FISHERY BULLETIN; VOL. 79, NO 4 



Table 7. — Major taxon composition of 19 box core' samples 

 containing 154 species taken at SG29 ( 119 m deep) and 8 box 

 core samples containing 97 species taken at SGIO i426 m deepi. 



'Effective sampling area is 0.083 m^. 

 ^Values found for the same location. 



optimal forager that 1) prefers more profitable 

 prey, i.e., prey whose ratio of food value to predator 

 search and handling time is highest, 2 ) feeds more 

 selectively when profitable prey are common, and 

 3) ignores unprofitable prey whose addition to the 

 diet lowers the net energy intake per time spent 

 searching and handling (Pyke et al. 1977; Krebs 

 1978)? 



The Dover sole is not a simple opportunistic 

 feeder, consuming all available prey in proportion 

 to their occurrence in the environment. Since the 

 percentage contribution of a major taxon in the 

 diet and in the environment (as reflected by box 

 core samples) was often significantly different, the 

 Dover sole can be termed a selective feeder. For 

 example, polychaetes and ophiuroids played a 

 more important role in the diet than molluscs and 

 crustaceans, despite the fact that polychaetes and 

 ophiuroids were not always most abundant in box 

 core samples. Moreover, trends in selectivity of 

 major taxa were qualitatively similar at both loca- 

 tions despite different abundances of prey. 

 Polychaetes and ophiuroids were always posi- 

 tively selected, occurring more often in the diet 

 than in the environment. Even though density of 

 pelecypod molluscs at SGIO was three times 

 greater than at SG29, the contribution of molluscs 

 to total diet was lower at SGIO than SG29. This 

 general consistency of diet between these two loca- 

 tions in the face of varying abundances of prey, 

 species composition, depth of benthic macrofauna 

 within the sediment, and depth of the station itself 

 does not support a hypothesis of the Dover sole as a 

 simple opportunistic feeder. 



The most profitable prey for a Dover sole in 



terms of food value ( gram-calorie per gram dry 

 weight) are first, molluscs and crustaceans; sec- 

 ond, polychaetes; and third, ophiuroids (Brawn et 

 al. 1968; Cummins and Wuycheck 1971; Tyler 

 1973). However, most observations of calories per 

 gram dry weight were made for shell-free molluscs 

 while polychaete weights included tubes. This is 

 probably why values for Lumbrineris fragilis, an 

 untubed polychaete, are comparable with those for 

 pelecypods ( =^ 4,500 g cal/g dry weight) and 

 greater than those for amphipods (e.g., 4,050 g 

 cal/g dry weight), while those for tubed 

 polychaetes (e.g., Pherusa plumosa, Pectinaria 

 hyperborea) are lower (2,200-3,500 g cal/dry 

 weight). Ophiuroids generally have lowest food 

 values (2,100 g cal/g dry weight) of the four major 

 prey taxa consumed (Brawn et al. 1968; Cummins 

 and Wuycheck 1971). Thus, an optimal diet based 

 only on maximum caloric value per gram of food 

 ingested would consist principally of crustaceans, 

 followed by molluscs and polychaetes, and lastly of 

 ophiuroids. However, observed diets of all sizes of 

 Dover sole consisted primarily of ophiuroids and 

 polychaetes, with relatively few molluscs and 

 crustaceans. Thus, food value alone does not ex- 

 plain the diet of Dover sole. 



The second factor determining profitability of 

 prey, the relative expense of acquiring and digest- 

 ing different prey, may play a more important role 

 than food value in structuring the diet of Dover 

 sole. Although no quantitative observations of 

 feeding behavior in terms of search and handling 

 costs have been made, some inferences can be 

 made based on knowledge of environmental condi- 

 tions and morphological features. Crustaceans 

 were not major components of the diet, perhaps 

 because 1) crustaceans such as amphipods may be 

 difficult to detect in dim or turbid bottom water, 2) 

 energy expended in pursuit of agile swimming 

 prey may be greater than that derived from their 

 digestion, or 3 ) jaw morphology may make capture 

 of swimming Crustacea difficult (Yazdani 1969, 

 based on Microstomas kitt). 



Molluscs may also require expenses in acquisi- 

 tion or digestion beyond their energetic benefits. 

 They may be more difficult to detect and less effi- 

 ciently digested than polychaetes and ophiuroids. 

 Because the shell is not digested, digestion of mol- 

 luscs such as pelecypods must take place slowly 

 through the apertures of the shell. Calcium from 

 the shell may also raise pH in the gut, thereby 

 reducing efficiency of gastric enzymes. The opti- 

 mal pH level for enzymes found in the stomachs of 



760 



