Fry: Sustenance of Farfantepenaeus aztecus in Louisiana coastal waters 
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Figure 7 
8 13 C and 8 15 N values (in units of %o) of proventriculus (gut content) samples collected 
in 2006. (A) Locations of sample collections. (B) Isotope values of samples. Arrows 
indicate the location (A) and isotope value (B) of the single offshore proventriculus 
sample that had isotope values similar to those of riverine samples (black diamonds) 
in the Bird’s Foot Delta, as presented in the Results section. 
patterns, but it is noteworthy that in the Bird’s Foot 
Delta region, the high 8 15 N values for brown shrimp 
extended over a greater distance than did the the low 
8 13 C values (Fig. 3). Stated another way, C isotope 
values returned to marine values more quickly than 
did N isotope values at both ends of the delta (Fig. 3). 
This difference between the N and C isotope patterns 
is expected because river N nutrient concentrations 
are very high and dominate freshwater-marine mixing 
dynamics (Fry, 2002). In contrast, river and marine 
sources have fairly similar inorganic carbon concentra- 
tions, so that riverine signals are diluted much more 
quickly for C than for N. 
Riverine shrimp with the unique CN isotope tags of 
high S 15 N and low S 13 C accounted for about half of the 
recent immigrants arriving offshore. There is little 
independent tagging information that could validate 
this isotope estimate. LADWF does not sample low- 
salinity habitats in the Bird’s Foot Delta and instead 
focuses on routine sampling of Barataria and Terre- 
bonne bays of the central Louisiana coast to help set 
the periods for shrimp season openings and closings. 
River-influenced areas along the Bird’s Foot Delta are 
not given a special focus by LADWF, but the isotope 
estimates from the present study may indicate that 
they deserve more focus in future work. This may be 
