Food habits and trophic structure of the demersal fish assemblages on the Mississippi-Alabama continental shelf

This study surveys the trophic interrelationships of 56 demersal fishes of the Mississippi-Alabama continental shelf. Fishes were collected from 12 stations at 20, 60, 100 and 200 meter depths along three transects. Counts of food items identified to lowest possible taxon and estimation of relative percent volume of food were carried out on 5,616 specimens. Food items were condensed into ecological food categories based on ecological characteristics of the food items. Specimens of each species were grouped by size into ontogenetic trophic units (OTUs). Food habits of the OTUs are discussed in reference to the ecological food categories by season, transect and depth. A guild and assemblage structure is proposed and diet overlap is analyzed.; Guild analysis of the OTUs identified ten guilds: Pelagic Planktivores, Pelagic Macromobile Carnivores, Medusae Tentacle Nippers, Hypo-planktivores, Bentho-pelagic Non-selective Carnivores, Bentho-pelagic Fish Carnivores, Benthic Macro-mobile Carnivores, Benthic Fish Carnivores, Benthic Non-selective Carnivores, Macro-worm Carnivores. All ten of the guilds were found in the summer collections, while nine were found in the winter collections. Cluster analysis identified a ten cluster model for the winter and summer collections. Principal components analysis substantiated some of the classifications, but fared poorly on the two non-selective carnivore guilds. Factor analysis indicated that the perceived vertical location of the food items in the water column was the most important factor in guild assignment.; Recurrent group analysis divided the OTUs into three winter assemblages (Inshore Assemblage, Mid-shelf Assemblage, Outer-shelf Assemblage) and four summer assemblages (Inshore Assemblage, Shallow Mid-shelf Assemblage, Mid-shelf Assemblage, Outer-shelf Assemblage). Using season, assemblage, guild and OTU hierarchical levels, diet overlap analysis on food item count data was performed using Pearson product moment correlation coefficients. Little overlap was found, with most of the overlap occurring among the benthic guilds. A conceptual diagram of the spatial and trophic structure of the winter and summer demersal fish assemblages shows the vertical relationship of the guilds in each assemblage.