METHODS USED FOR EVALUATION OF 

 STOMACH CONTENTS 



It seems appropriate to review briefly the 

 various methods of evaluating food components 

 that have been used in studies of the stomach 

 contents of fishes, birds, mammals, and other 

 animals. The four most common methods are 

 discussed here and comments are made on 

 their advantages and disadvantages. 



Numerical Method 



The numerical method is based on counts of 

 the food items present; each item is evaluated 

 as a percentage of the total number of all food 

 items. The chief disadvantage in this method 

 is that the size of forage organisms is not 

 considered. Small organisms suchas megalopa 

 stages, for example, which occur in tuna 

 stomachs in large numbers, often rank higher 

 in apparent importance than do fishes or other 

 large forage organisms. The method places 

 undue emphasis on organisms with resistant 

 parts--crustaceans, shelled mollusks, and 

 scaled fishes--and thus may give a distorted 

 picture of food components. Also, it is difficult 

 and time-consuming to count accurately the 

 many fragmented organisms found in stomachs, 

 andnumerous macroplanktonic organisms must 

 be either counted or estimated from sub- 

 samples. 



Percentage Frequency-of -Occurrence Method 



The number of fish that have ingested a par- 

 ticular food organism is calculated as a per- 

 centage of the total number of fish examined. 

 The method has the same disadvantages as the 

 previous one: the number and size of organ- 

 isms are not considered, small organisms 

 usually outnumber large ones, and large or- 

 ganisms lose their importance. Like the pre- 

 ceding method, this procedure is primarily 

 useful in evaluating organisms within a simi- 

 lar size range. The method also allows rough 

 estimates of availability of food organisms and 

 selectivity by predators. 



Volumetric, or Weight, Method 



The displacement volume or the weight of 

 each food item is expressed as a percentage of 

 the total volume or weight of the stomach con- 

 tents. This method probably depicts most reli- 

 ably the relative importance of various food 

 items. 



The volumetric method can be used in 

 various ways. Two of these--the "aggregate- 

 total- volume" and "average-percentage" pro- 

 cedures-- were described by Martin, Gensch, 

 and Brown (1946). In the first version, the 

 percentage of each kind of food is obtained by 

 dividing total food of each kind by total volume 



of the stomach contents of all fish; the volume 

 of food from each stomach influences the final 

 result in direct proportion to that volume. 

 The method accurately reflects the volumetric 

 importance of a particular food organism 

 regardless of how many or how few other 

 organisms are present. In the "average-per- 

 centage" version, equivalents are calculated 

 for each item of food, and each stomach is 

 taken as 100 percent, regardless of the volume 

 of its contents; variation in total volume of 

 food does not influence the results. 



In his study of the food of the smallmouth 

 bass ( Micropterus dolomieui ), Tester (1932) 

 combined the volumetric and the frequency-of- 

 occurrence methods for a graphic representa- 

 tion of the food items. Welsh (1949) used an 

 average percentage rating obtained by aver- 

 aging (1) the percentage of the total food vol- 

 ume contributed by the individual food item 

 (indicating food value), (2) the percentage of 

 total number of individual animals found in 

 stomachs (indicating abundance), and (3) the 

 percentage of the total stomachs in which 

 organisms were found (indicating availability). 

 When Welsh's method is used, the final figure 

 appears to be a combination of different terms, 

 but it may serve as a simplified and useful 

 index. 



"Points" Method 



Points are assigned to food items, to the 

 total volumes, and to the "degree of fullness" 

 of each stomach. Assuming that the organ- 

 isms are of equal size (or allowing for size), 

 the investigator assigns the greatest number 

 of points to the most easily digested organ- 

 isms. The chief disadvantage in this method 

 is the difficulty of establishing or maintaining 

 standards on which to base designation of 

 points throughout extensive surveys. 



Comments on Methods Used 



In most reports classified in the present 

 paper as notes, reviews, incidental publica- 

 tions, and reports on data, evaluation of 

 stomach contents was qualitative; information 

 consisted of lists of forage organisms, ac- 

 companied by general comments. Examples 

 are: De Monaco, 1888; Chevreux, 1893; Grand- 

 besancon, 1910;' Joubin and Roule, 1918a, 

 1918b; Le Danois, 1921; Nichols, 1922; De 

 Buen, 1927; Legendre, 1932, 1933; Long- 

 ley and Hildebrand, 1941; Priol, 1944; Le 

 Gall, 1949; Postel, 1950, 1954; Bigelow and 

 Schroeder, 1953; Rivas, 1954; Postel, 1955b; 

 Anderson, Gehringer, and Cohen, 1956a, 1956b; 

 Anderson and Gehringer, 1957a, 1957b, 1958a, 

 1958b, 1959a, 1959b, 1959c; Krumholz, 1959; 

 De Sylva and Rathjen, 1961; Klawe, 1961; 

 Alverson, 1963a; Postel, 1964; Zharov, Zhere- 

 benkov, Kadil'nikov, and Kuznetsov, 1964; Da 

 Cruz, 1965; Zharov, 1966; Bogdanov, Korzhova, 



