Temporal and spatial patterns of microhabitat use by fishes and decapod crustaceans in a Louisiana estuary

We used a 1-m beam trawl to characterize microhabitat use by fishes and decapod crustaceans in monthly samples collected in Vermilion and West Cote Blanche bays in central coastal Louisiana. Randomized sampling within strata characterized the distributions of species, size-classes, and environmental conditions throughout the coastal bays. Microhabitats were characterized by salinity, temperature, dissolved oxygen, depth, distance from shore, substrate type, and turbidity. In 211 short-duration tows conducted from August 1998 through July 1999, we identified no fewer than 40 fish and 10 decapod crustacean taxa from 22,732 specimens, most of which were juveniles. The five most common species accounted for 67.9% of all individuals and were found in more than one-half of the samples ( greater than or equal to55.5%). Three of the five species are of immense commercial importance; the other two are important to the ecological community and as forage species for sport and commercial fishes. The two most abundant, Atlantic croakers Micropogonias undulatus and brown shrimp Farfantepenaeus, represented 39.3% of all individuals. A rotated principal components analysis resolved seven environmental variables into three major axes that explained 67% of the environmental variation and were characterized as seasonal (temperature-dissolved oxygen), salinity-turbidity, and depth-distance axes. Separation of species and life history stages in three-dimensional principal components space reflected temporal and spatial segregation. In univariate analyses of size-related patterns of microhabitat use within the five common species, 13 of 35 variable comparisons showed evidence of ontogenetic shifts in which at least two size-class means differed significantly. When resource use patterns were compared without regard to size, significant differences were detected for all seven variables between some species. This suggests that water management that influences important environmental variables (e.g., salinity and temperature) will affect the dominant species and the overall community structure in the system.