Habitat selection by sympatric tonguefishes (Symphurus: Cynoglossidae) in coastal Louisiana, USA: unravelling seasonal, spatial, and size-specific patterns in resource utilisation

We examined patterns of resource utilisation between young-of-the-year blackcheek tonguefish (Symphurus plagiusa) and offshore tonguefish (S. civitatium), in Barataria Bay, Louisiana, USA. A stratified monthly sampling approach was implemented to facilitate sampling of the broad saline to brackish estuary along environmental gradients. At each site several environmental variables were determined: salinity, temperature, dissolved oxygen, bottom type, median depth and distance from shore. In 594 independent beam-trawl samples, 2897 offshore tonguefish and 631 blackcheek tonguefish were collected. The distributions of both species overlapped broadly on a seasonal basis (Schoenerʹs index of overlap=0.78). Both species were found in the study area in all seasons, with highest abundances during fall and winter months. On a spatial basis, the species did not overlap as much (index of overlap=0.35). Approximately 83% of blackcheek tonguefish were collected in the uppermost or landward three strata, whereas 64% of offshore tonguefish were collected in the lowermost or seaward stratum. When spatial and seasonal distributions were considered together, overlap was reduced to 0.30, and when size-class distributions were added, the overlap of comparably sized individuals was reduced to approximately 0.23. In a multivariate analysis of variance comparison of microhabitat use between species, several significant differences were detected (P<0.05): blackcheek tonguefish generally used lower salinities, higher temperatures, finer bottom types, shallower depths, and were found nearer to shore than offshore tonguefish. Among size classes there were significant differences within and between species for all six microhabitat variables. Size-related shifts in resource utilisation along environmental gradients were evident for both species.