Preliminary Report on the Feeding Habits of Tunas 

 in the Gulf of Guinea^ 



By 



PAUL N. SUND, Oceanographer 



and 



WILLIAM J. RICHARDS, Research Systematic Zoologist 



Tropical Atlantic Biological Laboratory 



Bureau of Commercial Fisheries 



Miami, Florida 33149 



ABSTRACT 



The stonnachs of 171 yellowfin tuna ( Thunnus albacares ) and 72 skipjack tuna 

 ( Katsuwonus pelamis ), captured in the Gulf of Guinea, contained mostly fishes, 

 cephalopods, and crustaceans. The percentage composition by volumeof these three 

 food categories was: 55, 19, and 8 for yellowfin tuna; 96, 1, and 2 for skipjack tuna. 

 The percentage frequency of occurrence was: 76, 40, and 53 for yellowfin tuna; 

 73, 14, and 22 for skipjack tuna. The occurrence of forage species varied little in 

 geographic areas within the region, but differences did occur between the food 

 species found in stomachs of fish captured in the "warm" and "cool" seasons. 

 A trennatode parasite of the genus Hirudinella was found in stonnachs of both tuna 

 species . 



INTRODUCTION 



A major objective of the Tropical Atlantic 

 Biological Laboratory is to aid the U.S. fish- 

 ing industry, and the peoples of nations border- 

 ing the tropical Atlantic Ocean, through studies 

 that will increase knowledge of marine food 

 resources. Recently emphasis has been di- 

 rected toward an investigation of the tuna re- 

 sources in the Gulf of Guinea. A significant 

 part of this program is the description of the 

 components of the nekton connmunity upon 

 which tunas feed, because the distribution and 

 movements of tunas often are directly related 

 to the availability of proper forage. 



This is a preliminary report on the forage- 

 predator relation of tuna stocks in the Gulf of 

 Guinea. Studies of the food of tunas are numer- 

 ous, but none exist that concentrate on the 

 Gulf of Guinea, and only a few deal with the trop- 

 ical Atlantic Ocean (Beebe, 1936;Postel, 1954; 

 Anderson, Gehringer, and Cohen, 1956a, b; 

 Anderson and Gehringer, 1956, 1957a, b, 1958a, 

 b, 1959a, b, c; Suarez-Caabro and Duarte- 

 Bello, 1961). Alverson (1963) reviewed the 

 literature on the food habits of tunas and 



' Contribution No. 41, Bureau of Commercial Fisheries 

 Tropical Atlantic Biological Laboratory, Miami, Fla. 

 33149. 



concluded that despite their widely varied diet, 

 tunas in a given locality ate in quantity only a 

 small variety of forage organisms. 



METHODS 



The samples on which this report is based 

 were collected during January- May and July- 

 October 1964, on cruises of the research 

 vessel Geronimo of the Bureau of Commercial 

 Fisheries. The collections were made in the 

 Gulf of Guinea between long. 7° 30' W. and 

 long. 3° 30' £!., within about 100 nautical miles 

 (185 km.) of the West African coast. The area 

 covered and the locations of tuna sightings and 

 captures of yellowfin tuna ( Thunnus albacares ) 

 and skipjack tuna ( Katsuwonus pelamis) are 

 shown in figure 1. 



During the surveys, 171 yellowfin and 72 

 skipjack tunas were examined. The plan of the 

 field program was to catch at least 25 fish 

 from every yellowfin and skipjack school 

 encountered. Actual nunnbers taken fronn each 

 school successfully fished ranged fronn 1 to 

 49, or an average of 5. 



The fork length, weight, and sex of each 

 fish were recorded, and the stomach and 

 gonads (fennales only) were rennoved and 



