Effects on Mammals 



Spiller and Chabreck (1975) compared abundance of selected wild mammals in upper Barataria 

 Basin in nine areas influenced by weirs and in nine control areas. Indices were used to compare 

 populations of nutria, muskrat, and swamp rabbit (Sylvilagus aquaticus). Two indices were used 

 to compare muskrat populations. One indicated higher populations in managed marshes and the 

 other indicated higher populations in unmanaged marshes, but neither was statistically different. 

 Three indices were used to compare nutria abundance and the results indicated no difference 

 between managed and unmanaged marshes. Only one index was used to compare swamp rabbit 

 populations. The index indicated more rabbits in managed marsh on all sampling dates, but on only 

 one date was it statistically different. Small mammal populations were compared by using removal 

 trapping. The only small mammal collected was the rice rat (Oryzomys palustrus). Fifty-five 

 percent of the rice rats caught were caught in managed marsh, but this was not statistically 

 significant. 



CONCLUSIONS 



Previous research clearly indicates the positive effect of weirs on aquatic vegetation in most 

 marshes. Two areas where weirs have not consistently produced abundant aquatic vegetation are 

 saline marshes and on Marsh Island, where salinities fluctuated drastically in response to discharge 

 from the Atchafalaya River. In all areas, mean salinities in managed ponds and unmanaged ponds 

 were similar, but turbidities were only slightly lower in managed ponds. This indicates that weirs 

 may increase the occurrence of aquatic vegetation by modification of other factors. The other 

 possible factors related to weirs are reduced tidal scouring, reduced salinity fluctuations, and 

 stabilized water levels. Because of a great difference in the amount of aquatic vegetation between 

 managed and unmanaged ponds in an area drastically altered by artificial channelization, we believe 

 that the reduction in tidal scouring may be the main mechanism by which weirs affect aquatic 

 vegetation. We also believe that stabilization of salinity is important. Previous researchers have 

 reported that weirs slowed the rate at which salinities changed, but unfortunately no data are 

 available to compare the magnitude of salinity fluctuations in managed and unmanaged ponds. 



Weirs apparently do not have a large effect on the species composition of emergent plant 

 communities. Only one publication reported a difference in the structure of plant communities 

 associated with weirs and control areas, and this difference was not evident when the areas were 

 sampled 7 years later. 



No direct data are available to assess the influence of weirs on plant productivity. Limited data 

 indicate that hydrogen sulfide production should occur at a shallower depth in managed marsh, but 

 dissolved hydrogen sulfide concentrations were the same in managed and unmanaged marshes. 

 Therefore, some mechanism was removing hydrogen sulfide as it was produced in both areas, 

 possibly through tidal flushing, or by precipitation with ferrous iron. From these data we conclude 

 that plant productivity was similarly affected by hydrogen sulfide in marsh affected by weirs and 

 marsh not affected by weirs. The direct effect of the slightly lower redox potential on plant 

 productivity was not evaluated. 



In one study, vegetative cover was measured at three sites; at two sites more cover was found 

 in control areas, and at the other site the marsh influenced by weirs had more cover. From these 

 data we conclude that either weirs have no effect on the amount of vegetative cover, or that cover 

 varies in relation to factors associated with the location. 



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