FLINT and RABALAIS: GULF OF MEXICO SHRIMP PRODUCTION 



tical Area 20, the shrimp landing reporting region 

 (U.S. National Marine Fisheries Service 1978) 

 most closely associated with the STOCS study 

 area. As mentioned previously, the shallower 

 waters of the south Texas shelf are more produc- 

 tive, and the maximum yield of the brown shrimp 

 fishery is within the depth range of this station 

 (Grant and Griffin 1979). This site was charac- 

 terized by one of the largest data bases of all 25 

 stations. Water column variables were sampled 

 monthly during 9 mo in 1976 and 1977, and benthic 

 variables were sampled monthly during 9 mo in 

 1976 and seasonally (winter, spring, and fall) in 

 1977. Details of sampling procedures are available 

 in Flint and Rabalais.^ 



The STOCS data base contained adequate data 

 on various biotic and abiotic components to allow 

 for an integrated investigation of ecosystem rela- 

 tionships, with the shrimp populations as the ul- 

 timate focus of this exercise. Our approach was 

 twofold. In the first step, we evaluated all vari- 

 ables in relation to one another using a correlation 

 matrix. These comparisons included components 

 within the pelagic environment, within the 

 benthic environment, and between the two envi- 

 ronments. Variables considered were similar to 

 those listed in table 3 of Flint and Rabalais (1981). 

 We looked for relative changes in population den- 

 sities and biomass of biota that might be expected 

 to be associated, such as phytoplankton and zoo- 

 plankton or brown shrimp and benthic macroin- 

 fauna. Also, tracers, such as hydrocarbons and 

 trace metals, identified relationships between 

 components of the ecosystem based on organism 

 body burdens and concentrations of the tracers in 

 water samples and sediments. 



Bivariate Pearson correlation coefficients were 



■'Flint, R. W, and N, N. Rabalais (editors). 1980. Environmental 

 studies, south Texas outer continental shelf, 1975-1977. Vol. III. 

 Final report to the Bureau of Land Management, Department of 

 the Interior, Wash., D.C. Contract AA551-CT8-51, 648 p. 



used (Sokal and Rohlf 1969). All correlation coeffi- 

 cients, either positive or negative, were evaluated 

 for their biological meaning. Those suspected of 

 ecosystem relationships were put in a two-by-two 

 correlation matrix, within which the number of 

 significant correlations (P<0.01) had to be more 

 than 5% of the total for us to conclude that they 

 were not chance produced (Bernstein et al. 1978). 



The goal of this first step was to develop a corre- 

 lational model of relationships in the data that 

 suggested patterns in trophic coupling between 

 shrimp populations and other biotic components. 

 The patterns derived in step one prompted us to 

 develop biomass estimates for the related compo- 

 nents. The goal of the second step was to develop a 

 theoretical model of energy flow in a trophic web 

 which included penaeid shrimp as our central 

 focus. Data from the STOCS Reference Station 

 were used to estimate floral and faunal biomass as 

 follows. 



Chlorophyll a was measured according to stan- 

 dard techniques (Strickland and Parsons 1968). 

 Biomass of zooplankton was determined from 

 oblique (surface to near bottom to surface) tow 

 samples taken with aim net of 223 /xm mesh. 

 Neuston biomass was determined from samples 

 collected in a 505 /u,m mesh neuston net towed in 

 surface waters for 15 min. Zooplankton and neus- 

 ton data were originally reported as ash-free dry 

 weights but were converted to wet weights using a 

 conversion factor of 0.15 for crustaceans (Lie 1968). 

 Microplankton samples were collected in a 50 1 

 Niskin bottle at the surface and at one-half the 

 depth of the photic zone. Wet weight biomass was 

 estimated by measuring volume displacement and 

 assuming that a cell density of 1 fx'^ equalled 10 ~^ 

 /xg wet weight. 



Benthic macroinfauna ( >0.5 mm) samples were 

 taken with a 0.1 m Smith-Mclntyre grab. Esti- 

 mates of benthic infaunal biomass (Table 1) in the 

 south Texas shelf area ranged between 0.5 g/m^ 



Table l. — Comparison of abundance and biomass of macrobenthos from the northwestern Atlantic 



Ocean and northwestern Gulf of Mexico. 



'Measures from Rowe et al (1974). 



^Measures from the South Texas Outer Continental Shelf Study, 1975-77. 



^Wet weight calculated from densities of organisms using the density to wet weight ratio of the respective values from Rowe 

 etal. (1974) 



739 



