FISHERY BULLETIN: VOL. 79. NO. 4 



(STOCS study) and 2.4 g/m^ (Rowe et al. 1974). 

 Since the data from Rowe et al. (1974) were based 

 on a single sampling effort and the measures from 

 the south Texas study were based on 12 separate 

 sampling periods, we biased our infaunal biomass 

 estimates towards the STOCS data and derived a 

 biomass figure from a regression between total 

 density and total biomass of infaunal samples (Ta- 

 ble 1). Epifaunal invertebrates and demersal fish 

 were sampled in 15-min bottom tows with a 10.7 m 

 Texas box otter trawl with a 25 mm stretched 

 mesh cod end. Wet weights were determined di- 

 rectly from the trawl samples. 



Because biomass measurements were made on 

 the penaeid shrimp during only one season in the 

 whole STOCS study, we felt the data were not 

 sufficient to completely characterize the biomass 

 levels for the shrimp. Thus, shrimp biomass data 

 were taken from Gulf Coast Annual Shrimp Land- 

 ing Reports (U.S. National Marine Fisheries Ser- 

 vice 1976, 1978). The shrimp fishery yields, how- 

 ever, did not represent the total production of 

 shrimp in the coastal gulf waters. Therefore, for 

 our model we estimated the biomass of shrimp 

 populations that was not reflected by the catch 

 statistics. A survival curve for the shrimp popula- 

 tion was calculated (Figure 2), based upon a total 

 population egg production rate of 10^^ [based on a 

 mean of 800,000 eggs/adult female >140 mm total 

 length and a 1:1 sex ratio (Perez Farfante 1969)] 

 with a survival rate for the hatch of 1%. This 



resulted in a recruitment rate of 10^ juveniles to 

 the population (Figure 2). The three additional 

 data points on the curve were determined by split- 

 ting the shrimp biomass from the catch statistics 

 (shaded area) into three size classes and calculat- 

 ing the number of shrimp of mean size within each 

 of these classes. The curve was then extrapolated 

 from recruitment through each of these data 

 points with the mean size at emigration from the 

 bays indicated (Figure 2). 



The results of this two-step exercise provided 

 information to estimate production and develop an 

 energy flow model for the components of the south 

 Texas shelf food web according to the ideas of 

 Steele (1974) and Mills and Fournier (1979). Pri- 

 mary production estimates on an annual basis 

 were calculated from chlorophyll a measurements 

 according to the methods of Ryther and Yentsch 

 (1957). A turnover ratio of 7 was used to convert 

 macrozooplankton standing stocks to annual pro- 

 duction ( Steele 1974). Certain factors, such as tows 

 failing to reach the bottom and net clogging caus- 

 ing <100% efficiency, contribute known biases to 

 zooplankton sampling methods (Hopkins 1963; 

 Wiebe and Holland 1968; Fasham 1978). Because 

 of this and the fact that the water column was 

 usually homogeneous in the shallow waters at the 

 Reference Station (Flint and Rabalais 1981), we 

 doubled the zooplankton production estimates. A 

 turnover ratio of 10 was used for the microzoo- 

 plankton standing stocks because we assumed a 



Figure 2.— Plot of the reported shrimp 

 iPenaeua aztecus) fishery yield accord- 

 ing to size class (shaded area) along 

 with an estimated survivorship curve 

 (solid line! for the south Texas continen- 

 tal shelf from NOAA Statistical Area 20 

 (see Figure ll. 



10 Eggs Produced assuming a population of 



40 « 10' adultsd:! sex ratio 

 with 500,000 eggs/female and. 

 U egg hatch survival . 



ESIINATEO SURVIVORSHIP CURVE fROH RECRUITMENT AND 

 THREE DATA POINTS CALCULATED FROM CATCH STATISTICS 



ESTIMATED SHRIMP BIOMASS FROM CATCH STATISTICS (NOAA). 

 AREA UNDER CURVE REPRESENTS 781 of TOTAL SHELF POPULATION 



Z 



INDIVIDUAL WEIGHT(g) 



740 



