Without detailed hydrological data, it is impossible to determine the extent to which the low- 

 level fixed-crest weir reduced the volume of inflow relative to the other sites. If inflow was 

 reduced more than that at Little Pecan Lake, that might explain why production of white shrimp 

 was lowest at the fixed-crest weir site. 



Independent of water exchange, fixed-crest weirs affect recruitment of estuarine organisms 

 through blockage of the lower and middle portions of the water column. A 122 -cm-diameter 

 variable-crest flap-gated culvert, when operated properly, would provide ingress opportunities 

 throughout a larger portion of the water column than would a low-level fixed-crest weir. 

 Therefore, if immigrating post-larval white shrimp (and other immigrating estuarine organisms) 

 are sensitive to blockage of the water column, then a water control structure such as the one at 

 Little Pecan Lake may offer greater recruitment opportunities than a fixed-crest or a slotted weir. 



Because of scant data and the assumptions that were made in this study, findings generated 

 herein should not be construed as scientifically based conclusions. Instead, these findings should 

 be viewed as hypotheses. It is hoped that through this study, researchers will see that additional 

 practical fishery or hydrology research is needed before we fully know how, and to what extent, 

 different kinds of water control structures affect recruitment and production of estuarine organisms. 



CONCLUSIONS 



Available data suggest that when properly operated, a large diameter variable-crest flap-gated 

 culvert, installed according to current Soil Conservation Service guidelines, may provide greater 

 recruitment opportunities for white shrimp than either a standard-level fixed-crest weir or a low- 

 level weir, installed according to current Soil Conservation Service guidelines. In comparing the 

 semi-impoundment not regulated by a water control structure versus the semi-impoundment 

 regulated by a variable-crest flap-gated culvert, it appeared that water exchange was severely 

 reduced by the flap-gated culvert. Our observations suggest however, that relatively severe 

 reductions can be affected in the volume of water exchange without a proportional reduction in 

 white shrimp production. Immigrating post-larval white shrimp and other estuarine dependent 

 organisms may utilize a combination of active locomotion and passive transport to orient toward 

 and immigrate into selected areas, despite unfavorable tidal/exchange conditions. To maximize 

 recruitment and production of white shrimp within an actively managed semi-impoundment, the 

 structure should be kept open throughout the recruitment season, if possible. 



LITERATURE CITED 



Arnold, G.P., and P.R. Cook. 1980. Fish migration by selective tidal stream transport: first results 

 with a computer simulation model for the European continental shelf. In J.D. McCleave, G.P. 

 Arnold, and W.R. Neill, eds. Mechanisms of migration in fishes. Plenum Press, New York. 



Bradshaw, W.H. 1985. Relative abundance of small brown shrimp as influenced by semi- 

 impoundment. M.S. Thesis. Louisiana State University, Baton Rouge. 61 pp. 



Chabreck, R.R., and G. Linscombe. 1978. Vegetative type map of the Louisiana coastal marsh. 

 Louisiana Department of Wildlife and Fisheries, New Orleans. 



Copeland, B.J., and M.V. Truitt. 1967. Fauna of the Aransas Pass inlet, Texas. Vol 2. Penaeid 

 shrimp postlarvae. Tex. J. Sci. 18(l):66-74. 



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