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About 200,000 hectares of Louisiana marsh are, or soon will be, semi‐impounded by private landowners. Additional large acreages have been semi‐impounded by public agencies, and the trend is expected to continue, To determine quantitatively the effect of semi‐impoundment on fisheries export, a 70‐hectare marsh pond was leveed to create two nearly identical 35‐hectare ponds. A weir (low dam) was placed on the outlet to semi‐impound one of the ponds. All fishes and crustaceans emigrating from the two ponds were trapped continuously and the traps were emptied daily. The trapping continued for 2 years, the weir being switched to the outlet of the other pond after the first year. Comparisons of catches from the weired and unweired ponds showed that the weir caused substantial reductions in both number and total weight of most of the important estuarine‐dependent fishes and crustaceans migrating back toward the Gulf of Mexico. A subsequent test indicated semi‐impoundment by a vertically slotted weir would allow more fisheries export.

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ABSTRACT/Structural marsh management, using levees and water-control structures, is used in the coastal zone for many objectives, for example, to reduce marsh loss, to enhance waterfowl habitat, to revegetate open-water areas, and to reduce saltwater intrusion. The literature was evaluated to categorize structural marsh management and to determine some of its effects on fishes and crustaceans. Structural marsh management had positive effects on standing stock of most resident organisms and negative effects on marine-transient organisms. Emigration was negatively affected for both resident and marine-transient organisms. Techniques such as opening structures at critical migration times, designing structures that offer the greatest management flexibility, and using structures to create a flow-through system could reduce these impacts. More effort should be put into monitoring managed areas to determine if the objectives are being met and to evaluate the effects on fishes and crustaceans. Although frequent manipulation could reduce these impacts, the costs and problems incurred may outweigh the anticipated benefits.Structural marsh management, using levees and water-control structures, has been used to control water levels in the coastal zone for many years. The objectives of water control in coastal marshes are quite varied and include reducing extreme water level fluctuations, preventing saltwater intrusion, manipulating water levels, reducing coastal erosion, minimizing turbidity, stabilizing salinity, revegetating shallow mudflat areas, enhancing growth of desirable vegetation, enhancing waterfowl habitat, decreasing saltmarsh mosquitoes, and improving human access. Many of these objectives are closely related; for example, reducing water-level and salinity fluctuations may enhance waterfowl habitat by increasing submerged vegetation.Much controversy exists over the use of marsh management among federal, state, and local agencies, as well as the various user groups. Considerable effort has been expended in recent years to understand the effects of marsh management on coastal fishes and crustaceans, as well as other aspects of the ecosystem.We will review the literature and (1) describe and categorize various structural marsh management techniques, (2) examine the literature for effects on