DISTRIBUTION AND MICROHABITAT USE BY FLATFISHES IN A LOUISIANA ESTUARY

We used a 1 m beam trawl to characterize microhabitat use of flatfishe s in monthly samples collected in Barataria Bay, Louisiana. Six strata were established along a salinity gradient from the nearshore zone al ong the Gulf of Mexico to approximately 30 km inland. Randomized sampl ing within strata characterized flatfish distributions and environment al conditions throughout the bay. Microhabitats were characterized by salinity, temperature, dissolved oxygen, depth, distance from shore, a nd substrate type. In 594 short-duration, beam-trawl samples collected from October 1992 through September 1994, we identified seven flatfis h species from 7046 specimens, most of which were juveniles. Four spec ies accounted for 98.3% of all flatfishes. The two most abundant, offs hore tonguefish and bay whiff, were euryhaline and widely distributed over the salinity gradient; however, offshore tonguefish were concentr ated in the lowermost stratum where bay whiff were uncommon. The third species, fringed flounder, was more abundant in middle and lower bay strata at higher salinities. The fourth species, blackcheek tonguefish , was most common in middle and upper bay strata and declined in abund ance in coastal strata. A rotated factor analysis resolved six environ mental variables into three major axes that explained 69 percent of th e variance and were characterized as seasonal, depth-distance, and sub strate-salinity axes. Separation of species and life history intervals in three-dimensional factor space reflected temporal and spatial segr egation. Within the four common flatfishes, 14 of 24 variable comparis ons showed clear ontogenetic trends in which at least two size-class m eans differed significantly. Among the common species, temporal differ ences were reflected by mean temperatures and dissolved oxygen concent rations and spatial differences were reflected by other environmental variables, including depth, distance, substrate, and salinity.