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 wou...