and some salmonids, noting that the greatest overlap oc- 

 curred when the prey was only identified to the order 

 level. In the present study the same effect may be 

 observed. If, for example, the percentage similarity is 

 calculated for the diet of Atlantic cod and silver hake at 

 the highest taxonomic level given in Table 2, the simi- 

 larity increases from 58' c (Fig. 3) to 89' r. On the other 

 hand, Keast (1977) justified the use of broader tax- 

 onomic groupings for sorting prey; he argued that the 

 body size of the prey rather than finer taxonomic iden- 

 tity is important for studies on fish food habits. It would 

 appear that there exists an optimal level of taxonomic 

 classification for prey identified for dietary overlap cal- 

 culations. This optimum is piobably primarily depen- 

 dent on the size of the prey, provided the prey shares a 

 similar ecological niche, since size dependent prey selec- 

 tion has been well-documented for fish (Keast 1965; 

 Tyler 1972; Daan 1973; see also Edwards 1976). Apart 

 from the level of taxonomic identification, sample size is 

 probably the second most important nonbiological factor 

 influencing dietar>' overlap calculations. The smaller the 

 sample the more variable the percentage similarity. This 

 effect was noted in the results for the marlin-spike and 

 longnose grenadier where a relatively small change in 

 sample size, and, consequently, in the quantity of prey 

 consumed, could have a large influence on the observed 

 composition of the diet and any resulting overlap esti- 

 mates. 



Bearing in mind some of the limitations discussed 

 above, an overall pattern emerges from calculating and 

 categorizing the percentage similarity between the 

 fishes' diets (Fig. 3) which is not as readily apparent from 

 a cursory examination of the tables on stomach contents 

 for each of the 15 predators alone. Generally, the great- 

 est block of similarity occurs in the upper left section of 

 Figure 3 while the lowest level of overlap is in the upper 

 right-hand section of the same figure. This reflects two 

 distinct feeding types as revealed by an examination of 

 the data in Table 2. Thus, fish populations such as 

 Atlantic cod, pollock, silver hake, white hake, offshore 

 hake, and cusk are decidedly piscivorous while the had- 

 dock, longfin hake, fourbeard rockling, marlin-spike, 

 longnose grenadier, fawn cusk-eel, and ocean pout are 

 characterized more as benthic, invertebrate feeders. Red 

 and spotted hake are intermediate, resulting in a very 

 similar diet which overlaps both the piscivores and 

 benthic invertebrate feeders. 



ACKNOWLEDGMENTS 



We thank Roland Wigley for his guidance in the pre- 

 paration of the manuscript and also Deborah Hartley, 

 Martha Hill, Ray Maurer, Thomas Morris, and Janet 

 Murphy who helped collect, analyze, and tabulate the 

 data. 



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22 



