FLINT and RABALAIS: GULF OF MEXICO SHRIMP PRODUCTION 



Table 2.— Procedures for calculating the amount of annual production for zooplankton components 



'Turnover ratio (TR) for zooplankton from Steele (1974). 



'Assume carbon equivalent equal to 6°o wet weight for metazoans (G. T Rowe pers. commun). 



^Assume higher turnover ratio for microplankton than TR = 7 from Steele (1974) 



per yr (Table 2). The estimated total production for 

 the zooplankton components of the food web on the 

 inner Texas shelf was 4.1 g C/m^ per yr (Table 2, 

 Figure 5). 



If we assume a minimum transfer efficiency of 

 20% between primary producers and zooplankton 

 as suggested by Steele (1974), which is more con- 

 servative than the 27-32% suggested by Mills and 

 Fournier (1979), then 20.6 g C/m^ per yr (Figure 5) 

 would be required to support these fauna and 82 g 

 C/m^ per yr of primary production would remain. 

 With the exception of a small proportion of this 82 

 g C/m^ per yr, which may support pelagic 

 planktivorous fish, we believe that the majority of 

 the primary production is directed elsewhere. 



We derived a biomass figure for the benthic mac- 

 roinfauna of 1.1 g/m^ which we then converted to 

 an annual production of 0.29 g C/m^ per yr (Figure 

 5). From shrimp catch statistics, we estimated 

 shrimp production at 0.03 g C/m^ per yr. Based on 

 the hypothesized survival curve (Figure 2), the 

 estimate of shrimp production from catch statis- 

 tics was found to represent 78% of the actual shelf 

 population production as indicated by the shaded 

 area under the curve. Therefore, wdth the addi- 

 tional 22% of unharvested shrimp biomass, the 

 annual shrimp production was 0.04 g C/m^ per yr 

 (Figure 5). Additional data from the STOCS study 

 indicated that demersal fish and invertebrate 

 epifauna composed 0.02 and 0.01 g C/m^ per yr 

 production, respectively (Figure 5). The combina- 

 tion of these amounts with the shrimp production 

 estimates accounted for 0.07 g C/m^ per yr pro- 

 duced by fauna living in the bottom waters. Com- 

 paring this trophic level with the infaunal produc- 

 tion (0.29 g C/m^ per yr) and assuming a 10% 

 transfer efficiency, benthic infaunal production 

 appears to be an insufficient food source to solely 

 support the demersal component of the inner shelf 

 food web. 



DISCUSSION 



Research emphasis on the populations of shrimp 

 that are fished in the gulf has been directed to- 

 wards migratory habits (e.g., Inglis 1960; Klima 

 1964; Kutkuhn 1966; Trent 1967), dockside catch 

 statistics (e.g., Gunter 1962; Caillouet and Patella 

 1978; U.S. National Marine Fisheries Service 

 1978), the development of models relating fishery 

 harvest to environmental factors such as freshwa- 

 ter inflow ( e.g., Hildebrand and Gunter 1952; Mar- 

 tin et al. 1980), and natural history (e.g., Heegard 

 1953; Iversen and Idyll 1960; St. Amant et al. 

 1966). Other studies have focused on behavior 

 under laboratory conditions (e.g., Aldrich et al. 

 1968; Lakshmi et al. 1976). More recently, simula- 

 tion of the shrimp fishery emphasizing different 

 management strategies has been attempted by 

 Grant and Griffin (1979). These studies, however, 

 fail to pinpoint which factors maintain the shrimp 

 production which supports a thriving fishery. 

 Sources and pathways of nutrition and ramifica- 

 tions of interruption of this flow still remain to be 

 determined. 



Other recent studies on important fishery areas 

 (Steele 1974; Mills and Fournier 1979; Arntz 1980) 

 point out the need to understand the general 

 structure of marine ecosystems and the trophic 

 webs which support species important to fisheries. 

 Our study consolidates information on primary 

 production, secondary production, and abundance 

 of benthic animals into a theoretical model of the 

 northwestern gulf shrimp fishery food web. 



The Texas shelf supports large phytoplankton 

 biomasses with high annual production, espe- 

 cially in inner shelf waters where plankton are 

 most abundant (Figure 6). Spring blooms in 

 phytoplankton biomass are correlated with 

 riverine inputs and nutrient maxima (Flint and 

 Rabalais 1981). The patterns of inner shelf phyto- 



743 



