Management of RWR
Today habitat conservation is achieved by water level and salinity manipulation within management units and prescribed marsh burning. The basic management scenario utilized on RWR is to conserve vegetation, stabilize water levels, and manage salinities to encourage growth of submerged aquatic vegetation. In the fresher units, spring and summer draw-downs encourage production of annual emergents. Annual emergent and submerged aquatic plants are prime waterfowl foods. There are over 40 water control structures and 200 miles of levees maintained by the RWR staff. These levees and structures are strategically placed to form 12 management units which total approximately 43,000 acres of the total 72,650 wetland acres. Management units range in size (740 to over 14,000 acres) and each is classified under one of three major hydrologic management programs. The distinguishing characteristic of each management program is the extent to which water and salinity levels are controlled in response to meteorological conditions. The overall objective of RWR management units is to promote specific vegetation communities (i.e., emergent annuals, emergent perennials and aquatics) to support wildlife, especially waterfowl.
Gravity Drainage (Units 3, 4, 5, 15, Price Lake).—These five impoundments are located near the chenier ridge and enclose brackish to intermediate marshes. The primary management objective for these units is to control water and salinity for the propagation of important wildlife food plants. Many of the preferred waterfowl foods are herbaceous annual plants that must be reestablished each spring by germination of their seeds. To induce seed germination for the majority of these plants, water levels must be drawn down to near the level of the marsh floor so that a moist, not dry, surface exists. Once germination is achieved and a young stand of annuals is established, it is usually desirable to reflood the unit with a few inches of water to enhance growth and survival. After plant maturity is reached in the fall, the impoundment is allowed to flood further to ensure the availability of foods for wintering waterfowl. The draw-down is initiated in the spring, usually in May, with reflooding scheduled for September. The success of this program requires proper maintenance of the gravity drainage structures (concrete variable crest reversible flap-gates, 36-inch gate culverts and 48-inch marine aluminum flap-gates) and favorable meteorological conditions (Wicker et al. 1983).
Gravity drainage can lower water levels only to the low-tide stage plus additional loss through evaporation. In some years, when heavy rainfall occurs in the spring, complete draw-down is impossible and there is no chance for germination and production of herbaceous annuals. In such cases, the unit may be maintained as a flooded impoundment with the objective being to produce stands of submergent vegetation such as pond weed (Potamogeton pusillus) and widgeongrass (Ruppia maritima). Conversely, during periods of drought, there may be insufficient rainfall to reflood the impoundment after germination and establishment of the seed-producing annuals to promote growth and survival. In these instances, it may be necessary to allow brackish water to enter the impoundment through the water control structures. The units under this management are drawn down one year out of three. This is accomplished beginning in February, when stop-logs are removed and the flap-gates are lowered slightly on the structures to begin releasing water. From March through June or July, the structure remains in this draw-down operation mode. Around August the unit is reflooded and stop-logs are added to raise the flap-gates and contain higher water levels. Water is then held at this level for growth of widgeongrass. These management techniques are staggered among the units to produce seed-bearing annuals in some units and aquatic plants in others. In this way, the refuge can provide a food source aimed at the various preferences of the many species of waterfowl throughout most of the wintering season that normally lasts from September through much of March.
For example, Unit 4 is a testament to the success of this type of management program as evident in percent coverage by perennial, annual, and aquatic vegetation. During several years, annual and submerged aquatic plants covered almost 40 percent of the area sampled. The dominant aquatic plant was widgeongrass, and the most abundant annual, due to the normally brackish water conditions present, was dwarf spikerush (Eleocharis sp.; Chabreck 1959, 1960b, 1961, 1962, 1963; Chabreck and Joanen 1964, 1965, 1966; Joanen et al. 1967, 1968, 1969, 1970, 1971, 1972, 1973, 1974). The leaves, stems, and seeds of both plants are fed upon by waterfowl (Bellrose 1976), and up to 100,000 ducks have been observed in this unit (Ensminger, pers. comm. cited in Wicker et al. 1983). The degree to which the impoundment dries in the spring and summer determines, to a large extent, whether widgeongrass or spikerush is produced. During extremely dry years the production of both annuals and aquatics is low. The impoundment needs to be dewatered approximately every third year to allow consolidation of the bottom in order to reduce turbidity upon reflooding and to encourage widgeongrass production. In years during which water is abundant and widgeongrass production poor, excessive turbidities may cause low productivity. However, because multi-use is emphasized in this unit, less rigid controls of water levels are practiced in order to allow ingress/egress of estuarine aquatic animals such as shrimp and fish.
