Tidal migrations of nekton in salt marsh intertidal creeks

Salt marsh intertidal creeks are important habitats for dozens of species of nekton, but few studies have attempted to quantify patterns of tidal movement. We used the sweep flume, a new sampling device, to investigate relationships between depth and movements of nekton inside the mouths of intertidal creeks. Sweep flumes located in three creek beds were used to collect nekton at 10 cm increments (10-100 cm of water depth) during flood and ebb tides in the North Inlet, South Carolina, salt marsh. Of the 37 taxa collected, 13 comprised >99.5% of the total catch and were the focus of the analysis. A nonlinear mixed modeling procedure was used to determine the depth at which each major taxon reached peak abundance during flood tides. With high degrees of spatial and temporal consistency, resident taxa entered early on the rising tide and transient taxa entered during mid to late tide. Depths of peak migrations varied among taxa and were consistent between creeks, days (within months), and years. As summer progressed, depths of peak migration increased for young-of-the-year Leiostomus xanthurus, Lagodon rhomboides, Mugil curema, Eucinostomus argenteus, and Litopenaeus setiferus as their median sizes increased. Within tides, depths of migration increased as a function of size for L. xanthurus and M. curema. Comparisons between flood and ebb tides indicated that most taxa exited the creeks at approximately the same depths at which they entered. Relationships between major taxa pairs suggested that biotic interactions may have contributed to the structure of the migrations observed in this study. Our results are the first to demonstrate quantitatively that the migrations of nektonic taxa into intertidal creeks are structured and related to depth.