McEachran 1976; Gulf of Mexico Fishery Management Council [GMFMC] 1980a) . 

 Historically, these attributes are believed to have enabled the 

 communities to withstand high rates of exploitation with little 

 detrimental effect. Many of the commercially and recreationally 

 important reef species and pelagic predators of the shelf assemblages 

 are typically larger, longer-lived, and characterized by lower annual 

 mortality rates than the forms described above, and are thus much more 

 susceptible to over exploitation. 



The demersal epifauna of the shelf and some pelagic and reef 

 species of commercial importance (e.g. Gulf menhaden, red snapper) 

 appear to be characterized by the same generalized life cycle. Spawning 

 occurs at sea within the habitat of the adult, but the larvae are 

 transported, or make their way to organic-rich, low-salinity, nearshore 

 areas which serve as nursery grounds. The young animals develop in 

 these areas, and then return to their respective marine habitats as 

 subadults or adults (Pearse and Gunter 1957; Gunter 1967; Copeland and 

 Fruh 1970; Gallaway and Strawn 1974). Species characteristic of the 

 white shrimp ground assemblage (e.g. white shrimp, Atlantic croaker, 

 Gulf menhaden, silver seatrout, star drum) and a few from the brown 

 shrimp ground assemblage (e.g. brown shrimp) penetrate into the 

 estuaries which are among the most productive and organic-rich of all 

 ecological systems. Many estuaries and/or rivers, particularly those of 

 the eastern Texas-Louisiana coast, are significant exporters of organic 

 materials and nutrients to the nearshore shelf which is generally 

 characterized by higher levels of organic materials and productivity 

 than more offshore areas. These nearshore areas are used as nursery 

 grounds by many species representative of the brown shrimp ground and 

 outer shelf assemblages (Rogers 1977). The above described life history 

 pattern has contributed strongly to the confusion over the placement of 

 the bathymetric boundaries between the faunal assemblages represented on 

 the Texas-Louisiana shelf. 



A simplified conceptual model for the generalized shelf assemblage 

 is shown in Figure 21 . Boundaries between the pelagic and demersal 

 components should not be considered as precise, particularly when the 

 water column is well mixed. Flint (1979) points out that many nektonic 

 species are pelagic as adults and larvae but have benthic eggs; and, 

 conversely, numerous benthic species produce floating eggs which hatch 

 into planktonic larvae and become dispersed by currents before settling 

 permanently to the bottom. He further notes that motile epifauna such 

 as demersal fish can (and do) swim into the pelagic zone to feed on 

 plankton, but both the benthic epifauna, which lack strong swimming 

 capability, and the benthic infauna depend upon the continued rain of 

 organic materials from the overlying waters for nourishment. 



The phytoplankton are depicted as the major primary producers in 

 the system with productivity governed by levels of dissolved and 

 suspended nutrients and toxins, and by levels of suspended solids which 

 decrease the availability of sunlight. An important source of dissolved 

 and suspended nutrients to the system is the discharge from rivers and 



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