156 
Fishery Bulletin 109(2) 
29° N- 
Gulf of Mexico 30m n 
28° N- 
200m 
1000m ^ 
0 50 
1 1 1 km 
□ □ W 
□ Offshore 
□ Mid-shelf transition 
B Mid-shelf hypoxia 
Proventriculus 15 N-enriched 
95° W 
94° W 
93° W 
92° W 
91° W 
90° W 
89° W 
30° N 
Figure 8 
Station groupings from cluster analysis of samples from 2006. Circled points had 15 N-enriched proventriculus 8 15 N 
>10.7 %o, greater than average values for offshore resident brown shrimp ( Farfantepenaeus aztecus). The polygon 
indicates the area of the long-term hypoxic zone documented by Rabalais et al. (2002), where hypoxia is present in 
>25% of summer surveys. Symbols indicate station groupings identified by cluster analysis. 
especially true because of loss of inshore habitat in 
the Bird’s Foot Delta (Britsch and Dunbar, 1993). Es- 
timates for riverine shrimp contributions to offshore 
fisheries were very similar for 2005 and 2006, with 
2005 having average river discharge and 2006 having 
about 60% average discharge (http://www.mvn. usace. 
Figure 9 
Trends in relation to depth for stable isotope averages (±1 
standard error) in proventriculus contents of brown shrimp 
( Farfantepenaeus aztecus ) collected from the Gulf of Mexico 
in 2006 (see also Table 2). 
army.mil/cgi-bin/watercontrol, accessed July 2010). 
The similar riverine contributions in the two different 
years may mean that it is the long-term structure of 
the deltaic marsh-bay platform rather than the annual 
river inputs that is more important for the shrimp sup- 
ply to the offshore fishery. 
There are various reasons why the isotope es- 
timates presented here could be overestimates 
for the contributions of riverine shrimp to off- 
shore populations. For example, inshore sampling 
showed that both riverine and bay shrimp popu- 
lations produce some shrimp that have the same 
isotope values as resident offshore shrimp (Fig. 4). 
The isotope accounting done here thus underes- 
timates the contributions of the inshore popula- 
tions, and if this underestimate is more severe for 
bay than riverine shrimp, this would lead to the 
apparent strong contribution of riverine shrimp. 
In the extreme, if all of the <125 mm offshore 
shrimp with isotope values inside the resident 
box of Figure 5 were actually misclassified and 
instead were all bay shrimp, the contribution of 
riverine shrimp would decline from 46% to 25%. 
Further research should include samples nearer 
the mouths of bays to check whether most animals 
leaving bays already have isotope values classi- 
fied here as resident offshore shrimp, but in the 
end, even a 25% contribution of riverine shrimp 
is probably noteworthy for management purposes. 
