(Hackney & Haines 1980 Riera & Richard 1996), and freshwater inflow may increase the total primary 

 production through nutrient inputs (Nixon et al., 1986). 



The food sources utili2ed by juvenile brown shrimp during migration are more difficult to determine than that 

 of adults, which live in deeper offshore environments that are generally phytoplankton-based systems (Frj' & 

 Parker 1979). As juvenile penaeids migrate, they occupy different habitats, which correspond to different 

 feeding grounds (Dall et al. 1990). Food availabilit}' can differ gready within and between these habitats (Frj^ 

 1981), and different food sources may be used preferentially by shnmp. For example, stomach content analyses 

 of small juvenile Penaeus semisculatus vittt composed of a large variety of prey including diatoms, meiofauna, 

 insect larvae and seagrass (Heales et al 1996). Moreover, Wassenberg & liill (1987) observed large intraspecific 

 differences in the food ingested by Penaeus esculentus collected from widely separated areas. Using immunological 

 methods, it was found that the Penaeus ai^ecus and Penaeus setiferus have a diverse diet (Hunter & Feller 1987). 



Stable isotope analysis has been used successfully to determine original food sources of marine and estuatine 

 invertebrates (Harrigan et al. 1989). Stable isotopes assess food sources assimilated over time (Fry & Sherr 

 1984), so they are valuable for feeding studies when material in gut contents are difficult to identify due to 

 digestion and trituration. \^ariarion in d -^C values of the migratory brown shrimp along the South Texas coast 

 has been investigated previously (Fry & Parker 1979, Fry 1981). These studies have pointed out that seagrass 

 meadows of shallow-water habitats were important feeding grounds for migratory juvenile brown shrimp. 

 However, littie is known about spatial and temporal variation of food sources encountered by brown shrimp 

 throughout a complete migration between oceanic waters and upper estuarine reaches. As suggested by Fry 

 (1981), habitats other than seagrass meadows may contribute significandy to the feeding of migrating shrimps. 

 Therefore, it is important to know which feeding habitats contribute the most to the growth and development 

 of the brown shrimp along the Texas coastline. 



The aim of the present study was to identify the trophic dynamic links of migratory Penaeus a^ecus with food 

 sources in various habitats along the South Texas coast. Shrimp migrations were followed from the Aransas 

 Pass to Corpus Christi and Nueces Bay and to the ultimate nursery ground in the Nueces Delta. A primary 

 objective was to determine if the p rim ary production in a coastal marsh, which is being currendy restored by 

 the re -introduction of freshwater inflow, can support the feeding and growth of juvenile brown shrimps. 

 Shrimp and potential food sources were determined by stable isotope analyses (6"C and 6'*N). 



Material and Method 

 Sampling Area 



The study was carried out in the Nueces Estuary (Fig.l). The estuary consists of the Nueces River, the Rincon 

 Bayou marsh and the Rincon Bayou mouth. The Nueces River empties into Nueces Bay, which is connected to 

 Corpus Christi Bay, which is connected to the Gulf of Mexico by the Aransas Pass. Historically, the Nueces 

 River fed into Rincon Bayou, which is in the center of the Nueces Delta. A dam was created early in the 20* 

 century to contain Nueces River water. Recendy, the dam was lowered to allow flood events from Nueces River 

 to flow into the Rincon Bayou to restore the Rincon Bayou marsh. Sampling locations into Rincon Bayou 

 marsh (Fig.l) were distributed from the freshwater entrance into Rincon Bayou marsh (River site) to the Rincon 

 Bayou mouth (Rincon mouth). Sampling stations also included "Up Marsh" and "Down Marsh" within Rincon 

 Bayou marsh, and "Aransas Pass" which connected Corpus Christi Bay to the Gulf of Mexico. These sites have 

 been considered because, as reported by Moffett (1970), they may lie on the migratory route of brown shrimps 

 that enter Corpus Christi Bay through Aransas Pass. 



Collection and Preparation of Samples 



Sampling for organic matter sources and brown shrimp was carried out during from December 1995 to July 

 1996. Shrimp were collected by otter trawls or hand-thrown cast nets. They were sorted by hand and kept in 

 seawater from the sampling site so that guts would be purged prior to analysis. At the laborator)', shrimp were 

 identified to the species levels using a magnifying glass and keys, measured (length) and were frozen (-80°C). 

 For stable isotope analyses shrimp were prepared according to Fry & Parker (1979). White muscle tissue was 

 dissected from the shrimp abdomen, acidified (10% HCl) to remove any residual carbonates from cuticules, 



E-2 ^ Utili:(ation of Estuarine Organic Matter During Gnwth 

 and Migration by Juvenile Brown Shrimp 



