Fishery Ecology Branch Current Research

Terracing is a relatively new habitat-restoration technique that is 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 is constructed by arranging these ridges in some pattern that maximizes intertidal edge and minimizes fetch between ridges. One arrangement is a checkerboard pattern of square cells with open corners. Cell size (chosen for convenience) in the original checkerboard terracing project constructed in Louisiana was 0.37 ha (61 m on a side) and cell ridges were 49-55 m long. Following construction, the intertidal ridges are planted with marsh vegetation, and seagrasses or other species of submerged aquatic vegetation (SAV) may be planted in subtidal areas between terrace ridges. The technique has been promoted as a means of enhancing deposition and retention of suspended sediments, reducing turbidity, increasing marsh edge habitat, increasing overall primary and secondary productivity, and maximizing access for marine organisms. However, little is known about potential negative impacts of disrupting productive shallow water habitats to create terrace fields.

We assessed the nursery value for fishery species of the original terracing project constructed in 1991 at Sabine National Wildlife Refuge (SNWR), Louisiana (Rozas and Minello 2001). The results of that study in a mesohaline marsh system indicate that habitat value within a terrace field for some species (e.g., brown shrimp, blue crab) will increase as the proportion of emergent marsh in a terrace field increases. One way to increase marsh area would be to reduce the size of each cell (relative to the SNWR design) within the terrace field, but the effect on habitat value of reducing cell size has not been examined directly. Reducing cell size will increase the density of marsh ridges in the terrace field and the amount of edge vegetation, and the abundance of fishery species that use marsh edge habitats should increase. Reduced cell size also should reduce fetch within the cells promoting increased sedimentation and reduced turbidity. However, reducing cell size will increase the percentage of disturbed bay bottom in each terrace cell, and the more complex patterns of connectivity within the terrace field may reduce access for transient organisms. Of course, decreasing cell size will also increase the construction cost per unit area of a terrace field.

In the spring and summer of 1999, terraces were constructed at Galveston Island State Park (GISP) for a restoration project funded by Apex NRDA restoration funds, CWPPRA, USFWS, Texas Parks and Wildlife Department, and the NMFS Community-Based Restoration Program. Terrace cells of three sizes: sides of 122 m (1.49 h pond area), 61 m (0.37 h), and 30 m (0.09 h) were constructed (Figure 1) in this project to allow an assessment of cell size effects on nekton use.

This restoration project provided an opportunity to test the effect of cell size on habitat quality, examine how cell size affects the efficiency of terrace construction, and evaluate marsh terracing as a method for restoring estuarine habitat and fishery production in Galveston Bay. We compared nekton density and biomass (as measures of habitat value) in marsh and open water habitat types among the three cell sizes of the GISP terrace fields. We also compared the habitat value of these terrace fields with the area before the terraces were constructed, with nearby SNB similar to that replaced by the terraces, and with natural marsh habitat. Animal abundance and biomass increased substantially in the project area following restoration by marsh terracing. An analysis of post-construction samples detected few statistically significant differences in animal density and biomass among cell sizes, but density, biomass, and species richness varied significantly by habitat type (i.e., higher in marsh vegetation than over SNB). Most taxa were not abundant over SNB; and within terrace cells, densities of most fishery species were similar between deep borrow and shallow pond areas. Using these post-construction density data, GIS, and population models for selected fishery species, we examined how nekton populations and cost effectiveness of terracing projects would vary with cell size. Populations of most fishery species increase as cell size decreases; but as cell size decreases, the cost of terrace construction increases much faster than population size. Our analysis shows that terrace fields constructed of medium or large cells would be more cost effective than terraces composed of small cells.