COASTAL RESTORATION: Rebuilding Fishery Habitat

Linda Benedict  |  5/5/2005 6:29:58 PM

Megan La Peyre, Christina Bush and John Plunket

Many fishery species rely on estuaries as critical habitat during early life stages, including redfish, menhaden, shrimp, blue crab, croaker and flounder. Because of the economic importance of many of these fishery species, the success of many coastal restoration projects is partially determined by the habitat provided for them. Researchers from the LSU AgCenter and the U.S. Geological Survey Fish and Wildlife Cooperative Research Unit are involved in several studies investigating the effectiveness of marsh and shallow water estuarine restoration projects in restoring fishery habitat.

Within estuaries, vegetated habitats are often favored by juveniles because their decomposition products are the basis of the food web for small crustaceans, shellfish and juvenile fish, which in turn support larger fish and wildlife. Furthermore, coastal vegetation and other structured habitats such as oyster reefs provide valuable three-dimensional habitat that supply settlement substrate, shelter and food for a wide variety of important fishery species.

With coastal land loss in Louisiana averaging 30 square miles a year, consequences for shallow-water estuarine fishery habitats are unavoidable. The Coastal Wetlands Planning, Protection and Restoration Act of 1990 (Breaux Act), combined with other legislation, provide monetary support for Louisiana to implement a large-scale, coast-wide restoration program. While the ultimate goal of coastal restoration is to reverse land loss and restore damaged habitats, the means to achieve this goal effectively remain elusive.

In response, numerous restoration techniques have been and continue to be developed to replace lost habitat. Ranging from large-scale freshwater and sediment diversions, such as the Caernarvon diversion, to smaller scale restoration projects such as hydrologic manipulations, marsh re-vegetation, dredge spoil removal and reef creation, these restoration approaches have had various levels of success based on wide-ranging criteria. Thus, new ideas for restoration of estuarine habitats are continually being tried and tested.

Restoration ecology in the past has relied on the general working hypothesis called “field of dreams,” which assumes that “if you build it, they will come.” With increased focus on fish habitats resulting from the passage of the federal Sustainable Fisheries Act (1996), which requires the protection and conservation of habitats important to fishery species, managers and scientists alike have begun to examine this assumption. As a result, there is increased focus on the ability of many restoration projects to replace and restore fish habitat.

Two approaches that have generated recent interest involve (1) marsh restoration through terracing, which is hypothesized to restore marsh edge, reduce wave energies and encourage submerged aquatic vegetation growth, resulting in increased fish habitat, and (2) the creation of oyster reefs, which provide three-dimensional structure for increased shelter, food availability and settlement substrate for many fishery species.

Fish assemblages in terraced marsh

Among the different estuarine habitats, the marsh edge is hypothesized to be favored by juveniles for several reasons: (1) the vegetation reduces current and wave energy, (2) the structure of marsh vegetation provides areas for fish to hide from predators and (3) food availability is high at the marsh edge because nutrients and organic matter are released into the water from decaying plants on the marsh surface.

Terracing is a relatively new method of restoring marsh edge habitat that is gaining popularity as a means to restore shallow open water areas of Louisiana to vegetated marsh.Terracing is a restoration technique used to replace marsh and encourage sediment accretion in surrounding open water areas. Terraces are built in open water areas of the estuary where marsh previously existed, and the distance between marsh edges has been slowly increasing. Dredged materials are piled to form a discontinuous linear ridge that is planted with a tolerant and fast-growing marsh plant such as smooth cordgrass, Spartina alterniflora. Unlike spoil banks, which are continuous and rise above normal tides, terraces are discontinuous and flood at high tide. Multiple terraces are built in a pattern to maximize marsh edge habitat and decrease wave energy across the open water.

Constructing terraces gained popularity as a restoration and mitigation technique following reports that terraces near the Calcasieu Ship Channel on Sabine National Wildlife Refuge reversed shoreline erosion and created almost 17 acres of salt marsh with an interface of almost 5,000 feet. Terraces may also increase the abundance of submerged aquatic vegetation (SAV) and, hence, waterfowl, fish and invertebrates, but supporting data are unavailable.

