significantly greater numbers of brown shrimp below a weir than above it and concluded that the 

 weir may have been an ethological barrier for some species even if it was not a total physical 

 barrier. Recruitment of several aquatic species (Herke 1971) and brown shrimp (Wengert 1972) 

 into semi-impounded areas was delayed, as was emigration from these areas. Moreover, Wengert's 

 average catch of brown shrimp per trawl was consistently almost 4 times as great from the 

 unimpounded areas. Weaver and Holloway (1974) found the standing stock of gulf menhaden, 

 white shrimp, and Atlantic croaker to be much lower in semi-impounded areas than in adjacent 

 unimpounded areas. In most cases "the detrimental effect [on estuarine-dependent fisheries] seems 

 to be magnified if semi-impoundment results in increased growth of submerged aquatic vegetation- 

 -and stimulation of such vegetative growth is a primary reason for creating semi-impoundments" 

 (Herke 1979). 



In the late 1970's a decision was made to install large concrete weirs in the two canals 

 connecting the marsh of Sabine National Wildlife Refuge to the western edge of Calcasieu Lake. 

 The weir crests were to be set at the conventional height, but unlike traditional weirs, there was 

 to be a radial arm gate in each. The gate, when lifted, was to provide a vertical opening about 

 10 ft wide from top to bottom of the channel. The U.S. Fish and Wildlife Service funded a study 

 to develop baseline biological and physical information so that Refuge personnel could manage the 

 gates for overall wildlife needs, yet reduce as much as possible the impact on estuarine-dependent 

 fisheries organisms. Passive traps, pictured in Herke and Rogers (1984), were mounted in each 

 canal as the principal sampling gear for our study. The traps fished the top, middle, and bottom 

 of the water column and caught organisms moving both to and from the marsh nursery. 



Although the study was not designed to determine the effects of weirs on fishery use of the 

 marsh, much pertinent information was obtained. The final report (Herke et al. 1984a) contained 

 an abundance of information on such things as the seasonal presence, relative abundance, 

 directional movements, and use of habitat types by numerous species. The canals were, used as the 

 migratory pathway between the marsh nursery and the Gulf of Mexico by 117 fish and 12 

 crustacean species (Herke and Rogers 1984). There was no time during the year when closure 

 of a water-control structure in one of these canals would not have interfered with the migratory 

 movements of a number of species. Figure 2 illustrates this point by showing the periods during 

 which only eight of the more important species were caught in the traps while moving either to 

 or from the marsh nursery. In the canals, most species moved mainly at dusk and at night. Cold 

 front passages and succeeding low water temperatures were followed by large emigrations 1 to 3 

 days later. (The absence of openings in a weir at such times could prevent such emigrations and 

 cause fish kills due to cold. Such openings can also allow escape during low water periods in hot 

 weather when low dissolved oxygen can cause fish kills; large scale emigration of brown shrimp took 

 place through the weir gate slot at water levels far below the weir crest-Schultz 1985.) Most 

 species moved primarily in the upper and middle levels of the water column, although some such 

 as white trout (sand seatrout, Cynoscion arenarius), Atlantic croaker, redfish, and speckled trout 

 moved mainly in the bottom half of the water column (Rogers and Herke 1985). This information, 

 coupled with lower speckled trout catches in semi-impounded areas than in nearby natural areas 

 in other studies, suggests that weirs inhibit immigration of speckled trout into semi-impounded areas 

 (Herke et al. 1984b). 



Most of the preceding studies estimated standing crop, which is not the same as annual 

 production, and thus gave only circumstantial evidence that semi-impoundment reduces fisheries 

 production. Standing crop is the number of individuals and the biomass in a given area at a 

 particular point in time. Rather than attempting to catch all the individuals to weigh and measure 

 standing crop, biologists usually take samples and use the results as an index of the standing crop. 

 Annual production is a function of the standing crop and how many times it "turns over" during 

 the year. For example, a hectare of unimpounded marsh and a hectare of semi-impounded marsh 

 may have the same standing crop of shrimp. Shrimp, however, need to cycle between the gulf and 

 the marsh, and some young shrimp are normally moving into the marsh while older ones are 

 moving toward the gulf. If a weir or other water-control structure inhibits these movements by 



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