36 Charles S. Manooch, III and Diane L. Mason 



other where ties do occur. The equation for tied food categories (Fritz 

 1974) was used: 



"^ - where Sx^ = — STx; Sy2 = zTy; 



2 Sx2 Xy^ N -^ N 



J - ^^ ' ^ ; N = numbers of ranks; d = difference between ranks; T = 



N 

 correlation factor for ties and t = number of observations tied at a given 

 rank. Pearson and Kendall's Tau B Correlation Coefficients, in addition 

 to the Spearman rank, were also derived to evaluate differences in the 

 diets. 



RESULTS AND DISCUSSION 



Composition of Stomach Contents 



Stomach contents of both species could be grouped into four prin- 

 cipal categories: fish, cephalopods, crustaceans and miscellaneous non- 

 food items (Tables 1, 2; Fig. 1). Major representatives of each group will 

 be discussed below under separate headings and will also be analyzed 

 later to identify differences in diets related to the species of predator and 

 its size. A graphic presentation of the overall contribution of selected 

 foods to the diet (IRI plots) is presented in Figure 2. 



Fish. — Fishes occurred in 77% of yellowfin and 67% of blackfin stom- 

 achs that contained food (Tables 1, 2; Fig. 2) and consisted primarily of 

 older larvae and juveniles often associated with floating Sargassum. In 

 all, 23 famiHes were identified. Adult exocoetids, scombrids and syngna- 

 thids were found occasionally in yellowfin, as were syngnathids, serran- 

 ids, sciaenids and stromateids in blackfin. For all life stages, fish that 

 occurred most frequently in yellowfin tuna were Scombridae (12.2%), 

 Balistidae (11.2%), Syngnathidae (8.2%), Diodontidae (5.1%) and Exo- 

 coetidae (4.6%). Fifty-three percent of stomachs with food contained 

 unidentifiable fish remains. Fish that occurred most often in blackfin 

 tuna stomachs were Balistidae (10.1%), Trichiuridae (5.6%), Carangidae 

 (4.5%) and Syngnathidae (4.5%). Unidentifiable fishes were found in 

 44.9% of the stomachs containing food. 



Cephalopods. — Cephalopods constituted almost all the molluscan 

 food of both species. One exception was unidentifiable mollusk tissue, 

 possibly cephalopod, from a yellowfin captured in the Gulf of Mexico. 

 Two groups were represented: Teuthidida and Octopodida. Teuthoids 

 (squids) were the most important by frequency of occurrence and by 

 volume: 50.5% and 41.0% for yellowfin, 31.5% and 21.5% for blackfin. 

 By comparison, octopodids, represented by the paper nautilus, Argo- 

 nauta argo, appeared in only 7.7% of the yellowfin tuna and 3.4% of the 



