larger fish fed mainly on C. irroratus. Lophius americanus was pri- 

 marily piscivorous and all sizes were included in Group I. 

 Citharichthys arctifrons fed mainly on corophiid amphipods, and 

 all sizes were included in Group III. 



DISCUSSION 



Comparison of present results with previous studies indicates that 

 although fishes select a certain type of prey, depending on their size 

 and habitat, the prey species consumed is dependent upon prey 

 availability and prey community structure. Generally, the impor- 

 tant higher prey taxa, e.g.. polychaetes, amphipods, decapods, 

 etc. , are important in the diet of shelf fishes throughout their range, 

 but the species consumed reflect availability of these prey items. 

 Raja erinacea fed mainly on amphipods and decapods in the waters 

 around Long Island (Smith 1950; Richards et al. 1963), a finding 

 duplicated by the present study. The amphipod Leptocheirus pin- 

 guis was the most abundant species in the diet in Long Island 

 waters, and Smith (1950) reported that this amphipod was a domi- 

 nant species in the benthos. Although locally common in muddier 

 habitats on the outer shelf. L. pinguis is seldom abundant in the 

 present study area (Boesch 1978) and was not abundant in the diet 

 of the little skate. McEachran et al. ( 1976) found no significant dif- 

 ferences in the higher taxonomic composition of the diet of R. 

 erinacea from four areas, including the Middle Atlantic Bight, 

 Georges Bank, the Gulf of Maine, and the Nova Scotian shelf. 

 However, species composition of the most important prey changed 

 from north to south, probably reflecting changes in the benthic 

 fauna. Previous reports of stomach contents of the other dominant 

 species from other localities show similar results (Hildebrand and 

 Schroeder 1927; Olsen and Merriman 1946; Bigelow and Schroe- 

 der 1953; Jensen and Fritz 1960; Richards 1963; Barans 1969; 

 Sikora et al. 1972; Vinogradov 1972; Langton and Bowman 1980, 

 1981). 



Diversity and Seasonality of Prey Availability 



The diets of several outer shelf fishes were quite diverse. Raja 

 erinacea fed on at least 107 species. U. chuss fed on about 1 30 spe- 

 cies. S. chrysops fed on 106 species, and C. arctifrons fed on about 

 70 prey species. Other predators were more specialized in diet, 

 such as U. regia (45 species of prey). M. bilinearis (5 1 species), M. 

 americanus (39 species), and P. oblongus (34 species). Lophius 

 americanus was the most specialized predator, feeding on only 24 

 pre) species, mostly fishes. 



Although many species of prey were consumed by the fish com- 

 munity, only a few species predominated in the diet of each preda- 

 tor. Most species important in the diet of any one predatorwere also 

 important for other predators as well. These species include the 

 amphipods Ampelisca vadorum, Byblis serraia. Erichilumius 

 rubricornis, and Unciola irrorata. Other important prey species 

 were Cancer irroratus, Crangon sepiemspinosa, Dichelopandalus 

 leptocerus, and Diastylis bispinosa. 



Predation can be an important factor controlling the structure of 

 benthic communities (Virnstein 1977. 1979; Peterson 1979). 

 Selective predation on prolific prey species keeps the population 

 levels of these species low. allowing more species to coexist in the 

 same habitat (Dayton and Hessler 1972). The heavy predation mor- 

 tality exerted through selective predation by demersal shelf fishes 

 on ampeliscid and. especially, corophiid amphipods may keep pop- 

 ulations of these prolific species from completely dominating the 

 benthic community, thus contributing to the high diversity (Boesch 



et al. 1977; Boesch 1978) in macrobenthic communities on the 

 outer shelf. 



In addition to those prey species that were important for many 

 predators, some prey species were important to only a few preda- 

 tors. These included such species as Ensis directus (preyed on 

 mainly by R. erinacea), Clymenura sp. A (prey for C. arctifrons), 

 Chone infitndibuliformis (prey for S. chrysops and C. arctifrons), 

 Ampelisca agassizi (preyed on mainly by 5. chrysops), fishes (fed 

 on by L. americanus and seasonally important for U. regia, H. 

 oblonga, and M. bilinearis), and cephalopods (important for U. 

 regia, M. bilinearis. and H. oblonga). 



Many prey species, mostly planktonic invertebrates, were only 

 seasonally important in the diet of some predators. These included 

 the copepods Centropages typicus and Paracalanus spp. . hyperiid 

 amphipods, decapod larvae, chaetognaths, larvaceans, and juve- 

 nile fishes. Seasonally important benthic invertebrates included 

 Diastylis sculpta. Ciro/ana polita, Trichophoxus epistomus, Mono- 

 culodes edwardsi, Dichelopandalus leptocerus, and Crangon sep- 

 iemspinosa. However, most benthic food items were equally 

 important in the diet of the predators during all seasons, reflecting 

 the temporal persistence (Boesch 1978) of populations of macro- 

 benthos on the shelf. The seasonal importance of planktonic prey in 

 the diet is related to two factors. The first is the size class composi- 

 tion of the predators. Smaller red and silver hake present in the fall 

 and summer consumed more copepods and hyperiids. Secondly, 

 seasonal importance of pelagic food items is related to abundance 

 of these taxa in the nearbottom plankton community. Chaetognaths 

 were probably concentrated near the bottom in winter and spring. 

 when they were important food for many demersal fishes. Larval 

 stages of decapods were also seasonally important; this is related to 

 the seasonal abundance of these stages in the plankton (Grant 

 1977). 



Seasonal prey switching (Murdoch et al. 1975; Love and Ebeling 

 1978) was evident for some predators. Forexample. S. elegans was 

 rare in the diet of large silver hake present in the study area in win- 

 ter. In spring, however. 5. elegans was the most abundant prey spe- 

 cies consumed by silver hake. This probably represents an 

 opportunistic switching in silver hake as chaetognaths become 

 abundant in the nearbottom plankton. This switching was indepen- 

 dent of predator size, i.e.. larger silver hake present in winter and 

 spring switched from a diet dominated by amphipods in the winter 

 to a diet dominated by chaetognaths in the spring. Other predators 

 switched seasonally between benthic and planktonic prey. Urophy- 

 cis chuss demonstrated a similar switching to chaetognaths, and C. 

 arctifrons switched to planktonic larvaceans in the spring. Love 

 and Ebeling (1978) noted that fishes they studied switched to a 

 more planktonic diet in winter and spring, when plankton volumes 

 were high in their study area or when other food may have been rel- 

 atively scarce. There were similar increases in plankton in spring in 

 the Middle Atlantic Bight. Increased feeding on S. elegans in 

 spring is related to an appearance of boreal zooplankton. which 

 were abundant following the severe winter of 1977 (Grant 1979. 

 1980). As concluded by Love and Ebeling (1978), seasonal 

 switching in prey selectivity, in this case to different prey types — 

 i.e.. benthic to pelagic— probably reflects an increased relative 

 abundance or availability of these prey species. Since benthic popu- 

 lations remain relatively constant (Boesch et al. 1977; Boesch 

 1978). this is probably due to an increase in nearbottom zooplank- 

 ton. especially chaetognaths and larvaceans. Increased importance 



'G- C. Gram. Acting Assistant Director. Virginia Institute of Marine Science, 

 Gloucester Pt„ VA 23062, pers. commun. 20 April 1980. 



