DRAGOVICH and POTTHOFF: FOOD OF SKIPJACK AND YELLOWFIN TUNAS 



on the differences in the occurrence of forage 

 organisms of skipjack and yellowfin tunas in the 

 Gulf of Guinea between these two seasons are 

 compared with ours. 



In our study, the fish families present in the 

 diet of both species of tunas only during the 

 "warm" period were Mullidae, Dactylopteridae, 

 Gonostomatidae, and Engraulidae. In the study 

 of Sund and Richards (1967), Dactylopteridae 

 were also present during the "warm" season 

 only. A number of crustacean taxa were present 

 in our study only during the "warm" season 

 and absent during the "cool" season. Grapsidae 

 (megalopal stages) , Petrochirus sp. and Streetia 

 challengeri, were found only during the "warm" 

 period and absent during the "cool" period. 

 Other prominent crustaceans observed by us in 

 stomachs of both species of tunas only during 

 the "cool" period were Vibilia armata, Scyllar- 

 ides sp., Scyllarus sp., and S. arctus. Some of 

 the crustaceans (Phronima sedentarin, Phrosina 

 semilunata, Euphausia sp.) occurred only in one 

 season in the observations of Sund and Richards 

 (1967), whereas we observed them in both sea- 

 sons. More extensive collections are needed be- 

 fore any final evaluation is made in regard to the 

 significance of the occurrence of these organisms 

 during different seasons. 



EVALUATION OF FOOD ORGANISMS 



In selecting the most important food organ- 

 isms in a given area, many variables have to be 

 considered. Reintjes and King (1953) stated 

 that food items that rank high in number, high 

 in volume, and high in frequency of occurrence 

 are important foods — at the time and in the area 

 sampled. Using these criteria plus the geo- 

 graphic distribution in evaluation of food or- 

 ganisms of both species of tunas, we have cal- 

 culated dispersal and abundance indices and 

 mean displacement volumes for each food taxon 

 and ranked them accordingly. 



The entire investigation area was divided into 

 27 one-degree squares. If a taxon was present 

 in one square it was assigned a value of one. 

 Using the data from both cruises and for both 

 species of tunas combined, the number of oc- 



currences of each taxon in 27 squares was divid- 

 ed by the number of squares — the quotient was 

 called the dispersal index. An abundance index 

 was calculated by dividing the total number of 

 individuals in each taxon by the total number 

 of all organisms. An approximation of biomass 

 of each food item was represented by the mean 

 displacement volume. The mean displacement 

 volume of food items represented in Figure 5 

 varied from 0.1 to 0.7 ml. 



Since a large number of taxa are represented, 

 we have selected the 32 taxa with the highest 

 dispersal and abundance indices and presented 

 them in a descending order of magnitude (Fig- 

 ure 5) . Vinciguerria nimharia and Anchoviella 

 guineensis, although with low dispersal indices, 

 were included in the diagram because of their 

 high abundance indices. From Figure 5 it is 

 obvious that Stomatopoda, Phrosina semilunata, 

 Teuthoidea, Carangidae, Serranidae, and meg- 

 alopal stages were the most important identifi- 

 able food items throughout the investigation 

 area while V. nimharia, Euphausia hanseni, and 

 A. guineensis were of great local importance. 



In the evaluation of forage organisms by the 

 present method we consider the geographic dis- 

 persal of food organisms to be the most impor- 

 tant criterion for the survival of skipjack and 

 yellowfin tunas, particularly since these tunas 

 are migratory and widely distributed. In our 

 study the tuna forage organisms were both 

 widely distributed and abundant in the area of 

 sampling as indicated in Figure 5. High abun- 

 dance indices were usually associated with high 

 dispersal indices. Thus these food organisms 

 may be considered to be important in the food 

 chain of skipjack and yellowfin tunas for the 

 given time and area. 



The disadvantage of the method is that the 

 estimated geographic distribution of forage 

 taxa as calculated from stomach contents may 

 not represent the true distribution. The only 

 other information nearest to the natural distri- 

 bution of certain forage organisms found in our 

 study was obtained from zooplankton double 

 oblique tows which were made at about the same 

 time of the capture of tunas from which stomach 

 samples were taken. The preliminary analysis 

 of the composition of zooplankton from these 



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