FISHERY BULLETIN: VOL. 79, NO. 4 



shown by the chlorophyll a concentrations in the 

 water column (Figure 7). These increases with 

 depth indicate that much of what is produced in 

 the pelagic zone reaches the bottom and accumu- 

 lates. 



The lack of stratification of the water column on 

 the inner shelf during most of the year makes this 

 conclusion reasonable. The water column is al- 

 most always well mixed in shallower waters <30 

 m depth (Flint and Rabalais 1981, figure 4), allow- 

 ing for a direct transport of photic zone primary 

 production to the bottom. This characteristic is 

 also ideal for processes related to benthic-pelagic 

 coupling, which we suspect are important in this 

 coastal ecosystem. 



Because mixing in the shallow shelf waters re- 

 sults in a relatively homogeneous water column, it 

 is reasonable to propose a trophic coupling 

 hypothesis for shrimp which includes both pe- 

 lagic and benthic components. The relationships 

 based on nickel tracers (zooplankton-^sedi- 

 ment^'shrimp) support this scheme. Also, zoo- 

 plankton fecal pellets are a major input to the 

 marine detritus pool (Gushing 1966; Steele 

 1974). Under the hydrographic conditions pres- 

 ent, the discrimination between pelagic and ben- 

 thic parts of the ecosystem is decreased and the 

 potential for trophic coupling between the sea 

 floor and overlying waters becomes more 

 meaningful. 



A key question about shrimp production con- 

 cerns the component(s) of the benthic community 

 from which shrimp derive their nutrition. Several 

 studies have attempted to determine the role of 

 benthic infauna as a food source for commercially 

 important demersal species. Boesch (in press) 

 found alterations in macrobenthic communities 

 that resulted in reductions in populations which 

 were dominant food items for demersal fishes and 

 invertebrates. However, the contribution of energy 

 flow to higher trophic levels from these popula- 

 tions and whether the larger fish and inverte- 

 brates were severely affected were unknown. 

 Additional observations such as those of Mcln- 

 tjrre et al. (1970) on molluscan siphon cropping 

 by a commercially important fish species, 

 and Arntz (1980) on changes in benthic infau- 

 nal biomass directly associated with demersal 

 fish predation, implicate the benthos as impor- 

 tant food items for species of commercial value. 



Based upon our theoretical model of production 

 estimates and energy flow for the south Texas 

 coastal environment, benthic infaunal production 



appears to be an insufficient food source to solely 

 support the demersal component of the inner shelf 

 food web. If we assume a minimum 10% transfer 

 efficiency for the infaunal biomass produced, 

 which is 0.29 g C/m^ per yr, this trophic level could 

 not support the 0.07 g Clw? per yr of total produc- 

 tion by fish, shrimp, and other invertebrates, nor 

 the 0.04 g C/m^ per yr annual production of P. 

 aztecus. Our calculations do not include 

 meiofauna production. Even if this component 

 were known, there probably would still not be 

 enough carbon production by fauna in the benthos 

 to directly support all higher trophic levels in the 

 bottom waters. In addition, the correlation analy- 

 sis did not identify any significant correlations 

 between shrimp densities and densities of fauna 

 inhabiting the sediment. 



The correlations between shrimp body burdens 

 and tracers in the sediment, however, provide evi- 

 dence for another means of shrimp gaining nutri- 

 tion, the detritus pool. These correlations support 

 conclusions from other studies (Cook and Lindner 

 1970; Caillouet et al. 1976) that shrimp rely upon 

 food provided by the marine detritus pool for at 

 least some of their nutrition. Condry et al. (1972) 

 observed that brown shrimp ate dead diatoms and 

 algal mat material in an estuarine habitat. Mor- 

 iarty (1977) recorded microbial feeding by shrimp 

 from detritus substrates, and Foulds and Mann 

 (1978) found evidence that crustaceans are able to 

 digest cellulose. The dependence of these popula- 

 tions on the detritus pool is a reasonable conclu- 

 sion. 



Another potential contribution to the detritus 

 pool comes from the discards — small shrimp, fish, 

 and other invertebrates — from the methods of 

 harvest employed by the shrimp fishery. According 

 to Bryan and Cody,*^ approximately 116 million kg 

 of catch-associated organisms are discarded an- 

 nually on the shelf off south Texas. Most of this 

 material eventually reaches the bottom and be- 

 comes an additional source of food for scavengers, 

 such as shrimp, to supplement the food sources 

 from the benthic habitat. 



The theoretical food web we propose for the 

 penaeid shrimp fishery of the shallow nearshore 

 waters of the south Texas continental shelf is in 

 contrast to the food web described by Steele (1974) 

 for the North Sea ecosystem and its related 



"Bryan, C. E,, and T. J. Cody. 1975. Discarding of shrimp 

 and associated organisms on the Texas brown shrimp (Pe/zoe-i/s 

 aztecus) grounds. Final Rep. to Texas Parks and Wildlife De- 

 partment PL88-309, Project 2-276R, 38 p. 



746 



