Table 4. White shrimp landings from inshore waters of Cameron 

 Parish, Louisiana. 



Annual catch Effort Catch per effort 



Year (kg) (number trips) (kg/trip) 



However, Little Pecan Lake is located along the interface between fresh and intermediate marsh, 

 whereas the Grand Bayou sites are located within brackish marsh (Chabreck and Linscombe 1978). 

 Catches of white shrimp in a lake along the fresh-intermediate marsh interface of the Barataria 

 Basin were found to be substantially less than catches in brackish and saline marshes (Smith 1979). 

 It was assumed that similar abundance patterns occur throughout coastal Louisiana. Therefore, 

 white shrimp recruitment and abundance at Little Pecan Lake should be naturally less than that 

 at Grand Bayou. The exaggeration of white shrimp production at Little Pecan Lake, relative to 

 that from the Grand Bayou sites, may well be counterbalanced by the above-mentioned location- 

 related effect. If such is the case, we could then qualitatively compare the effect of water control 

 structures on white shrimp production within these semi-impounded marshes. 



Three hypotheses have been proposed to explain how weirs and other water control structures 

 reduce the recruitment of estuarine organisms (Rogers et al. 1987). The most commonly accepted 

 hypothesis is that immigrating estuarine organisms move passively with tidal currents. Therefore, 

 structures that reduce tidal exchange should also reduce recruitment. Implied in this explanation 

 is that, because of their passive transport, any reduction of water exchange will cause a proportional 

 reduction in recruitment of estuarine organisms. A second hypothesis proposes that behavioral 

 characteristics of ingressing estuarine organisms may cause them to avoid passing through or over 

 water control structures even though physically able. The third hypothesis proposes that ingress 

 of estuarine organisms is impeded when water control structures block that portion of the water 

 column that organisms prefer to use. 



Assuming that tidal amplitude is similar at each site, an index of turnover or water exchange 

 for each site can be estimated as the ratio of the water control structure's cross-sectional area 

 (below marsh level) at its greatest constriction versus the watershed of the semi-impounded area. 

 Therefore, at the unmanaged Grand Bayou site, for each square meter of the chute's cross-sectional 

 area at its greatest constriction, there are 9.5 ha of watershed. Similarly, for each square meter 

 of cross-sectional area above the low-level fixed-crest weir, there are 52.6 ha of watershed. At 

 Little Pecan Lake, there are 148.7 ha of watershed for each square meter of culvert cross-sectional 

 area. These indices provide a relative measure of the degree to which water exchange is reduced 

 among sites (Table 5). 



Water levels within a semi-impoundment regulated by a fixed-crest weir did not fall as low as 

 levels within a semi-impoundment regulated by a slotted weir (Rogers et al. 1987) or a semi- 

 impoundment not regulated by a water control structure (Herke et al. 1987b). The period of 

 flooding tide (recruitment period) was therefore longer for these semi-impoundments compared 



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