| Summary: | The demersal deep-sea fish fauna of the temperate eutrophic Middle Atlantic Bight (MAB) is compared with that of the tropical oligotrophic Bahamas region between depths of 900 and 5500 m. Of 136 species collected, 52 are shared. A taxonomic dichotomy between the two faunas reveals a fundamental pattern of differentiation among Atlantic deep-sea bottomfishes. Taxa of cold temperate-boreal origin with energetically expensive life histories (Moridae, Macrouridae) predominate beneath eutrophic waters. Taxa of warm temperate-tropical affinities with energetically conservative life histories (Ophidiidae, Chlorophthalmidae, Halosauridae) prevail beneath oligotrophic waters. The dichotomy appears to relate to regional differences in benthic macrofaunal prey availability.Numbers and biomass of fishes decline with depth in both regions. Population densities are higher in the MAB than in the Bahamas at slope depths, but are comparable below 3000 m. Biomass is uniformly higher in the MAB by about an order of magnitude at all depths, paralleling a comparable regional disparity in macrofaunal biomass. Mean fish weight displays a "bigger-deeper" trend only in the MAB. Both large and small species are important at all depths in the Bahamas. Species replacement takes place continuously, but at a variable rate. Bathymetric replacement gives the appearance of zonation with depth, but well-defined discontinuities are not apparent. Faunal richness is very stable across station groups and between regions, suggesting that "diversity" does not decline with depth. Species tend to space themselves randomly in the environment; individuals and units of biomass tend to be mildly aggregated. Such patterns suggest independent foraging and selective feeding among species. On the outer rise and abyss two alternative feeding modes prevail: euryphagy by large mobile species or nektonic microphagy by small sedentary species.Competitive exclusion is suggested by bathymetric complementation both within and between regions. This conclusion is reinforced by the limitation of and stability of faunal richness over depth. The longstanding generalization of demersal deep-sea fishes as non-selective, non-competing trophic generalists is inadequate.
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