FISHERY BULLETIN; VOL. 87, NO. 4, 1989 



other abundant sparid fishes from the same habi- 

 tat. 



METHODS 



Stomachs of fish analyzed for food habits were 

 collected during six trawl cruises on the conti- 

 nental shelf in 1980 and 1981. Stomachs of white- 

 bone porgy were taken at 11 hard bottom sta- 

 tions distributed among 3 depth zones represent- 

 ing the inner shelf (16-22 m depth, three sta- 

 tions), middle shelf (23-37 m depth, four sta- 

 tions), and the outer shelf (46-69 m depth, four 

 stations). Dehneation of depth zones was based 

 on distribution of fish and invertebrate species 

 assemblages as noted in previous studies and on 

 community analysis of trawl catches used in the 

 present study (Struhsaker 1969; Miller and 

 Richards 198(); Sedberry and Van Dolah 1984; 

 Wenner et al. 1984). Fishes were collected pri- 

 marily in trawl tows as described elsewhere 

 (Sedberry and Van Dolah 1984; Wenner et al. 

 1984; Sedberry 1985). A few specimens were 

 collected with trap or hook and line. Sampling 

 for fishes was conducted on hard bottom habitat, 

 which was mapped for each station using under- 

 water television (Sedberry and Van Dolah 1984). 



Whitebone porgy were measured (standard 

 length, SL) at sea, and their stomachs were pre- 

 served in 10% seawater formahn. Contents of 

 individual stomachs were then sorted in the lab- 

 oratory by taxa, counted, and measured volu- 

 metrically. The relative contribution of food 

 items to the diet was described by percent fre- 

 quency occurrence (F), percent numerical abun- 

 dance (A^), and percent volume displacement iV). 

 F, N, and V were calculated for prey species and 

 for prey items grouped into higher taxonomic 

 categories, for 50 mm intervals of SL. 



In order to determine the selectivity or de- 

 pendence of demersal fishes on hard bottom prey 

 organisms, stomach samples were compared 

 with benthic samples using Ivlev's index of elec- 

 tivity (Ivlev 1961). Electivity values range from 

 -1 to +1. Negative values imply that the prey 

 species is avoided by the predator or that it is 

 unavailable to the predator. Positive values im- 

 ply that the predator prefers the prey species 

 or that it is feeding on prey species that occur in 

 a different habitat than that sampled by the 

 benthic sampler. A value near zero implies no 

 selectivity by the predator; i.e., the fish is feed- 

 ing on the prey in proportion to the prey's 

 relative abundance in samples taken in the 

 habitat. 



The electivity index was calculated for species 

 that were numerically dominant in benthic sam- 

 ples or in fish stomach samples which were 

 pooled by depth zone (inner, middle, and outer 

 shelves) for comparison. Benthic samples were 

 obtained at the 11 hard bottom sites during 1980 

 and 1981 with a suction device (inner and middle 

 shelves) or a grab (outer shelf). Divers obtained 

 five replicate suction samples at each inner and 

 middle shelf station by scraping the hard sub- 

 strata enclosed by a 0.1 m^ quadrat box, while 

 simultaneously sucking with an airlift device 

 similar to that described by Chess (1979). Suc- 

 tion samples were collected in 1.0 mm mesh 

 bags. At the outer shelf stations where water 

 depth precluded the use of the suction device 

 operated by divers, quantitative samples (five 

 copies) were collected with a 0.1 m^ Smith- 

 Mclntyre gi-ab. After retrieval, each sample was 

 placed into a 1.0 mm sieve and washed to remove 

 the finer sediment. 



Sampling motile benthic invertebrates with 

 the suction sampler proved to be a very simple, 

 yet effective, technique. Samples were quantita- 

 tive because suctioning effectively collected 

 everything within the confines of the walled box 

 placed on the substratum. The Smith-Mclntyre 

 gi-ab, which was substituted for the suction sam- 

 pler at deeper stations, was somewhat less quan- 

 titative because the sampler is not as effective on 

 hard substrate, and the actual area sampled was 

 unknown. In spite of these limitations, the grab 

 sampler was the only feasible means of sampling 

 the benthos at outer shelf stations and provided 

 the only benthic collections with which to calcu- 

 late electivity. 



Similarity in diet between whitebone porgy 

 and four other co-occurring and frequently col- 

 lected sparids was also investigated. Stomach 

 samples of these additional species were col- 

 lected at the same time as the whitebone porgy 

 stomachs and were analyzed in a similar manner 

 (South Carolina Wildlife and Marine Resources 

 Department 1984; Sedberry 1987). These other 

 sparids were sheepshead, Archosargus probato- 

 cephalus; pinfish, Lagodon rhotnboides; red 

 porgy, Pagrus pagrus; and southern porgy, 

 Ste)ioto)nus aciileattis. 



Similarity in diet between these sparids was 

 measured using the Bray-Curtis measure (Bray 

 and Curtis 1957). Because sample sizes of preda- 

 tors were unequal, abundance of prey items was 

 standardized as percent numerical abundance for 

 each predator, resulting in values of percent sim- 

 ilarity in composition of diet between pairs of 



936 



