.iu of Commcrrial Fisheries Tropical 

 Atlantic liioloKiial I.aljoiutoiy. Miami. Fla. :i:il4'J. 

 rublishcil Ni.vcmb.r 1U70. 



of forage organisms found in tuna stomachs; 

 others contain lists of forage organisms and 

 morphometric measurements ; few include 

 quantitative analyses of data, or consideration 

 of the relation of forage organisms to geo- 

 graphic areas, to the size of tunas, or to the 

 habitat. 



MATERIALS AND METHODS 



Of 1,671 tuna stomachs examined (table 1), 

 about 55 percent were collected on cruises by 

 the research vessels Gcronimn and Undaunted 

 of TABL; 23 percent were supplied to TABL 

 by ORSTOM (OflRce de la Recherche Scienti- 

 fique et Technique Outre-Mer), Pointe Noire, 

 Africa; and the rest came from other BCF 

 laboratories and commercial vessels. All data 

 were collected during 1965 and 1966, from 

 three arbitrarily defined areas of the Atlantic 

 Ocean (fig. 1). 



In area 1, skipjack tunas were caught by 

 purse seine and yellowfin tunas on longlines ; in 

 areas 2 and 3, all tunas were caught at the 

 surface by live bait and trolling. 



The fork lengths of the fish measured (figs. 

 2 and 3) ranged from 22 to 81 cm. for skipjack 



FISHERY BULLETIN: VOL. G8. NO. 3 



445 