The benefits of terraces to fishery species have not been well studied, although the technique is being heralded as one new and effective approach to restoring fish habitat. Beginning in the summer of 2001, LSU AgCenter researchers started examining the value of fish habitat provided by terraces built in 1999 at Sabine National Wildlife Refuge. Using a throw trap, the created terrace field and surrounding natural marsh areas have been sampled for fish and large invertebrates. The throw trap is a circular net thrown so that it traps all organisms in the water column in a 1 square meter area and has been used effectively in past studies to identify and compare assemblages of small fish using various shallow water habitats.

Samples are taken to represent the natural marsh edge, terraced marsh edge and open water habitat that would have been present before the terrace was built. Initial results suggest that terraces do increase habitat availability for fishery species, but the numbers of fish present may not be as high as at natural marsh edge habitat. Research is ongoing to increase our understanding of which species use the habitat provided by terraces. Researchers hope to identify aspects of the natural marsh and the created terraces that may be highly valued (used) by specific fish species and fish assemblages. The information gained from both the fishery portion and the submerged aquatic vegetation portion of the study can then be used to develop more effective means and patterns of marsh terracing and increase the effectiveness of our marsh restoration program.

Fish assemblages on restored oyster reefs

In subtidal estuarine areas, oyster bottoms may be particularly significant as fish habitat, because they often provide the only complex structure in the midst of flat mud bottoms. A number of studies indicate that oyster reef communities are highly diverse, supporting complex ecological communities that include species not found in adjacent areas of soft bottom habitat.

Bottom-dwelling fish such as blennies, gobies and toadfish are associated with oyster reefs, as are shrimp, crabs and many other invertebrates. Surveys of oceanic fish in other parts of the country have found that more than 70 species, including many economically important species such as red drum and seatrout, are often associated with three-dimensional oyster reef complexes.

The creation of artificial reefs in subtidal estuarine areas has become of increasing interest to many in Louisiana. In coastal Louisiana, dramatic land loss, changing salinity regimes and, in some areas, past shell-dredging activities have all affected the integrity and structure of many natural oyster reefs. One approach to coastal restoration that many, especially recreational fishing groups, are promoting is the creation of three-dimensional reefs in shallow-water areas where shell reefs once existed.

Despite more extensive studies in other parts of the country, relatively few surveys of oyster reef fauna have taken place in Louisiana, and few data exist documenting distinct differences in the abundance, diversity and feeding habitats of fish and invertebrates on natural oyster bottoms versus mud bottoms.

In August 2001, three oyster and three mud-bottom habitat areas in middle Barataria Bay were identified using side-scan sonar data. Using several different gear types, oceanic fish and bottom-dwelling animals are being sampled monthly to quantify any potential differences in abundance and diversity of fish and invertebrates using these different habitats. Preliminary results have shown a greater abundance and diversity of pelagic fish species at oyster reefs than mud bottoms, although sampling is ongoing.

A related project being developed by AgCenter researchers and the Coastal Conservation Association involves comparing abundance and diversity of fish associated with created reefs. In Louisiana, intertidal and subtidal reefs are being created with limestone rocks and oyster shells. Given the proliferation of artificial reef programs in the Gulf of Mexico, it behooves us to document the role of reefs as habitat for fishery species and identify characteristics of natural and created reefs that may be attractive to fishery species.

Maximizing the effectiveness of coastal restoration projects involves an adaptive cycle of project development, implementation, evaluation and adjustment. Understanding the consequences of various new techniques on fishery habitat and understanding the relationship between specific habitat characteristics of natural and created habitats can only serve to increase the effectiveness and overall success of Louisiana’s large-scale coastal restoration program.

Acknowledgments Funding comes from the U.S. Fish and Wildlife Service, Sabine National Wildlife Refuge, Louisiana Department of Wildlife and Fisheries, the National Fish and Wildlife Foundation and the National Oceanic and Atmospheric Administration, National Marine Fisheries Service.
Megan La Peyre, Adjunct Professor; Christina Bush and John Plunket, both graduate students, U.S. Geological Survey Fish and Wildlife Cooperative Research Unit, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.

(This article was published in the spring 2002 Louisiana Agriculture Magazine.)

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