GULF OF MEXICO SHRIMP PRODUCTION: A FOOD WEB HYPOTHESIS' 



R. Warren Flint and Nancy N. Rabalais^ 



ABSTRACT 



The desire to better understand the dynamics of commercial shrimp populations which support an 

 important regional fishery on the south Texas outer continental shelf stimulated us to investigate an 

 extensive data base for links in the various ecosystem components that related to these dynamics. A 

 correlational model was developed that suggested relationships between pelagic and benthic compo- 

 nents of the south Texas marine ecosystem. Utilizing tracers, such as nickel concentrations in biota, 

 sediment, and water, we identified pathways of natural transfer between zooplankton, the benthos, and 

 coastal shrimp populations. These results stimulated us to develop a theoretical food web for the shrimp 

 populations, focusing on transfer of carbon. The results of this exercise indicated that the majority of 

 primary production ( approximately 809r ) is diverted to the benthos. Furthermore, it appeared that the 

 secondary production of benthic infauna was not sufficient to alone support the coastal shrimp 

 populations. We concluded that at least part of their nutrition was derived from the detritus pool which 

 was maintained by the excessive amount of primary production diverted to the benthos. The evidence 

 presented here suggests that the marine ecosystem in the coastal waters of south Texas functions 

 differently than other ecosystems studied in recent years and pinpoints the need for a better un- 

 derstanding of the basis upon which our marine living resources are supported, in order to predict not 

 only fishery yields but also effects of environmental disturbance. 



The commercial shrimp fishery in the U.S. waters 

 of the Gulf of Mexico is one of the most productive 

 fisheries the United States pursues. This fishery 

 provides better than 20% of the gross dollar value 

 for the total U.S. harvest (U.S. National Marine 

 Fisheries Service 1976) and represents the largest 

 fishery in terms of weight harvested and effort 

 expended along the gulf coast. For example, from a 

 coastal area of Texas covering 10,000 km^, an av- 

 erage of 5.7 X 10^ kg/yr of brown shrimp, Penaeus 

 aztecus, was landed in 1975-76, which represented 

 an annual value of $18 million. A decline in this 

 fishery could cause economic loss, at least on a 

 regional scale. 



Research emphasis on the populations of 

 penaeid shrimp that support the commercial 

 fishery in the northwestern Gulf of Mexico has 

 been directed towards laboratory behavioral 

 studies, migratory habits, and the development of 

 models relating harvest to environmental factors 

 and management strategies. Although the data 

 derived from these studies contribute to our un- 

 derstanding of the natural fluctuations that occur 

 in the fishery, they do not provide adequate infor- 

 mation about where the penaeid populations fit 



'The University of Texas Marine Science Institute Contribu- 

 tion No. 503. 



^University of Texas Marine Science Institute, Port Aransas 

 Marine Laboratorv, Port Aransas, TX 78373. 



Manuscript accepted May 1981. 



FISHERY BULLETIN: VOL. 79, NO. 4, 1981. 



into the trophic structure of the marine ecosystem 

 and how these populations function. Due to this 

 lack of knowledge, environmental managers 

 would not be able to predict with confidence how a 

 major perturbation in the Gulf of Mexico would 

 affect the shrimp populations. 



For years information has been accumulating on 

 primary production, zooplankton biomass, and the 

 distribution of benthic fauna in important marine 

 fishery areas. Attempts to quantify links between 

 these components have been provided by Steele 

 (1974) for the North Sea ecosystem and by Mills 

 and Fournier (1979) for the Scotian shelf. Arntz 

 (1980) more recently attempted to relate benthic 

 production with that of commercially important 

 demersal fishes in the Baltic Sea. With the comple- 

 tion of a 3-yr multidisciplinary environmental 

 study of the south Texas continental shelf (Flint 

 and Rabalais 1981), one more fishing area has been 

 characterized. 



The Texas shelf ecosystem is a dynamic system 

 driven by a complex aggregation of meteorologic 

 and oceanographic events. Superimposed upon 

 these phenomena are influences from local rivers 

 and estuaries as well as from distant points such 

 as the Mississippi River and the deep oceanic 

 waters of the gulf basin (Flint and Rabalais 1981). 

 The shallower waters of the Texas shelf are biolog- 

 ically a critical part of this ecosystem because of 



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