147 
Abstract — Brown shrimp (Farfan- 
tepenaeus aztecus) are abundant 
along the Louisiana coast, a coast- 
line that is heavily influenced by 
one of the world’s largest rivers, the 
Mississippi River. Stable carbon, 
nitrogen, and sulfur (CNS) isotopes 
of shrimp and their proventriculus 
(stomach) contents were assayed to 
trace riverine support of estuarine- 
dependent brown shrimp. Extensive 
inshore and offshore collections 
were made in the Louisiana coastal 
zone during 1999-2006 to document 
shrimp movement patterns across 
the bay and shelf region. Results 
showed an unexpectedly strong role 
for nursery areas in the river delta 
in supporting the offshore fishery, 
with about 46% of immigrants to off- 
shore regions arriving from riverine 
marshes. Strong river influences also 
were evident offshore, where cluster 
analysis of combined CNS isotope 
data showed three regional station 
groups related to river inputs. Two 
nearer-river mid-shelf station groups 
showed isotope values indicating 
river fertilization and productivity 
responses in the benthic shrimp food 
web, and a deeper offshore station 
group to the south and west showed 
much less river influence. At several 
mid-shelf stations where hypoxia is 
common, shrimp were anomalously 
15 N depleted versus their diets, and 
this 5 15 N difference or mismatch may 
be useful in monitoring shrimp move- 
ment responses to hypoxia. 
Manuscript submitted 10 July 2010. 
Manuscript accepted 07 January 2011. 
Fish. Bull. 109:147-161 (2011). 
The views and opinions expressed 
or implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National Marine 
Fisheries Service, NOAA. 
Mississippi River sustenance 
of brown shrimp ( Farfantepenaeus aztecus ) 
in Louisiana coastal waters 
Brian Fry 
Email address: bfry@lsu.edu 
Department of Oceanography and Coastal Sciences 
Louisiana State University 
Baton Rouge, Louisiana 70803 
Brown shrimp ( Farfantepenaeus az- 
tecus) have an estuarine-depen- 
dent life history that is well known 
(Gaidry and White 1 ). Adults spawn 
offshore, and postlarvae enter bays 
to settle as benthic juveniles. The 
juveniles typically reside in bays for 
1-3 months until they reach about 
70-100 mm total length, then leave 
for offshore shelf areas where they 
may double in length before complet- 
ing a largely annual life cycle. Both 
estuarine and offshore phases of this 
life cycle have been studied in detail; 
recent shrimp studies in estuaries 
have focused on loss of marsh nurs- 
ery habitats (Peterson and Turner, 
1994; Haas et ah, 2004), and offshore 
studies have focused on bottom water 
hypoxia that can impede shrimp 
migrations and decrease overall habi- 
tat area for brown shrimp (Craig et 
ah, 2005). 
The brown shrimp fishery of Loui- 
siana is one of the largest fisheries in 
the United States and occurs down- 
stream of Mississippi River inflows 
that fertilize the Louisiana coastal 
zone (Moore et ah, 1970). Nutrient 
loading from the Mississippi River 
has increased at least 2—4 times in 
recent decades in contrast to histori- 
cal background levels (Turner and 
Rabalais, 1991; Turner et ah, 2007), 
and this increase in fertilization of 
the coastal zone may be affecting off- 
shore shrimp dynamics. In this study, 
1 Gaidry, W. J., Ill, and C. J. White. 
1973. Investigations of commercially 
important penaeid shrimp in Louisiana 
estuaries. LA Wildl. Fish. Comm. Tech. 
Bull. #8, 154 p. LA Wildl. Fish. Comm., 
New Orleans, LA. 
stable isotopes were used to trace how 
the river currently supports brown 
shrimp populations because isotopes 
are increasingly used to trace link- 
ages between riverine nutrients and 
coastal fisheries (Schlacher et ah, 
2005; Leaky et al., 2008). 
The Mississippi River is one of 
the world’s largest rivers in terms 
of catchment size, total discharge, 
and sediment load (Deegan et ah, 
1986; Rabalais et ah, 1996). Most 
of the river flows into northern Gulf 
of Mexico in the Bird’s Foot Delta 
south of New Orleans, and also into 
Fourleague Bay west of New Orleans 
where the Atchafalaya River carries 
30% of the river flow into the Gulf 
of Mexico. During spring and early 
summer months when brown shrimp 
are found in coastal bays, most flow 
of the river is to the west along the 
coast and has typically high produc- 
tivity and high chlorophyll levels in 
the shallow offshore waters within 
5—10 km of the barrier islands (Walk- 
er and Rabalais, 2006). Eddies force 
some river water into bays where 
phytoplankton use nitrates from the 
river. Tides subsequently export pro- 
ductive phytoplankton to the Gulf of 
Mexico (Das et al., 2009). The high 
nutrients and strong water column 
mixing create conditions for high 
shrimp productivity that are similar 
to those observed in shrimp aquacul- 
ture ponds, but the coasts and bays 
are more open and allow extensive 
brown shrimp migrations at 10-100 
km scales. In these open systems, it 
can be difficult to trace the connec- 
tions between life history stages and 
populations that are important for 
managing fisheries. 
