Recent research has shown that low-level, fixed-crest weirs reduce recruitment and production 

 of estuarine organisms in semi-impounded marshes (Bradshaw 1985; Herke et al. 1987b). Marsh 

 management practices have recently evolved from passive techniques to the use of variable-crest 

 weirs and flap gates. When of sufficient capacity, properly installed, and operated, these structures 

 can be used to provide a wide range of management flexibility. For example, these structures can 

 be effective in reducing loss of vegetated wetlands and promoting reestablishment of emergent 

 wetlands, especially in fresh and intermediate marshes (Wicker et al. 1983; Hess et al. 1988; T. 

 Joanen, Louisiana Department of Natural Resources, Baton Rouge; pers. comm.; R. Chabreck, 

 Louisiana State University, Baton Rouge; pers. comm.). These structures can also be operated to 

 maintain wetland functional values and allow moderate usage of semi-impounded marshes by 

 estuarine organisms (Davidson and Chabreck 1983; Hess et al. 1988). Given the fact that Louisiana 

 is losing as much as 10,360 ha of marsh per year (Gagliano et al. 1981), the installation and 

 operation of these structures, where needed, may be one way to reduce marsh loss. However, 

 preservation and restoration of coastal marshes using these actively managed structures and 

 techniques would reduce ingress and productivity of estuarine organisms within such managed 

 marshes. Although no research has been conducted to quantify the extent to which estuarine 

 organism production is reduced within actively managed marshes, one might expect the reduction 

 would be at least equivalent to that attributed to usage of fixed-crest weirs. Because of the 

 operational flexibility of actively managed structures, many managers feel that they can preserve 

 coastal marshes while allowing greater use by estuarine organism than in marshes managed by 

 passive, fixed-crest weirs. 



Virtually no data presently exist concerning production of estuarine organisms within actively 

 managed marshes. In this paper we compare white shrimp production under three different 

 management regimes: an actively managed semi-impoundment, a passively managed semi- 

 impoundment, and a semi-impoundment where water exchange was not regulated by a water 

 control structure. A variable-crest flap-gated water control structure was utilized in the actively 

 managed semi-impoundment. A low-level, fixed-crest weir was used in the passively managed 

 semi-impoundment. The unmanaged semi-impoundment provided the best available estimate of 

 white shrimp production in a relatively unaltered system. 



METHODS 



Site Descriptions 



Little Pecan Lake is located within the Lower Mermentau River basin at Little Pecan Wildlife 

 Management Area, Cameron Parish, Louisiana. Little Pecan Lake, a nearly circular 174-ha lake 

 bordered by fresh and intermediate marsh (Chabreck and Linscombe 1978) was managed primarily 

 via a 122-cm-diameter variable-crest flap-gated culvert (Figure 1). 



In the early 1950's, Little Pecan Lake was an isolated freshwater lake. Later, an oil access canal 

 was dredged connecting Little Pecan Lake and Little Pecan Bayou. In an attempt to preserve 

 Little Pecan Lake's freshwater habitat and associated natural resources, a plug and a water control 

 structure were installed in the canal to prevent the free exchange of water between the lake and 

 the Little Pecan Bayou, which at times was brackish. Table 1 provides a listing of plants within 

 the Little Pecan Lake semi-impoundment. 



The other two sites used in the comparison are located adjacent to Grand Bayou, within Sabine 

 National Wildlife Refuge (Cameron Parish, Louisiana). Both areas, with 26.5 ha of water each, 

 are semi-impounded and were sites of extensive studies of estuarine organism movements (Herke 



171 



