DRAGOVICH and POTTHOFF: FOOD OF SKIPJACK AND YELLOWFIN TUNAS 



plankton samples showed that the major con- 

 stituents were copepods and arrowworms. Ar- 

 rowworms were entirely absent in the diet of 

 both species of tunas. Although copepods were 

 present in the stomachs of a few skipjack tunas, 

 they may have been remains of the stomach 

 contents of the ingested fishes. Among the 

 minor constituents of zooplankton, 60 species 

 and 10 genera of amphipods and 20 species and 

 3 genera of euphausiids were present in the 

 plankton tows. Although all of the amphipods 

 and euphausiids found in the tuna stomachs 

 were also present in the plankton tows, their 

 number represented only a small fraction of the 

 number of taxa found in the plankton tows, thus 

 suggesting selectivity in feeding of skipjack and 

 yellowfin tunas. Our findings support those of 

 Blackburn (1965), who stated that no species 

 of tuna consumes all the species of net-caught 

 micronekton or zooplankton. 



COMPARISONS WITH RESULTS OF 

 OTHER INVESTIGATORS 



Investigations concerned with the food of 

 skipjack and yellowfin tunas off the west coast 

 of Africa are numerous (Postel, 1954, 1955a, 

 1955b; Marchal, 1959; de Jager, de V. Nepgen, 

 and van Wyk, 1963; Penrith, 1963; Sund and 

 Richards, 1967; de V. Nepgen, 1970; and 

 Dragovich, 1970). Reliable qualitative compar- 

 isons of tuna forage between different studies 

 are difficult because identification of organisms 

 is usually incomplete. Quantitative compari- 

 sons between various studies of tuna forage 

 usually consist of comparisons between the 

 major food categories (fishes, crustaceans, and 

 cephalopods) . The nearest areas of the Atlantic 

 Ocean to our investigation area for which valid 

 comparisons can be made were investigated by 

 Marchal (1959), Sund and Richards (1967), 

 and Dragovich (1970). 



The diet of yellowfin and skipjack tunas in the 

 Gulf of Guinea was studied by Marchal (1959) 

 and Sund and Richards (1967), respectively. 

 From a long list of forage organisms presented 

 by Marchal only several fishes (Vomer setapin- 

 nis, Euthynnus alletteratus , Sternoptyx diaph- 

 ana, Hyppocampiis sp., Ophidion barbatum, 



Brotulidae, Chiasmodontidae) , a few crusta- 

 ceans (Stomatopoda, Heterocarpus ensifer, 

 Glaucothoe, megalopae (Brachyura) ), and salps 

 were common to both studies. All fish families 

 in the diet of yellowfin tuna and skipjack tuna 

 observed by Sund and Richards (1967) were also 

 observed by us. The differences in the compo- 

 sition of tuna food between our study and that 

 of Sund and Richards were on generic and spe- 

 cific levels except for cephalopods, where our 

 findings differed entirely. 



A striking similarity in the food of skipjack 

 and yellowfin tunas was observed between our 

 study and that of Dragovich (1970). Skipjack 

 and yellowfin tunas in the study by Dragovich 

 were captured off the coast of West Africa from 

 Sierra Leone to Angola. All forage-fish families 

 (21) in the diet of skipjack tuna noted by Drag- 

 ovich were also observed by us. The most prom- 

 inent fish families (Carangidae, Scombridae, 

 Gempylidae) in terms of volume of frequency 

 of occurrence observed by Dragovich were equal- 

 ly important in our study. We found the same 

 groups of crustaceans as Dragovich. In the 

 cephalopod diet ommastrephids were the prin- 

 cipal food in both studies. 



Postel (1955a) examined contents of stomachs 

 of yellowfin tuna caught off the coast of Senegal. 

 Of 30 species and 7 genera of fish and 12 ceph- 

 alopod taxa listed by Postel, only Euthynnus 

 alletteratus, Katsuivonus pelamis, Sphyraena 

 sp., Cranchia scabra, and Argonauta sp. were 

 observed by us. None of the identified species 

 and genera of crustaceans by Postel was ob- 

 served by us. The pronounced taxonomic dif- 

 ferences of forage between our study and that 

 of Postel may be partially explained by the dif- 

 ferent oceanographic regime off the coast of 

 Senegal. 



Postel (1955b), in his report on Katsuwonus 

 pelamis off Cape Verde Islands, identified Sar- 

 dinella aurita, S. sp., Myctophidae, Hemiram- 

 phus sp., Hyporamphus sp., Gephyroberyx dar- 

 ivini, Scomber colias, Aphanopus sp., and Mul- 

 lidae in the diet of this tuna. Myctophidae and 

 Mullidae were also observed by us in the diet 

 of skipjack tuna. From cephalopods, only Illex 

 illecebrosus coindeti was listed; this species was 

 not identified in the diet of skipjack by us. 



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