Lawrence P. Rozas, Thomas J. Minello, Denise J. Reed (UNO), Shea Penland (UNO), and Bob Cashner (UNO)

Current projections of Louisiana coastal land loss estimate that an additional 640,000 acres of marsh and swamps will convert to open water by 2050 unless the scale of restoration efforts escalates. In response, State and Federal agencies have developed a strategic plan for coastal Louisiana, ‘Coast 2050’, which seeks to provide a sustainable coast. An essential element of this plan is the diversion of fresh water from the Mississippi River into adjacent swamps, marshes, and coastal bays. Such projects are expensive – a structure to divert 10,600 cfs has recently been completed at Davis Pond, Louisiana at a cost of over $100 million. State and federal resource agencies know they must act swiftly in coastal Louisiana and funding has already been allocated under the Coastal Wetlands Planning and Protection Act (CWPPRA) for more diversions. The U. S. Army Corps of Engineers and National Marine Fisheries Service are developing plans for a diversion of 10-15,000 cfs to be constructed in the vicinity of Myrtle Grove, Louisiana to rejuvenate and sustain the deteriorated marshes of the Barataria Basin. While knowledge of coastal processes supports such strategies as the best approach for combating land loss, the specific impacts and benefits of the project have to be evaluated during this planning phase to both fulfill NEPA requirements and to optimize project design and operation.   

Production models accept physical and biological factor

Such large-scale coastal restoration in Louisiana is expected to affect fishery resources, but effects are difficult to predict given the current state of our knowledge. Fishery benefits of existing smaller-scale restoration efforts are based largely on generalized relationships and inferred from changes in total wetland area. Implementing large complex restoration projects, such as the Delta Building Diversion at Myrtle Grove, under CWPPRA and Coast 2050 requires knowledge of the effects of landscape-scale change in marsh patterns on fishery species in order to plan and implement these projects in an ecosystem context. Further, this information is required to provide realistic expectations about project outcomes to the public and resource agencies.
The density of juvenile fishery species appears to be a good indicator of habitat value in estuarine systems. While relationships between nekton density, habitat value, and secondary productivity are complex, density patterns also appear to be reasonable indicators of fishery productivity. Fine-scale (1-10 m) studies of nekton density patterns have shown that juveniles of many species (including brown shrimp, white shrimp, spotted seatrout, red drum, blue crab, and gulf menhaden) are closely associated with shoreline areas and the marsh surface in coastal wetlands. Although the factors that affect the use of these environments have not been completely defined, distance to the vegetation-water interface, salinity, marsh surface elevation, and tidal connectivity have all been identified as important regulators of habitat use and value. Thus, the spatial configurations of wetlands, their location in the landscape, the salinity regime, and a small number of other important environmental variables determine the value of nursery habitats for these species. However, our understanding of landscape-scale fishery impacts is hampered by the disconnect between observed fine-scale nekton-use patterns within individual marshes and the basin-scale changes in marsh characteristics associated with large restoration projects.

To support this project, NOAA has collected intensive, habitat-specific data required for linking marsh-water landscape patterns to fishery density patterns. These data were acquired by quantitatively sampling wetlands and water bodies across the estuarine gradient using 1-m2 drop samplers. This sampling effort was directed toward identifying density patterns in relation to pond size and bathymetry, marsh vegetation and elevation, the presence and species of SAV, and the distance from the marsh-water interface. These density patterns were then examined in relation to salinity regime and landscape-scale patterns of marsh-water.

Our analyses showed that saline and brackish salinity zones contained higher brown shrimp, white shrimp, and blue crab densities than the intermediate zone. Habitats associated with medium and large ponds supported higher densities of most species than small ponds. Most species were associated with vegetation structure, either concentrated among plant stems at the marsh edge or within submerged aquatic vegetation in shallow ponds. Brown shrimp in the lower estuary, however, were most abundant over shallow non-vegetated bottom near pond edges. These results indicate that small-scale density patterns of some nekton species in the Barataria System differ from patterns observed elsewhere in the Northern Gulf of Mexico.