Fry: Sustenance of Farfantepenaeus aztecus in Louisiana coastal waters 
153 
misclassified as immigrants from deltaic 
regions. In a test of this idea, proventric- 
ulus contents from near-delta offshore 
areas (open squares and open triangles 
in Fig. 7A) were sampled but generally 
did not show this riverine dual isotope 
label, i.e., 15 of 16 near-delta samples 
did not have the dual isotope delta label, 
but were relatively enriched in 13 C and 
followed the same isotope trend as that 
found for other samples collected from 
deeper offshore areas (Fig. 7B). Only one 
offshore proventriculus sample collected 
very close to the shore west of the Atcha- 
falaya Delta had the dual-label riverine 
combination of low high 5 15 N and low 
5 13 C (see open triangles with arrows, 
Fig. 7, A and B). 
Because the inshore bay and delta 
regions contained geographic isoscape 
distinctions that were useful in follow- 
ing shrimp movements, the offshore 
data for residents also were examined 
for possible geographic patterns. Clus- 
ter analysis was used to identify sepa- 
rate groups by using multivariate data 
for 48 stations sampled in 2006 where 
measurements included CNS isotope 
values for proventriculus samples and 
parallel CNS isotope values for muscle 
samples. For the cluster analysis, the 
muscle averages were compiled by us- 
ing only larger animals (>125 mm total 
length) that, as above, had C and N iso- 
tope values within the range of largest 
(>175 mm) resident animals, and there- 
fore were classified as offshore residents. 
The resulting cluster analysis identified 
three general regional offshore groups 
of shrimp: two mid-shelf groups inshore 
and closer to the river, and one offshore 
group farther away from the river to the 
south and west (Fig. 8). The two mid- 
shelf groups were mostly in or near the 
area identified by Rabalais et al. (2002) 
as regularly affected by summer hypoxia 
and linked to inputs from the Missis- 
sippi River (Fig. 8, polygon), whereas 
the offshore group was largely on the 
southwest side of this region, away from 
river inputs (Fig. 8). The offshore group 
was significantly different in average 
isotope values from the inshore group 
in all cases for the mid-shelf transition 
group and in all but one case for the 
mid-shelf hypoxic group (Table 2). 
Relative to this offshore group, the 
mid-shelf groups both showed signifi- 
cant enrichment in proventriculus 15 N 
and 13 C, and depletion in 34 S (Fig. 9, 
16 i 
4 H i i i i i i i i i i i i i ~ n 
-26 -21 -16 -11 
8 13 C 
Figure 5 
8 13 C and 8 15 N values (in units of %o) for smaller brown shrimp ( Farfan- 
tepenaeus aztecus ) (<125 mm total length) collected offshore and that 
had recently arrived from inshore estuaries. Shrimp were classified 
into three groups by considering the combined § 13 C and 8 15 N data: 
riverine shrimp (gray squares), bay shrimp (triangles), residents (x’s 
and diamonds, with diamonds indicating likely residents of the hypoxic 
zone, see Discussion section). The boxed values indicate the range of 
values observed in largest (>175 mm) offshore resident shrimp. 
