Fry: Sustenance of Farfantepenaeus aztecus in Louisiana coasta! waters 
159 
in the hypoxic zone to come to equilibrium with the 
15 N-enriched foods. This idea is reasonable given recent 
fisheries studies that show hypoxia is often displacing 
brown shrimp populations to areas of higher bottom- 
water oxygen (Craig and Crowder, 2005; Craig et al., 
2005). In future studies, the disequilibrium or mis- 
match between shrimp and proventriculus 8 15 N may 
help identify areas that do not continuously sustain 
brown shrimp populations. Areas where proventricu- 
lus 8 15 N is higher than shrimp muscle 8 15 N may be 
less suitable habitat that can be visited only briefly by 
brown shrimp. The isotope signals in diets of brown 
shrimp and their prey are built up over several weeks, 
so that the isotope measurements may provide longer- 
term information about shrimp use of hypoxic areas 
than do trawls that provide a more instantaneous snap- 
shot of how brown shrimp are using an area (Craig et 
al., 2005). 
However, occasional feeding in the hypoxic area 
should lead to somewhat elevated 8 15 N values, so that 
higher 8 15 N could develop in offshore resident shrimp. 
Several offshore shrimp were observed with high 8 15 N 
that could indicate some feeding in the hypoxic zone. 
These animals also had high 8 13 C values (less negative 
than -18%c; open diamonds in Fig. 5) expected for off- 
shore residents rather than for migrants from inshore 
regions, and were accordingly classified as offshore 
residents for purposes of estimating movement and 
inshore contributions to offshore fisheries. 
An interesting feature of this study was that offshore 
brown shrimp diets appeared to be linear mixtures 
between two sources, and variation in the source con- 
tributions accounted for most of the isotopic variation 
across the shelf (Fig. 10). The nature of these sources 
is not completely clear and may involve multiple factors. 
For example, high S 15 N values may reflect both a high 
value of Mississippi River nitrate at the base of coastal 
food webs (Fry and Allen, 2003; Wissel and Fry, 2005), 
and the presence of high trophic level consumers in 
the proventriculus contents. Conversely, low 8 15 N may 
reflect relatively low values for offshore marine nitrate 
and prey from low trophic levels. Unfortunately, isotope 
values for specific prey taxa have not yet been measured 
for this shelf ecosystem, and therefore trophic-level ef- 
fects for isotopes cannot be directly evaluated. Nonethe- 
less, some inferences can be made from the measured 
isotope data for shrimp and their proventriculus con- 
tents, as follows. 
The farthest offshore animals had high S 34 S values 
(Table 2) characteristic of mostly plankton-derived 
sulfur in the diet, with little contribution of benthic 
sulfides. These high 8 34 S values are consistent with 
relatively oligotrophic conditions across the deeper 
shelf, and with lower 8 34 S values indicating more eu- 
trophic conditions inshore. Carbon isotope TEFs be- 
tween offshore shrimp and proventriculus contents 
were surprisingly large at 3.2-5.2%c (Table 2), espe- 
cially compared to the general expectation that the 
carbon isotope TEF is near 0 %c for animals and their 
diets (Peterson and Fry, 1987) and compared to a 
measured carbon isotope TEF near 1 %o for estuarine 
Louisiana brown shrimp (Fry et al., 2003). The off- 
shore shrimp muscle 8 13 C values are fairly constant 
near -17.3%c, so that it is the very negative proven- 
triculus values that lead to the large observed TEFs. 
Nonetheless, the proventriculus 8 13 C values are near 
the long-term -22%c value associated with offshore 
marine primary production (Fry and Sherr, 1984), and 
may represent a realistic marine background value. If 
this is the case, then mass balance calculations would 
indicate that the labile foods near -17.3%e that are be- 
ing assimilated out of the -22%c marine background 
are likely a small part of the proventriculus contents. 
A consistent picture for the C and S results is that 
background, low-productivity pelagic conditions deter- 
mine the food availability at the offshore stations, but 
labile fractions that are depleted in 34 S and enriched 
in 13 C are increasingly found in the proventriculus 
contents at the more eutrophic inshore stations (Fig. 
10). These ideas need further study with taxonomic 
analyses of prey and with further studies on isotope 
changes during assimilation of offshore foods (Fry et 
al., 1984). 
In conclusion, further studies of both CNS isotopes 
and proventriculus contents in offshore brown shrimp 
could supplement annual summer water quality assess- 
ments of hypoxia and help determine hypoxia effects on 
living resources. Brown shrimp transit the mid-shelf 
hypoxic areas and isotopes in shrimp caught offshore 
show strong spatial signals that likely vary between 
years with high and low river flow. Isotope signals have 
been used as early warning indicators of the effects of 
eutrophication in coastal bays (McClelland et al., 1997), 
and it is possible that monitoring shrimp isotopes may 
help assess the effects of hypoxia on Louisiana shrimp 
populations. Adding benthic shrimp isoscape monitoring 
to ongoing water quality monitoring programs generally 
may be helpful for understanding changes in fisher- 
ies productivity and animal movements in this and 
other river-influenced marine ecosystems (Leakey et 
al., 2008). 
Acknowledgments 
K. Johnson and B. Pellegrin of the NOAA Pascagoula 
Laboratory kindly provided the offshore shrimp samples 
from SEAMAP cruises. G. Peterson assisted in shrimp 
collections made in the Bird’s Foot Delta region. LSU 
undergraduates E. Ecker, E. Gallagher, M. Grant, T. 
Pasqua, R. Sylvestri, and J. Wheatley helped with labo- 
ratory preparation of samples and data entry. J. Lentz 
drafted the maps used in this article. Brittany Graham 
read an initial draft of this manuscript and made 
helpful comments. This work was supported in part by 
Louisiana Sea Grant Projects R/CEH-13 and R-EFH-07, 
NOAA Multistress award NA 16OP2670, NOAA Coastal 
Ocean Program grant NA06NOS4780141, and NOAA 
grant 412 NA06OAR4320264 06111039 to the Northern 
Gulf Institute. This is N-GOMEX contribution 138. 
