COLVOCORESSES and MUSICK: CONTINENTAL SHELF DEMERSAL FISHES 



the spring cold-water group B, one member of the 

 cryophilic group, and Squalus acanthias, a 

 ubiquitous dominant in the spring found only in 

 the northern portion of the study area in the fall. 

 The ubiquitous spring group (C) persists with Mer- 

 luccius bilinearis the major dominant, and two 

 other common members from the spring group, 

 but the fall group is no longer exclusively boreal- 

 resident in faunal affinity and the group is dis- 

 tributed primarily in more northerly and deeper 

 waters. Two warm-temperate species, Peprilus 

 triacanthus and Urophycis regia, join this group as 

 major dominants, while the other warm- 

 temperate species, dominated by Prionotus 

 carolinus and Stenotomus chrysops, continue to 

 occur in the same group (A) but show a dramatic 

 change in distribution, occurring on the inner 

 shelf rather than the outer as in the spring. The 

 shelf break group (E) shows the same composition 

 and distribution as in the spring, while the fifth 

 group (D), which did not occur in the spring, is 

 composed of nondominant eurybathyic species 

 which occur sporadically across all but the south- 

 ern inner site group. 



It is obvious that although the two sampling 

 periods included the two extremes of average 

 water temperatures in the study area, the fall 

 (warm extreme) is a much more dynamic period 

 than the spring (cool extreme) for the fish com- 

 munities in the region. This appears to be related 

 in large part to the much less stable thermal re- 

 gime encountered in the fall, particularly in shal- 

 lower portions of the study area. Thermal gra- 

 dients developed during the warmer months on 

 the inner shelf are much stronger than those en- 

 countered on the mid-shelf during the spring, and 

 because cooling waters mix or turn over while 

 warming waters stratify, the fall gradients break 

 down much more quickly than those in the spring. 

 As a result, a fish community in this region may be 

 subjected to rapidly changing environmental 

 temperatures by a number of factors. A relatively 

 small shift of water masses in the vicinity of a 

 strong thermal gradient, migration across a gra- 

 dient, or rapid cooling and mixing along the gra- 

 dient all subject these communities to abrupt 

 changes of temperature (Parr 1933), and it is not 

 surprising that the site groupings based on faunal 

 similarities found in the inner portion of the study 

 area during the fall exhibited wide temperature 

 ranges (Fig. 7). Parr (1933) pointed out that the 

 temperature-related distributions of organisms in 

 the vicinity of a strong thermal gradient may be 

 more influenced by the magnitude of short-term 



temperature changes than by the actual tempera- 

 tures encountered. This concept may well have 

 application to the formation of the three inner- 

 most site groups identified during the fall; for al- 

 though the groups strongly overlap with respect to 

 the temperature ranges encountered, there is a 

 considerable difference in the strength of the 

 thermal gradients and presumably the short-term 

 temperature variations encountered within each, 

 with group I being primarily sited in the region of 

 the sharpest gradients and group III being located 

 in the most thermally stable area. 



The distributional patterns noted in this paper 

 lead to the conclusion that continental shelf de- 

 mersal fish communities in the Middle Atlantic 

 Bight are largely structured by temperature on 

 the inner- and mid-shelf and by depth on the outer 

 shelf and shelf break. This is not at all unexpected 

 considering the sedimentary and topographical 

 uniformity of the inner- and mid-shelf (Emery and 

 Uchupi 1972) and the large annual variation in 

 bottom-water temperature in the inshore region, 

 with the converse holding true along the outer 

 shelf and shelf break. Scott (1982) found the dis- 

 tributions of a number of groundfish species on the 

 Scotian Shelf to be related to bottom sediment 

 type. Although substrate preference indices were 

 not generated during the present study, compari- 

 sons of species group distribution with bottom sed- 

 iment type maps do not indicate any strong species 

 group-sedimentary relationships. This contrast 

 may be the result of two major differences between 

 the continental shelves in the Middle Atlantic 

 Bight and off Nova Scotia; there is a much more 

 variable sedimentary environment and a consid- 

 erably smaller annual range of bottom-water 

 temperatures on the Scotian Shelf. 



Tyler (1971) examined latitudinal variation in 

 the regular and seasonal components of several 

 nearshore Atlantic marine fish communities, and 

 concluded that the proportion of seasonal and 

 occasional components to regular components var- 

 ied directly with annual variation in water tem- 

 perature. The results of the present study are cer- 

 tainly in accord with this conclusion, in that the 

 most highly variable area in terms of annual wa- 

 ter temperature variation (the southern inner- 

 and mid-shelf) was also the most variable area in 

 terms of community composition, but it is also 

 evident that Tyler's statement cannot be taken 

 axiomatically The outer shelf, although very 

 homothermic, was also subject to considerable 

 seasonal variation in community structure be- 

 cause of the changing relationship between the 



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