FLINT and RABALAIS; GULF OF MEXICO SHRIMP PRODUCTION 



plankton productivity are paralleled by zooplank- 

 ton biomass (Figure 6) with peaks in shallow 

 waters and decreases in an offshore direction. 

 Likewise, both infaunal and epifaunal (rep- 

 resented by P. aztecus) benthic organisms are 

 more numerous along the inner shelf (Figure 6) 

 where general productivity is greatest in response 

 to larger food supplies, greater habitat heteroge- 

 neity, and nutrients. These productive shallow 

 waters are critical to the shrimp fishery popula- 

 tions. Greatest shrimp harvests for this part of the 

 Gulf of Mexico are recorded for these shallow 

 waters (Grant and Griffin 1979). 



Although our estimate of primary production 

 ( 103 g C/m^ per yr) may be low because one spring 

 bloom (1977) was missed by sampling frequency, 

 this value is similar to values reported for other 

 fish-producing areas. Mills and Fournier (1979) 

 reported 102 and 128 g C/m^ per yr for the Scotian 

 shelf and slope, respectively, and Steele (1974) re- 

 ported 90 g C/m^ per yr for the North Sea ecosys- 



tem. With the exception of a small portion which 

 may support pelagic planktivorous fish, we believe 

 that the majority of the northwestern gulf pri- 

 mary production is directed to the bottom. The 

 amount of pelagic fish production supported by 

 primary production on the Texas inner shelf is 

 unknown; however, the amount of zooplankton 

 biomass measured is not sufficient to support 

 large populations of pelagic planktivorous fish. As 

 indicated by the lack of a commercial fishery, the 

 planktivorous fish that primary production could 

 support represent small standing stocks in this 

 area of the Gulf of Mexico. Thus, almost 80% of the 

 total primary production biomass remains and 

 presumably much of this reaches the bottom in 

 coastal waters. 



Further evidence for this conclusion is shown by 

 the phytoplankton biomass distributions in the 

 water column (Figure 7). The bottom waters sup- 

 port equal or greater biomass of primary pro- 

 ducers than the surface or middepth waters as 



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FIGURE 7.— Plot of surface ( 1), one-half the depth of the photic zone (2), and bottom ( 5l water chlorophyll a concentrations for the south 

 Texas continental shelf environmental study between 1976 and 1977 at the Reference Station in Figure 1 (from Kamykowski, D.L., and 

 S. Milton. 1980. Phytoplankton and productivity In R. W. Flint and N. N. Rabalais (editors). Environmental studies, south Texas outer 

 continental shelf, 1975-1977, Vol. Ill, p. 231-284. Final report to the Bureau ofLandManagement, Wash., D.C. Contract AA551-CT8-.51). 



745 



