COLVOCORESSES and MUSICK: CONTINENTAL SHELF DEMERSAL FISHES 



(Combinatorial Polythetic Agglomerative Hierar- 

 chical Program) developed at the institution. Out- 

 put was in the form of similarity matrices and 

 computer generated dendrograms. 



The choice as to which branches in the dendro- 

 grams were to be identified as biologically signifi- 

 cant groups was based on the following procedure. 

 Each branch of the dendrogram, composed solely 

 of fusions involving only one entity as at least 

 one-half of each fusion, was considered to consti- 

 tute a minimal grouping. The distribution of each 

 minimal grouping was then map-plotted, with 

 logarithmic keyed symbols being used for plots of 

 abundances of minimal species groupings. The 

 plot of each grouping was then compared with that 

 of the grouping with which it next fused; if no 

 significant differences in distribution were evi- 

 dent, the fusion was considered to be intragroup. 

 This procedure was repeated until all minimal 

 groupings had been fused into groups showing 

 evident distributional differences. In cases where 

 there was any doubt as to whether two groups 

 should be fused, nodal analysis diagrams were 

 generated and compared for the two cases and the 

 decision producing the "crisper" result (Clifford 

 and Stephenson 1975) utilized. While this method 

 obviously involves some subjectivity in the recog- 

 nition of groups, it has been pointed out by several 

 authors that all methods of interpreting numeri- 

 cal classifications require a certain degree of sub- 

 jectivity and that fixed stopping rules are espe- 

 cially inappropriate with fusion strategies which 

 introduce a group size-dependent element into in- 

 tergroup relative affinities (Boesch 1977; Pielou 

 1977; Clifford and Stephenson 1975). 



Two methods of nodal analysis were performed. 

 The patterns of "constancy" and "fidelity" of 

 species groups to site groups were expressed as 

 relative densities of cells of a two-way table 

 (Stephenson et al. 1972). Constancy is the propor- 

 tion of the number of occurrences of each species 

 group in the site group to the total number of 

 occurrences possible (Boesch 1977). The index has 

 a value of 1 when all members of a species group 

 occur in all collections in a site group and a value 

 of when a species group does not occur in a 

 given site group. Fidelity is a measure of the 

 degree to which species groups are limited to site 

 groups. The fidelity index used in this study was 

 the constancy of a species group within a site 

 group divided by the average constancy over all 

 site groups. This index is unity when the con- 

 stancy of a species group in a site group is equiv- 

 alent to its overall constancy, >1 when its con- 



stancy in the site groups is greater than that 

 overall, and between and 1 when its constancy 

 is less than its overall constancy. A chi-square 

 test was applied to the fidelity value of each cell 

 to determine whether it varied significantly (a = 

 0.05) from 1. Fidelity values significantly >1 

 indicate a positive association of species in a 

 group with a site group, while values significant- 

 ly <1 suggest a "negative" association. In the 

 present analyses, a highly positive (or strong) 

 association was inferred if the number of occur- 

 rences of a given species group within a site 

 group was twice that necessary to produce a fidel- 

 ity value significantly >1, and a highly negative 

 association was assumed when the number of 

 occurrences was less than half that necessary to 

 produce a fidelity value significantly <1. All 

 nodal diagrams were drawn with the width of the 

 rows and columns proportional to the number of 

 entities in the respective site and species groups. 



Species Dominance 



Numerically dominant species have been used 

 by ecologists for many years to characterize com- 

 munities (Thorson 1957), and changes in domi- 

 nant species often reflect faunal changes. In the 

 present study, we have compared patterns of 

 species dominance among site groups. A species 

 was included in dominance comparisons if it oc- 

 curred among the five most abundant species 

 in at least 20% of all the stations from a site 

 group. 



Faunal Affinities 



The faunal affinities of fishes captured were de- 

 termined by examining published records of their 

 usual ranges of occurrence (Bigelow and 

 Schroeder 1953; Leim and Scott 1966; Struhsaker 

 1969; Musick 1972). Most warm-temperate species 

 had resident populations south of Cape Hatteras 

 in the "Carolinian" faunal province (Hazel 1970) 

 and had their normal northern range limit some- 

 where within the Middle Atlantic Bight south of 

 Cape Cod. Boreal species had permanent popula- 

 tions north of Cape Cod, and most had their south- 

 ern range limit somewhere within the Middle At- 

 lantic Bight north of Cape Hatteras. A few boreal 

 species transcend Hatteras through bathymetric 

 submergence. Certain components of the fauna 

 tended to be residents on the inner shelf 

 (Scophthalmus aquosus) or outer shelf (Par- 

 alichthys oblongus). Many species were resident 

 on the shelf edge and upper slope (Musick 1976). 



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