THREATS TO COASTAL FISHERIES 



William H. Herke 

 U.S. Fish and Wildlife Service 



and 



Barton D. Rogers 



School of Forestry, Wildlife, and Fisheries 



Louisiana Cooperative Fish and Wildlife Research Unit 



Louisiana State University Agricultural Center 



Baton Rouge, LA 70803 



ABSTRACT 



Many saltwater fishes and crustaceans caught in Louisiana spawn in the Gulf of Mexico, but their 

 young must migrate into the coastal marshes, which they use as a nursery. There they grow rapidly 

 into juveniles or subadults and then migrate back to the gulf. Levees and water-control structures 

 are being used in an attempt to slow the rapid erosion of the marsh. Our studies show that these 

 structural measures interfere with the migratory movements of aquatic organisms and greatly reduce 

 fisheries production in the areas semi-impounded by the structures. We know of no documented 

 studies proving such structural measures actually reduce marsh erosion, and there is some evidence 

 they may actually hasten marsh loss. Therefore, we believe marsh management permit applications 

 should be placed on hold until the most important questions concerning semi-impoundment can be 

 answered. In 1987, two Acts of the Louisiana Legislature authorized use of the marsh nursery for 

 mariculture. In our opinion, these acts are not in the public interest and should be repealed. 



INTRODUCTION 



Many saltwater fishes and crustaceans caught in the Louisiana commercial and sport fisheries 

 have a similar life cycle. Adults spawn in the Gulf of Mexico; eggs hatch in a short time; and 

 larvae or tiny juveniles are carried by the current or swim toward the coast and usually proceed 

 through the bays into the marsh where they grow rapidly. After a few weeks or months, the 

 juveniles or subadults begin their migration back toward the gulf, where the survivors eventually 

 spawn to complete the cycle (Figure 1). Examples of species that complete the cycle from gulf to 

 marsh and back to gulf in 1 year are brown shrimp (Penaeus aztecus), white shrimp (Penaeus 

 setiferus), gulf menhaden {Brevoortia patronus), and Atlantic croaker (Micropogonias undulatus). 

 Other species such as redfish (red drum, Sciaenops ocellatus) and speckled trout (spotted seatrout, 

 Cynoscion nebulosus) may take longer to complete the cycle but still make similar migratory 

 movements. Weirs have long been used in Louisiana to regulate marsh water levels for purposes 

 of increasing waterfowl food production, improving trapper access, and the like. Traditionally, these 

 weirs have been solid dams placed across tidal channels. The crest of these weirs is usually about 

 15 cm below the average ground level of the surrounding marsh. Limited water flow can occur 

 only when the water level exceeds the crest level; when water level is at or below crest level, the 

 water behind the weir is impounded; thus the area controlled by the weir is said to be semi- 

 impounded (Herke 1971). Although limited water exchange over the weir crest can occur, the 

 weirs have the potential of reducing fishery production because they are in the migratory pathway; 

 the larvae and tiny juveniles must pass over the weirs to reach the semi-impounded marsh nursery, 

 and the surviving juveniles and subadults must again pass over them on their return to the gulf. 



FISHERIES STUDIES 



We began studying the effects of weirs on fisheries over 20 years ago, and we soon noted 

 circumstantial indications that weirs reduced production of some fishery species. Herke (1968) took 



1% 



