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Fishery Bulletin 109(2) 
Previous studies of stable isotopes for offshore brown 
shrimp were conducted mostly along the south Texas 
shelf, showing that isotope “tags” allow some estimates 
about those estuarine habitats that are most important 
in supporting offshore populations (Fry, 1981, 1983). 
In particular, seagrass meadows produced shrimp with 
high 5 13 C values and small shrimp entering the off- 
shore fishery often had these distinctive isotope tags, 
indicating a strong linkage between inshore seagrass 
meadows and offshore populations (Fry, 1981). Offshore 
shrimp populations in Texas waters and the deeper 
Gulf of Mexico had very uniform carbon isotope values 
within a 2 %o range, consistent with relatively uniform 
average isotopic compositions of phytoplankton and phy- 
todetritus that support offshore benthic food webs (Fry, 
1983; Fry et al., 1984). Estuarine shrimp had a much 
greater (>5x) diversity of isotope values, reflecting the 
much more diverse set of food types supporting benthic 
food webs in estuaries (Fry, 1981), but shrimp arriving 
offshore as immigrants gradually lost these divergent 
estuarine labels and their isotope values converged to 
relatively uniform offshore values. Experiments showed 
that this change in isotope label was due to shrimp 
replacing their old estuarine biomass during normal 
metabolism, while also acquiring new biomass from 
offshore foods (Fry and Arnold, 1982). Calculations indi- 
cated that a 2-4x increase in mass was generally suffi- 
cient to lose the estuarine isotope tags for rapidly grow- 
ing shrimp that had switched to a new diet (Fry and 
Arnold, 1982; Fry, 2006). This tag loss could occur in 
1-3 weeks for smaller-size (<125 mm) shrimp that grow 
at rates near 1 mm/day and occurs over a longer (3-8 
week) period for any larger immigrant animals that 
grow more slowly offshore, but all immigrants gradually 
become residents as they acquire the distinctive offshore 
isotope tags. Experimental and field results thus both 
indicated that this type of food-related “disappearing” 
isotope tag had a relatively short life for rapidly grow- 
ing brown shrimp, but work with the isotope tags was 
nonetheless interesting because shrimp acquire the 
isotope tags naturally without handling or stress, all 
shrimp are tagged instead of just a few, and the isotope 
tags provide information about origins that is very dif- 
ficult to obtain otherwise (Fry 1981; 1983; 2008). These 
initial studies and much subsequent research has shown 
that isotopes can be used as tracers, tags, or labels for 
studying animal diets, origins, and movements (Hobson 
and Wassenaar, 2008; West et al., 2010). 
Given that previous studies of shelf areas off Texas 
and deeper waters of the Gulf of Mexico show uniform 
carbon isotope values in areas that lack strong river 
inputs, a comparative examination of isotopes in Loui- 
siana waters was undertaken in the present study to 
identify river impacts on brown shrimp origins and 
diets. A first goal was to test origins of shrimp along 
the Louisiana coast. Seagrass meadows that are hot 
spots of shrimp abundance in Texas waters are largely 
lacking along the Louisiana coastline owing to turbid 
waters, but Louisiana brown shrimp are nonetheless 
common in open bays and areas near salt marshes. 
Brown shrimp are especially abundant in Barataria 
and Terrebonne bays along the central Louisiana coast 
and these bays are sampled regularly by personnel of 
the Louisiana Department of Wildlife and Fisheries 
(LADWF) to help set various opening and closing dates 
for shrimp fishing seasons. These bays have relatively 
little input from the Mississippi River but are often 
considered the major estuarine source regions for Loui- 
siana shrimp production (Gaidry and White 1 ). However, 
during the course of this study brown shrimp were 
found to be also abundant in delta marshes in the Bird’s 
Foot Delta around the mouth of the Mississippi River, 
just to the east of the central coast and Barataria and 
Terrebonne bays. To test whether bays of the central 
coast or riverine marshes were more important shrimp 
source areas for the offshore fishery, small shrimp were 
collected as they arrived as immigrants to the offshore 
system and tested for their isotope tags. A second goal 
of this study was to test for a distinctive riverprint or 
isotope landscape (“isoscape”; West et al., 2010) by map- 
ping offshore shrimp isotopes to trace river subsidies 
to benthic food webs. The Mississippi River supplies 
most (>90%) of the freshwater in the Gulf of Mexico so 
that any river-related signals could be expected to be 
stronger in areas closer to the river. 
Combinations of C, N, and S stable isotope measure- 
ments were investigated as possible tracers of river 
influences. Carbon isotopes were used to investigate 
bay origins and linkages to offshore productivity, with 
low 8 13 C values (<-18%©) generally indicating estua- 
rine origins, and highest offshore values (near — 15%o) 
indicating higher phytoplankton productivity at the 
base of the food web (Fry, 1981; Fry and Wainright, 
1991). For nitrogen isotopes, studies of nitrates in 
the Mississippi River show a relatively high average 
value near 8 %o (Fry and Allen, 2003), so that estua- 
rine food webs incorporating nitrates became enriched 
in 15 N, a bottom-up labeling of whole food webs also 
observed in other human-impacted systems (Schlacher 
et al., 2005). Higher 8 15 N was expected for shrimp 
from river-influenced delta marshes than for shrimp 
from Barataria and Terrebonne bays that have much 
smaller river inputs. Sulfur isotopes also can provide 
an interesting label when high productivity in the wa- 
ter column leads to more organic matter settling to the 
seafloor and more sulfate reduction in benthic sedi- 
ments (Peterson and Howarth, 1987). Pelagic plants 
and animals have high S 34 S values near the +21%c 
value of marine sulfate (Rees et al., 1978; Peterson et 
al., 1985), but sulfides that are produced in sediments 
from sulfate reduction have low 8 34 S values and enter 
benthic food webs, resulting in lower 8 34 S values of 
5-15%e for animals such as estuarine brown shrimp 
(Fry, 2008). Geochemical studies in the northern Gulf 
of Mexico indicate that most sedimentary sulfides 
are bound with iron (Lin and Morse, 1991), but it is 
still possible that some of these sulfides are used by 
benthic bacteria and enter the organic food web, so 
that lowest shrimp S 34 S values might be expected for 
eutrophic river-influenced areas. 
