Food Habits and IVophic Relationships of a Community of Fishes 



on the Outer Continental Shelf 1 



GEORGE R. SEDBERRY 2 



ABSTRACT 



The demersal fish community of the Outer Continental Shelf in the Middle Atlantic Bight consists of resi- 

 dent species {Lophius americanus* Citharichthys arctifrons, Paralichthys oblongus), seasonal species with 

 boreal affinities (Raja erinacea* Urophycis chuss, Merluccius bilinearis, Macrozoarces americanus), and sea- 

 sonal species with warm-temperate affinities (Urophycis regia, Stenotomus chrysops). Although most dominant 

 demersal fishes of the Outer Continental Shelf feed primarily on dense, stable macrobenthic invertebrate com- 

 munities, some feed on fishes, cephalopods, and planktonic invertebrates. In addition to seasonal changes in 

 pre} species preference, food habits change considerably with fish size. Most predator species share many prey 

 species. Overlap in diet among predators varies seasonally, with overlap relationships changing as species and 

 size-class composition of the predators changes. Intraspecific diet overlap between size classes is low, but 

 higher interspecific overlap occurs between species of similar size. Dietary overlap is lowest in the spring, when 

 planktonic and nektonic organisms are consumed by most size classes of dominant predators. Although many 

 important prey species are consumed by several predators, some are selectively consumed by only a few preda- 

 tors, so that there is never complete dietary overlap between two species. 



INTRODUCTION 



Studies of the food habits of fishes are essential to a complete 

 understanding of the functional role of fishes in aquatic ecosys- 

 tems. Research in this field has resulted in an abundance of papers 

 dealing with the food habits of individual species, but fewer studies 

 have related food habits to community structure, including patterns 

 of competition, resource partitioning, or prey selectivity. Some 

 experimental and field studies have focused on resource partition- 

 ing, including food subdivision in closely related species (McEach- 

 ran et al. 1976; Werner and Hall 1976; Chao and Musick 1977; 

 Ross 1977; Langton and Bowman 1980). Fewer studies have dealt 

 with the feeding ecology of entire marine fish communities (Tyler 

 1972; Gatz 1979). Study of diet overlap is essential to understand- 

 ing competitive coexistence and species diversity (Pyke et al . 1 977) 

 and hence community structure. 



The continental shelf areas of the Middle Atlantic Bight have 

 been a focus of benthic biological research in recent years (Boesch 

 1972; Pratt 1973; Steimle and Stone 1973; Pearceetal. 1976), and 

 distribution and abundance of fishes have also been examined 

 (Tyler 1971; Musick 1974; McEachran and Musick 1975; Clark 

 and Brown 1977; Musick and Mercer 1977; Musick et al. 1979). 

 The fish fauna on the continental shelf of the Middle Atlantic Bight 

 consists of a highly migratory component of boreal and warm- 

 temperate species and a small resident component (Tyler 1971; 

 Musick et al. 1979). Stomach contents of some of the dominant 

 species on the Middle Atlantic Outer Continental Shelf have been 

 reported in faunal and taxonomic works and in life history studies 

 (Bigelow and Welsh 1925; Nichols and Breder 1927; Olsen and 

 Merriman 1946; Bigelow and Schroeder 1953; Fitz and Daiber 



'VIMS Contribution Number 1090 and South Carolina Marine Resources Center 

 Contribution Number 142. 



2 Virginia Institute of Marine Science, School of Marine Science, College of Wil- 

 liam and Mary, Gloucester Point, Va; present address: South Carolina Marine 

 Resources Research Institute, P.O. Box 12559. Charleston, SC 29412. 



1963; Richards et al. 1963; Barans 1969), and some food habits 

 studies have been done (Jensen and Fritz 1960; Sikora et al. 1972; 

 Vinogradov 1972; McEachran et al. 1976; Langton and Bowman 

 1980, 1981). 



The purposes of this report are to describe the food habits of dom- 

 inant demersal fishes on the Middle Atlantic Outer Continental 

 Shelf, to describe diet overlap patterns, and to relate these patterns 

 to predator size and seasonality and to seasonal prey abundance. 



METHODS 



Two areas were selected for intensive fish sampling: Area B 

 (approximately 735 km 2 ) off Atlantic City. N.J. ; and Area E 

 (approximately 540 km : ) off Delaware Bay (Fig. 1). These areas 

 were chosen for the great habitat variety of their complex topogra- 

 phy. The bottom of both areas is characterized by a series of ridges, 

 swales, scarps, and flats which support different benthic inverte- 

 brate assemblages (Boesch 1978), and an attempt was made to sam- 

 ple each bottom type. Both study areas were divided into 1 1 strata, 

 based mainly on depth data taken from U.S. Geological Survey 

 charts and also on available data on the distribution of bottom sedi- 

 ments and previous sampling of macrobenthos. 



Sampling for fishes consisted of tows of 15-min duration (at 

 about 6.5 km/h) with a lined, semiballoon ottertrawl having a 13.7 

 m (45 ft) headrope and the following stretch-mesh dimensions: 

 4.45 cm in the wings, 3.81 cm in the body, 3.96 cm in the cod end. 

 and 1.27 cm in the cod end liner. Six stations, three day and three 

 night, were randomly selected in each stratum for each cruise. Sam- 

 ples were collected seasonally on four cruises, utilizing the RV 

 Cape Henlopen (fall 1976, spring and summer 1977) and the RV 

 James M. Gilliss (winter 1977). 



All fishes captured were identified, measured to the nearest milli- 

 meter, and weighed. Standard length (SL) was taken on all domi- 

 nant species with the exception of Raja erinacea (disc width = 

 DW) and Macrozoarces americanus (total length = TL). Each fish 

 was dissected and its stomach excised if not conspicuously empty. 



