Table 5. Regression analyses relating net primary production (NPP) and inshore shrinp 

 production (1955-74) in hydrologic units to various physical parameters. NPP was calcu- 

 lated from the mean productivity and area of each habitat type (Costanza et al . 19S3). 

 Shrimp catch is from Barrett and Gillespie (1975). R is the proportion of the varia- 

 bility in the dependent variable accounted for by variations in the independent vari- 

 able. 



per unit area is very closely related to 

 the proportion of marsh in the unit 

 because -narsh productivity is higher than 

 aquatic productivity; therefore, average 

 productivity increases with the proportion 

 of marsh. 



Total net primary production is, as 

 might be expected, closely related to the 

 total area of the hydrologic unit. In 

 contrast, inshore shrimp catch, which in 

 these estuaries is quite a good index of 

 total shrimp yield (R. Condrey, LSU Center 

 for Wetland Resources; pers. comm. ) , is 

 poorly related to most single factors in 

 the analysis. This may be because of the 

 animal's complex migratory life history. 

 For example, shrimp yield is not related 

 to total hydrologic unit area, nor to 

 total net primary production. The best 

 relationship is to the marsh area and to 

 the total marsh edge length in the unit. 

 This suggests that accessibility to the 

 marsh and marsh refugia are important 

 fishery productivity, 

 indicated by the marsh 

 area ratio) increases 

 the del ta lobe . Si nee 

 marsh area decreases as the delta de- 

 grades, the total accessible marsh is 

 maximum in the early destructional geo- 

 logic phase. 



These tentative correlations between 

 marsh edge length and fisheries productiv- 



components of 

 Accessibility (as 

 edge length:marsh 

 with the age of 



ity need to be verified with additional 

 research, but the implications are inter- 

 esting and important. First, they support 

 Gagliano and Van Beek's hypothesis and 

 provide a reason why biological productiv- 

 ity peaks in degrading basins. 



Second, if the hypothesis is correct, 

 it has significant implications for the 

 future of Louisiana fisheries. We are 

 currently enjoying the results of past 

 delta building by the Mississippi River. 

 Modifications of the river have signifi- 

 cantly affected its ability to build new 

 wetlands. As a result we are not now 

 producing the geological resource for our 

 future fisheries. If there is a signif- 

 icant lag time before new delta growth can 

 support efficient fishery production, we 

 can not afford to wait until the present 

 bounty disappears before encouraging new 

 delta fonnation. 



SPATIAL GRADIENTS 



Within any delta basin a spatial 

 gradient is set up by the land's slope and 

 by the source and magnitude of freshwater 

 compared to marine water inflow. In the 

 Barataria basin the mean water slope from 

 the coast to the swamp forests 80 km 

 inland is about 2 mm/km (Byrne et al . 

 1976). Since coastal marsh elevations 

 approximate the local mean water level 



32 



