Marsh terracing as a method for restoring estuarine habitat in Galveston Bay, Texas
Rozas, Lawrence P., and Thomas J. Minello
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Terracing is a relatively new wetland-restoration technique used to convert shallow subtidal bottom to marsh. This method uses existing bottom sediments to form terraces or ridges at marsh elevation. A terrace field composed of these ridges arranged in some pattern that maximizes intertidal edge and minimizes fetch between ridges is constructed, and the intertidal area is planted with marsh vegetation. Recently, marsh terracing was used to restore estuarine habitat at Galveston Island State Park, and additional restoration projects using this technique in Galveston Bay are planned for the future. To assess whether marsh terracing can enhance fishery habitat, we examined the habitat value of a terraced area constructed in 1991 at Sabine National Wildlife Refuge (Sabine NWR), Louisiana. We quantified and compared nekton densities in spring and fall 1999 in a terrace field and nearby reference or control area at Sabine NWR using a 1-m2 drop sampler. Decapod crustaceans were more abundant than fishes, composing 62% and 95% of all organisms we collected in spring and fall, respectively. White shrimp Litopenaeus setiferus, dagger blade grass shrimp Palaemonetes pugio, blue crab Callinectes sapidus, and brown shrimp Farfantepenaeus aztecus accounted for 94% of all crustaceans, whereas 60% of all fishes were gulf menhaden Brevoortia patronus. Mean densities of white shrimp (fall), dagger blade grass shrimp, blue crab, and brown shrimp (spring) were significantly greater in terrace marsh than over the control pond bottom. Moreover, densities of gulf menhaden and white shrimp were greater at terrace pond sites than control pond sites. Terrace marsh, however, was not functionally equivalent to natural marsh, as mean densities of dagger blade grass shrimp (fall), brown shrimp (spring), and blue crab were higher at control marsh sites than terrace marsh sites. When marsh terracing is used for compensating unavoidable damages to coastal wetlands, mitigation ratios could be adjusted to account for this lack of functional equivalency. Future restoration projects that include design changes to increase the proportion of marsh in the terrace field could enhance the habitat value of marsh terraces for fishery species. Because marsh terraces provide nursery habitat and support higher densities of most fishery species than shallow nonvegetated areas, replacing nonvegetated bottom within Galveston Bay and other northern Gulf of Mexico estuaries with terraces should benefit most species by increasing the total area of nursery habitat.