FISHERY BULLETIN: VOL. 80, NO. 4 



Scotia. Details of this food-habit survey were de- 

 scribed in an International Council for the Ex- 

 ploration of the Sea document and will not be re- 

 peated here (Langton et al. 1980 3 ). 



Diet overlap, expressed as the percentage simi- 

 larity between diets, was calculated according to 

 the formula of Shorygin (Ivlev 1961) and has 

 been described in several other papers as a 

 means of evaluating the diet of northwest Atlan- 

 tic finfish (Langton and Bowman 1980; Grosslein 

 et al. 1980) although there are other methods of 

 indexing like diets (Lipovsky and Simenstad 

 1978). The calculation is quite simple and is done 

 by summing the smaller value, as a percentage 

 weight in the present case, for all prey shared by 

 the two predators. The computed value ranges 

 from to 100%, with 0% representing no diet 

 overlap and 100% representing identical diets. 

 The final overlap value is sensitive to the taxo- 

 nomic level at which the prey was identified and 

 for this paper the finest taxonomic breakdown 

 available was used, i.e., prey identified to species 

 whenever possible. Because of the sensitivity of 

 this overlap measure to the taxonomic break- 

 down of prey, statistical methods of evaluating 

 absolute overlap values are not practical. Instead, 

 the values have been classified as low, 0-29%; 

 medium, 30-60%; or high, >60% for the purpose 

 of discussion. 



The computation of diet overlap was auto- 

 mated and the actual computer program checked 

 by running diet overlap for any one predator 

 against itself. In this case the computer gener- 

 ates a value of 100% overlap for fish in the same 

 size class and then a mirror image of values on 

 each side of the 100% line. This is shown graph- 

 ically in Figure 1 where Atlantic cod is plotted 

 three dimensionally versus Atlantic cod. The 

 plotting program only considered size classes up 

 through class 25 (125 cm maximum fork length), 

 but by truncating the output at this level little 

 data was eliminated (a total of 5 cod out of 1,714 

 examined, for example). In fact, since the fish 

 were taken randomly from the catch, the major- 

 ity of the samples came from the most frequently 

 occurring size classes which, even for Atlantic 

 cod, did not approach this maximum size. Fur- 

 thermore, size classes that did not include a sam- 



ple size of at least 10 fish were eliminated before 

 the data were plotted since very small samples 

 would not necessarily be representative of the 

 size class. 



This study concentrates on two of the major 

 fish predators in the northwest Atlantic, silver 

 hake and Atlantic cod, and on the questions of 

 their diet overlaps with 15 other finfish species. 

 The food habits information presented is limited 

 to an explanation of the prey shared by the preda- 

 tors which results in the observed overlap values. 

 Detailed descriptions of the diet of the fish col- 

 lected by the Northeast Fisheries Center are in 

 preparation or have been given elsewhere. Die- 

 tary information on the northwest Atlantic Gadi- 

 formes and Pleuronectiformes can, for example, 

 be found in Langton and Bowman (1980, 1981), 

 Bowman and Bowman (1980), and Durbin et al. 

 (1980) 4 while data on fish from other taxa are 

 described in Edwards and Bowman (1979) and 

 Grosslein et al. (1980). 



RESULTS 



Atlantic Cod — Little Skate 



Atlantic cod, Gadus morhua Linnaeus, and lit- 

 tle skate, Raja erinacea Mitchell, show relatively 

 little similarity in diet (Fig. 2A). The maximum 

 value of 48% was found for the overlap between 

 size class 3 (11-15 cm) Atlantic cod and size class 

 3 little skate. The prey shared by these predators 

 are primarily small crustaceans, in particular 

 amphipods such as Unciola. Unfortunately, 

 slightly more than 10% of the diet of each of these 

 fish was unidentifiable with a resulting increase 

 in the overlap values. As can be seen from Figure 

 2A, apart from the peak of 48%, medium levels of 

 dietary overlap exist between Atlantic cod 11-20 

 cm (size classes 3 and 4) and little skate up to 45 

 cm total length (size class 9). Medium overlap 

 values again occur between little skate 36-55 cm 

 (size classes 8-11) and Atlantic cod 31-65 cm (size 

 classes 7-13). This overlap can generally be at- 

 tributed to the preponderance of a variety of 

 crustaceans in the diet of both predators. For 

 larger Atlantic cod the overlap values with little 

 skate are extremely low, primarily because of a 



3 Langton, R., B. North, B. Hayden, and R. Bowman. 1980. 

 Fish food habit studies-sampling procedures and data process- 

 ing methods utilized by the Northeast Fisheries Center, Woods 

 Hole Laboratory, U.S.A. Int. Counc. Explor. Sea, CM. 1980/ 

 L:61, 16 p. 



4 Durbin, E., A. Durbin, R. Langton, R. Bowman, and M. 

 Grosslein. 1980. Analysis of stomach contents of Atlantic 

 cod (Gad us morhua) and silver hake (Merluccius bilinearis) for 

 the estimation of daily rations. Int. Counc. Explor. Sea, CM. 

 1980/L:60 (revised), 21 p. 



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