Foods of Yellowfin and Blackfin Tuna 35 



North Carolina during the spring, summer and fall of 1980, 1981 and 

 1982. A few additional samples, indicated in parentheses as yellowfin 

 and then blackfin, were obtained from locations along the southeast 

 Atlantic and Gulf of Mexico coasts: South Carolina (31,8), Georgia 

 (3,1), east coast of Florida (0,2), northwest Florida (3,1), Mississippi- 

 Louisiana (0,6), and south Texas (0,25). 



Samplers at all locations apportioned their efforts to coincide with 

 local charter boat activities, primarily April through October. Port 

 samplers met boats at the docks as a day's catch was being unloaded. 

 Most fishermen either wanted to save their fish whole for mounting, or 

 to have them filleted and packed on ice or frozen upon returning to the 

 dock. Data were obtained only from the latter group, either in exchange 

 for cleaning the fish, or from fish cleaners who worked at local markets. 

 Fish were measured to the nearest millimeter (FL) and weighed to the 

 nearest tenth of a kilogram. Stomachs and gonads were placed in 

 labeled cloth bags or cheese cloth and preserved in 10% formaUn. 



In the laboratory, stomach contents were identified to the lowest 

 possible taxon and were enumerated, thus providing the relative number 

 of each food type in the stomachs. Frequency of occurrence of materials 

 was determined by counting every stomach that contained at least one 

 specimen or part of a specific item (taxon). Empty stomachs were 

 excluded. The volume of each taxon was obtained by water displace- 

 ment and was later converted to weight by a linear regression equation. 

 Larval and juvenile fish in the stomachs were identified after they had 

 been cleared and stained following the methods discussed by Dingerkus 

 and Uhler (1977) and Taylor and Van Dyke (1978). Crustaceans were 

 identified by Steven G. Morgan and Joseph W. Goy, Duke University 

 Marine Laboratory, Beaufort, North Carolina. Parasites, encountered 

 only occasionally, were separated from food items, counted, identified 

 and preserved. A stomach containing only parasites was considered 

 empty. 



All data were analyzed as percent frequency of occurrence, percent 

 of total number, and percent of food volume. Once frequencies, volumes 

 and numbers of the various foods were obtained, an index of relative 

 importance (IRI) was used to estimate the contribution of major food 

 groups to the diet (Pinkas et al., 1971). The index was calculated as: IRI 

 = (N + V) F, where N = numerical percentage of a food, V = its volumet- 

 ric percentage, and F = its percentage frequency of occurrence. 



The Spearman rank correlation (r^) was used to evaluate differences 

 in diets of the two species based on IRI values of foods from fish col- 

 lected in the same geographic area and over approximately the same 

 period of time. Two different equations may be used. One, where there 

 are no ties (rankings are equal for two or more food categories), and the 



