FISHERY BULLETIN: VOL. 70, NO. 4 



are important in studies concerned with energy 

 that is converted into caloric equivalents of en- 

 ergy utilized for growth. We have the infor- 

 mation only on the volumes of the stomach con- 

 tents. This information may be of interest to 

 the investigators concerned with energy budgets 

 of tunas and with studies on transfers of energy 

 within the food web. In our study, in the ma- 

 jority of observations, the displacement vol- 

 umes of the total stomach contents of skipjack 

 and yellowfin tunas varied from 0.1 to 20.0 ml 

 and from 0.1 to 60.0 ml, respectively (Table 4). 

 Information on the volumes of stomach contents 

 of tunas in the Atlantic Ocean is found only in 

 a limited number of investigations, as cited in 

 the review of studies of tuna food in the Atlantic 

 Ocean by Dragovich (1969). Dragovich (1970) 

 noted volumes of stomachs of less than 20 ml 

 in 75% of skipjack tuna sampled and in 85% 

 of yellowfin tuna. Volumes of stomach contents 

 of yellowfin tuna caught by longline off the coast 

 of east Africa (Williams, 1966) were generally 

 higher than those in our study. The majority of 

 the volumes measured by Williams fell within a 

 range of 3.0 to 499.9 cc. Higher volumes of 

 stomach contents observed by Williams may be 

 partially explained by the fact that tunas caught 

 on longline are usually larger than fishes caught 

 by surface methods. 



To determine the relation between the volume 

 of stomach contents and body weight of skip- 

 jack and yellowfin tunas, we have assumed that 



1.0 ml of stomach contents is equivalent to 1.0 g. 

 Comparisons on this basis were made between 

 the estimated weight of the stomach contents 

 and the body weights of tunas. Our calculations 

 have shown that the total volume of stomach 

 contents for both species of tunas in almost all 

 observations was well below 1.0% of the body 

 weight. This observation is in agreement with 

 the findings by Dragovich (1970). The results 

 of these calculations suggest that there was little 

 difference in the total amount of food found in 

 the stomachs of both species of tunas as related 

 to the body weight. Possible explanations for 

 such low volumes of stomach contents may be 

 rapid digestion of food, long periods between 

 the feedings, scarcity of food, and the fact that 

 most of the forage organisms are very small 

 macrozooplankton. 



SEASONAL CHANGES IN 



TAXONOMIC COMPOSITION OF 



FORAGE ORGANISMS 



Cruise UN6801 took place in the Gulf of 

 Guinea during what is sometimes called the 

 "warm" season (February, March, and April) 

 and UN6802 during the "cool" season (Septem- 

 ber, October, and November). Berrit (1961) 

 in his study on seasonal variations of oceano- 

 graphic conditions introduced these terms. Re- 

 sults on studies by Sund and Richards (1967) 



Table 4. — Distribution of the volumes of total stomach contents in 711 skipjack tuna and 132 yellowfin tuna 

 stomachs. The data were collected during cruises UN6801 and UN6802. 



1096 



