Fry: Sustenance of Farfantepenaeus aztecus in Louisiana coastal waters 
151 
-150 -75 0 -75 
Distance from western edge of the delta (km) 
150 
Figure 3 
Average § 13 C and 5 15 N values (in units of %o) for inshore brown 
shrimp ( Farfantepenaeus aztecus) collected in Terrebonne Bay 
(-100 km), three regions of Barataria Bay (west bay at -60 
km, central bay at -30 km, and east bay at 0 km), and in the 
Bird’s Foot Delta region (5-110 km). The north-south dividing 
line between Barataria Bay and Bird’s Foot Delta shown in 
Figure 1 marks the zero-km reference used here. Values are 
means ±standard error from Table 1. Squares represent 5 15 N, 
circles 5 13 C. 
(Fig. 2). The spread in 5 15 N values for small shrimp 
was an indication that different estuarine source 
regions might be involved, source regions with 
higher and lower 8 15 N values than the offshore 
values. In contrast, smaller immigrant shrimp 
with S 13 C and 8 34 S values divergent from those 
of the largest offshore animals had values mostly 
lower than the offshore values, so that estuarine 
source regions seemed likely to be similar in 8 13 C 
and S 34 S values. 
Shrimp were collected over several years to 
test these ideas about possible isotopic differ- 
ences among estuarine source regions. Surveys 
of inshore Louisiana bays showed that shrimp 
from the Bird’s Foot Delta had a combination of 
relatively high 8 15 N values and low 8 13 C values in 
contrast to shrimp from Terrebonne and Barataria 
bays (Fig. 3). Highest S 15 N values were reached in 
the central delta and extended along the eastern 
side of the delta. Stations along the northwest 
side of the delta at the margin of Barataria Bay 
showed the beginnings of an increase in S 15 N, but 
the coordinated pattern of higher S 15 N and lower 
8 13 C developed about 20 km farther east of this 
margin (Fig. 3, Table 1). This same dual isotope 
pattern of high S 15 N and low S 13 C values was also found 
in another river-influenced bay system, at a station 
sampled in upper Galveston Bay near inflows from the 
Houston Ship Channel (Table 1, station Upper Galves- 
ton Bay vs. other Galveston Bay stations). 
Both shrimp size and isotope information were used 
to estimate immigrant origins in offshore populations. 
First, shrimp were selected that were relatively small 
(125 mm or less, 13 g wet mass or less). These shrimp 
were closest in size to shrimp collected in inshore bays, 
where the inshore shrimp averaged 85 mm and 4.7 
g wet mass, whereas the <125 mm shrimp collected 
offshore averaged 109 mm and 9 g wet mass. It was 
the <125 mm offshore shrimp that were expected to 
have arrived most recently offshore and therefore best 
reflect prior feeding in inshore bays (Fry, 1981; Fry 
and Arnold, 1982), and it was these smaller animals 
that accounted for most of the variation in the offshore 
isotope values (Fig. 2). Secondly, the C and N isotope 
information for large offshore shrimp was used to set 
bounds or cut-off values expected for resident shrimp 
that had grown for longer periods of time offshore and 
had time to equilibrate with the offshore diets. As with 
approaches used earlier (Fry, 1981, 1983), data for larg- 
est shrimp were used as a second criterion to define 
isotope ranges characteristic for offshore residents, and 
shrimp >175 mm (>35 g wet mass) ranged from -15.3%c 
to -18.4%c for 8 13 C and from 9.1%c to 12.2%o for S 15 N. 
Overall, offshore residents were defined as >125 mm 
shrimp with isotope values between -15.3%e to -18.4%c 
for 8 13 C and between 9.1%c to 12.2%c for 8 15 N. Isotope 
values for residents fell within the boxes in Figures 4 
and 5. 
Inshore studies showed that riverine shrimp from the 
Bird’s Foot Delta generally had higher 8 15 N and lower 
S 13 C than resident offshore animals (Fig. 4). Inshore 
shrimp from Barataria and Terrebonne bays had more 
diverse isotope values, but always had S 15 N values 
less than 11.6%c (Fig. 4). Based on these isotope dis- 
tributions, two types of immigrant shrimp to offshore 
systems were identified: riverine immigrants with high 
8 15 N and low 8 13 C (>11.6%e and <— 18.4%c, respectively; 
solid squares in Fig. 5) and bay immigrants with lower 
8 15 N plus 8 13 C values that were outside the range of 
offshore resident values (triangles in Fig. 5). A last 
group of shrimp was considered likely to resident (open 
diamonds in Fig. 5, see Discussion section). 
Over the two years of summer collections, 406 shrimp 
were collected offshore that were <125 mm, and accord- 
ing to the above isotope-based criteria for distinguish- 
ing immigrants and residents, 185 of these shrimp 
were classified as residents at the time of collection and 
221 were immigrants. About 46% of these immigrants 
had a riverine origin and 54% had a bay origin. The 
fraction of riverine immigrants was very similar in the 
two years, 48% in 2005 and 45% in 2006. The <125 
mm immigrants were present mostly as mixed popu- 
lations (bay+riverine+residents) along the inner and 
mid-shelf, and riverine shrimp were dominant (50% or 
greater of the <125 mm shrimp) at stations nearest the 
Bird’s Foot Delta and along the central coast at stations 
to the south and west of the Atchafalaya River (Fig. 6). 
The dual isotope label present in shrimp from the 
Bird’s Foot Delta, as high 8 15 N and low 8 13 C, was also 
present in proventriculus contents, i.e., both delta 
shrimp tissues (Fig. 4) and shrimp diets (Fig. 7) had 
relatively high 8 15 N and low S 13 C values. The question 
was whether this riverine dual label would persist in 
offshore foods, so that animals feeding offshore might 
acquire this riverine dual label offshore and thus be 
