to freshwater flow have been con- 

 ducted by the Louisiana Department 

 of Wildlife and Fisheries. 



Louisiana Estuaries 



The total Mississippi drainage 

 area covers 3.2 million square kil- 

 ometers and includes one-eighth of 

 North America. The Mississippi River 

 and its main distributary, the Atcha- 

 falaya River, deliver an annual aver- 

 age of 17,400 m /s of fresh water to 

 the estuarine zone of Louisiana, an 

 area of approximately 13,866 km 

 (Barrett and Gillespie 1973). 



Within this estuarine system, 

 the salinity of upper bays is af- 

 fected primarily by local rainfall, 

 which averages 1,473 mm annually, 

 peaking in July, August, and Septem- 

 ber, with a low in March and April. 

 The influence of Mississippi-Atcha- 

 falaya flows on salinity predomi- 

 nates in the outer bays and near- 

 shore open shelf area. River flow, 

 a function of rainfall and snowmelt 

 over the entire watershed, reaches 

 maximum levels during April, May, 

 and June and minimum levels during 

 October, November, and December. 

 The Mississippi-Atchafalaya dis- 

 charges are responsible for 90 per- 

 cent of the freshwater inflow in the 

 Louisiana coastal system (Barrett 

 and Gillespie 1973). Mississippi 

 River discharge is a major contri- 

 butor of land-derived organic com- 

 pounds and nutrients such as nitro- 

 gen, phosphorus, and silica to 

 Louisiana coastal waters (Ho and Bar- 

 rett 1977). 



According to Barrett and Gilles- 

 pie (1973), shrimp recruitment is 

 sensitive to salinity and is strongly 

 influenced by the combination of lo- 

 calrainfall and river discharge. From 



1967 to 1972, brown shrimp yields 

 were greatest during years with min- 

 imal spring river discharges and 

 rainfall. White shrimp yields were 

 greatest during years with minimal 

 summer river discharges and rainfall. 

 Good years for brown shrimp and white 

 shrimp generally coincided. Barrett 

 and Gillespie (1973) suggested that 

 excessive spring and summer river 

 discharges and rainfall may lower 

 estuarine and nearshore salinities 

 below the tolerance limits of penaeid 

 shrimp and substantially limit op- 

 timum nursery areas. This conclu- 

 sion is supported by their observa- 

 tion that the area of inshore sur- 

 face water utilized each year as 

 fishing grounds varies from 8,000 km 

 to 11,500 km , depending upon the sa- 

 linity regime. Maximum acreage is 

 utilized during years of minimal riv- 

 er flow and local rainfall; whereas 

 minimal acreage is utilized during 

 years of high river flow and local 

 rainfall. Barrett and Ralph (1976) 

 suggested a relationship between an- 

 nual brown shrimp catches and the 

 number of acres of Louisiana estu- 

 arine surface waters above 10 ppt 

 salinity during the spring. The most 

 successful brown shrimp catches occur 

 when the number of square kilometers 

 above 10 ppt salinity exceeds 6,000. 

 Water temperatures also affect brown 

 shrimp recruitment success, but only 

 for a brief period in April (Barrett 

 and Gillespie 1973). 



A regression equation based on 

 research by Barrett and associates 

 is found in the Gulf of Mexico Shrimp 

 Fishery Management Plan (Gulf of Mex- 

 ico Fishery Management Council 1980). 

 The equation relates brown shrimp 

 landings to (1) average water tem- 

 perature at Grande Terre, Louisiana, 

 April 16 to 22 (positive relation- 

 ship); (2) average Mississippi River 

 discharge from March to May (negative 

 relationship); and (3) fishing ef- 



4 1 2 



