frozen. Later, in the laboratory, the wet-dis- 

 placement volume of the contents of each 

 stomach was measured, and the food organ- 

 isms were identified to the lowest possible 

 taxonomic unit. The volume and number of 

 specimens in each taxon were recorded to 

 deternnine the relative volume and the numer- 

 ical importance of the various forage organ- 

 isnns and groups. Bait fishes used for chum- 

 ming were not included in the calculations. 



STOMACH CONTENTS 



Food was present in 83 percent of the yellow- 

 fin tuna stomachs and 51 percent of the skip- 

 jack tuna stomachs. The contents consisted 

 nnainly of three categories: fishes, cephalo- 

 pods, and crustaceans (table 1). Other itenns 

 included salps, pteropods and heteropods, and 

 parasites. 



Table 1. — Percentage composition by volume and 

 percentage frequency of occurrence of the 

 food consumed by yellowfin and skipjack 

 tunas captured from the Gulf of Guinea, 

 igS'i. Fish with empty stomachs were not 

 included . 



Certain species of forage nekton appeared 

 in the stomachs of tunas more often than 

 others (table 2). Organisms con-imonly found 

 in yellowfin tuna stomachs were: Vomer seta- 

 pinnis , Auxis sp., Acanthurus sp., Dactylop- 

 terus volitans, Abralia veranyi , Bathypolypus 

 sponsalis , HI ex illecebrosus coindetii, Ony- 

 coteuthis banksi , Brachyscelus sp., pagurid 

 larvae, brachyuran larvae, and stomatopod 

 larvae. Skipjack tuna stomachs frequently con- 

 tained D. volitans and pagurid larvae. 



Some forage species were found exclusively 

 in yellowfin tuna stomachs, but others were 



used as food by both skipjack and yellowfin 

 tunas. The use of the sanne species might be 

 interpreted as an example of direct competi- 

 tion for certain forage organisms, but com- 

 petition is extremely difficult to document 

 because of the many factors that tend to re- 

 duce it (Cause and Witt, 1935), The possibility 

 that these two species may compete for food 

 should be considered in future investigations. 



The occurrence of forage species varied 

 little in geographic areas within the region. 

 Some species were found only off the Republic 

 of Ivory Coast or off Ghana but were taken so 

 seldonn that no conclusions on their occur- 

 rence are justified. Stomatopod larvae were 

 abundant in stonnachs of tunas collected off 

 Ivory Coast during January-May and off Ghana 

 during July-October, possibly because the 

 larvae were transported westward by the 

 Guinea Current, or because the spawning area 

 of the adult population shifted. 



Seasonal differences were apparent in the 

 occurrence of some organisms in the tuna 

 stomachs. Anchoviella guineensis , Bathypoly- 

 pus sponsalis , and Abralia veranyi were found 

 only in stomachs of fish taken during the "cool" 

 season. Hippocampus sp., Trichiurus sp., an 

 unidentified gonastomatid, Oxyporhamphus 

 micropterus , Atlanta sp., Ill ex illecebrosus , 

 and a species of unidentified salp were present 

 only in samples taken during the "warm" 

 season. Other species of fishes and inverte- 

 brates either did not differ in time of occur- 

 rence or varied so much between samples 

 that their seasonal distribution could not be 

 determined. 



The relation of hydrographic conditions to 

 feeding of tunas is undetermined. The surface 

 waters of the Gulf of Guinea exhibit seasonal 

 variations in physical and chemical charac- 

 teristics (Longhurst, 1962). In the "warm" 

 season (February- April) the waters are 

 characterized by higher temperatures (28°- 

 31° C.) and salinity values of 34.0 to 35.0 

 p.p.t. (parts per thousand). In the "cool" sea- 

 son (July-Septen-iber), the water temperatures 

 are from 22° to 25° C, and the salinities range 

 from 32 to 36 p.p.t. 



Stomachs from both yellowfin and skipjack 

 tunas contained an unidentified species of 

 Hirudinella (Trematoda). This parasite was 

 found 5 times (in 4 yellowfin tuna and 1 skip- 

 jack tuna) in stomachs collected during Jan- 

 uary-May, and 14 times (in 12 yellowfin tuna 

 and 2 skipjack tuna) in stomachs collected 

 during July-October. Both sexes and a wide 

 size range of each species were infected. 



