Full document / COSOC-W-86-002 - the National Sea Grant Library

Estuarine and CoastalManagement - Toolsofthe
Trade. Proceedingsof theTenthNationalConference
of TheCoastal Society. October 12-15,1986. New
Orleans.LA. Copyrightby TheCoastalSociety
1987.
EFFECTS OF FIXED-CREST WATER CONTROL STRUCTURES
ON THE ABUNDANCEOF FISH AND CRUSTACEANS
MIGRATING FROM A SHALLOW MARSH NURSERY
TOWARD THE GULF OF MEXICO
William H. Herke, E. Eric Knudsen, and BartonD. Rogers
LouisianaCooperative Fun and Wildlife Research Unit
School cf Forestry, Wildlife, and Fisheries
Louisiana State University Agricultural Center
Baton Rouge. LA 70303-6202
Abstract only
Weirs (low-level dams with their crests usually IS cm below marsh soil level) are
commonly used in Louisiana to improve wildlife habitatand stabilize eroding coastal marshes. We
suspected that these weirs may impede the migrations ofestuarlne-dependent organisms. To test
this hypothesis, a 70-ha marsh pond was leveed to create two nearly identical 35-ha ponds. The
single outlet from each pond connected througha common vestibule with a major tidal bayou. One
outlet contained aweir and the other didnot. Allfish, shrimp, and crabs emigrating from the two
ponds were trapped continuously. The trapping continued for two years, with the weir being
switched from the outlet of one pond to that of the other pond afterthe first year. Comparisonsof
catches, for most species, indicated that the standard, fixed-crest weir caused major reductions in
both numbers and biomass ofimportant estuanne-dependent fisheries resources migrating back
toward the GulfofMexico.
Once the adverse effect of the weirs on fisheries resources had been established, we
began a second investigation ofan alternative weir design which might reduce negative fisheries
impacts. Using the same study area we installed a standard fixed-crest weir in one outlet and an
identical weir, except with a vertical slot, in the other outlet We compared the catches ofanimals
leaving the pond having a standardfixed-crestweir to the catches from the pond with a verticallyslotted,
fixed-crest weir. Comparative catches indicated that a weir with an opening extending to
the bottom ofthe channel will increase the use ofthe marsh nursery behind the structure by about
SOpercent for all species combined.
791