Refuge biologists monitor postlarval brown and white shrimp recruitment. When high concentrations of postlarvae are discovered in the vicinity of structures, the gates are opened for a short period of time to permit ingress of the postlarval shrimp. Before water levels within the unit begin to rise appreciably, the gates are switched back to the outflow position. Estuarine organisms are able to exit when the flap-gates are discharging at low tide. A report of Davidson and Chabreck (1983) indicated substantial utilization within Unit 4 by brown and white shrimp, blue crab (Callinectes sp.), white trout (Cynoscion arenarius), black drum (Pogonias cromis), Atlantic croaker (Micropogonias undulatus), and bay anchovy (Anchoa mitchilli). Although more species occurred in the canal outside of Unit 4, usage within the impoundment was substantial for most of the important sport and commercial species. Indications are that Unit 4 is functioning as a viable nursery ground which has, in turn, spawned an enthusiastic cast net fishery for shrimp for local sportsmen (Perry et al. 1993). Perry et al. (1993) reported shrimp production in managed coastal areas of southwest Louisiana to equal approximately 23 lbs./ac., which would total approximately 131,000 pounds for Unit 4. It has been estimated that Unit 4 may produce as much as 300,000 pounds of shrimp in some years (Ensminger, pers. comm. cited in Wicker et al. 1983).
Forced Drainage (1, 2, 8, 10/13, 14).—Currently five impoundments are managed with forced drainage. Units under forced drainage management have encircling levees, double divergent, low-lift diesel pumps, and water is controlled on a seasonal basis similar to that practiced under gravity drainage. Forced drainage allows better control of water levels than gravity drainage, but encourages similar plant communities. This is the most expensive management program because of the cost of maintenance and fuel to operate the pumps.
Pumping to remove water begins in a spring month, such as May, and water levels are drawn down during late spring and summer. After annual seed producing plants are well established, water levels are allowed to increase a few inches, thereby maximizing plant growth. Excess water is removed by resuming pumping or, at times, simply by removing stop-logs. During dry years, water can be pumped into the impoundment from the canal system as long as water salinities are not prohibitively high. Reflooding of the impoundment to a depth of 6-9 inches is usually initiated in September to make the mature seed crop available to waterfowl, which begin to arrive during fall migration. In years when spring rains are heavy and it is impossible to dewater the impoundment even with pumping, the unit remains flooded throughout the year for production of widgeongrass or other aquatics.
Controlled Estuarine (Unit 6).—Controlled estuarine management is centered around multi-use by both estuarine fisheries species and waterfowl species. The specific objectives are to allow adequate numbers of postlarval shrimp and fish to enter the unit during times of peak abundance and, at the same time, to produce conditions that favor growth of food plants for waterfowl.
A set of 40-foot locks and two radial arm, steel-gated cement structures are also used to manage water levels and salinities in the Unit 6 area of RWR. These three large water control structures on the refuge also impact adjacent privately owned marshes of the Mermentau Basin. Management of these structures mainly strives to relieve flooding and allow metered saltwater introductions, while maintaining sufficient water levels during times of drought.
The controlled and passive estuarine management units are nearer the Gulf of Mexico and contain brackish-to-saline marsh zones as opposed to the lower salinity intermediate-to-fresh marsh zones more common to the forced drainage and gravity drainage units. The major distinction between the passive and controlled estuarine management programs is that under passive management no scheduled effort is expended in achieving management objectives. In the controlled estuarine management unit, large scale control structures, implanted in the levees, can be manipulated on a seasonal basis to permit multi-use of the units by estuarine organisms and wildlife species.
Uncontrolled Drainage.—Approximately 30,000 acres of RWR are considered “unmanaged” by water control structures and/or levees. These areas include southeastern parts of the refuge (mostly in Vermilion Parish), areas south of Unit 6, and areas south and west of Price Lake Unit. Unmanaged areas on RWR consist primarily of brackish/saline marshes with interspersed tidal creeks, which are dominated by oystergrass (Spartina alterniflora), saltgrass (Disticlus spicata), and black needle rush (Juncus roemerianus). These areas are one of the few remaining examples of “pristine” brackish-salt marsh within the Chenier Plain, primarily due to the lack of human modifications (i.e., canals, dredging, levees). This area is important to some species of wintering waterfowl (i.e., Northern Shoveler, Gadwall, Mottled Ducks, Mergansers) and it provides further habitat diversity to the refuge. It also provides optimal habitat to other salt-marsh dependent species, including Seaside Sparrows (Ammodramus maritimus), Clapper Rails (Rallus longirostris), and Diamondback Terrapins (Malaclemys terrapin; W. Selman, pers. obs.).