150 
Fishery Bulletin 109(2) 
+1.1 %o for S 13 C and -0.5 %c for S 15 N to account for the 
effects of formalin (Edwards et al., 2002). 
In the laboratory, shrimp were thawed, total length 
and blotted wet mass were measured, and white muscle 
tissue was dissected from the tail area. Muscle tissue 
was cleaned by rinsing it under running tap water, 
then soaking the tissue in deionized water in glass vi- 
als for 15-60 minutes to remove saltwater. The water 
used for soaking was discarded, tissues were dried at 
60°C, and then pulverized with a Wig-L-Bug automated 
grinder (Dentsply International, York, PA). Proventricu- 
lus contents were obtained by dissection, acidified with 
10% HC1, centrifuged, and the stomach contents pellet 
was kept, and the acid was discarded. To further rinse 
and remove acid and traces of seawater, the pellet was 
resuspended with 20 mL of deionized water and then 
centrifuged again. This rinsing process was repeated 
three times before final drying of the pellet at 60°C. 
Shrimp were analyzed as individuals, but proventricu- 
lus contents were pooled by station to obtain enough 
material for analysis. Samples were analyzed according 
to established procedures for stable C, N, and S isotopic 
determinations (Fry, 2007, 2008). These procedures in- 
volve combustion of samples to C0 2 , N 2 , and S0 2 gases 
in an elemental analyzer, followed by chromatographic 
separation and measurement of these gases with an 
isotope ratio mass spectrometer. Results are reported 
in d notation as a %o difference from standards accord- 
ing to the formula 
8 (in %c)= (R S AMPLE /i? STANDARD - D '1000, 
where standards are PDB (PeeDee Belemnite) limestone 
for 8 13 C, nitrogen gas in air for S 15 N, and CDT (Canyon 
Diable troilite) for S 34 S, and corresponding R values are 
13 C/ 12 C, 15 N/ 14 N, or 34 S/ 32 S (Fry, 2007). 
Possible continued digestion during long-term storage 
in freezers and repeated washing of acidified proven- 
triculus samples undoubtedly removed some 
labile organic matter from the proventriculus 
samples, but samples were treated similarly 
and used for between-sample and between- 
station comparisons. Shrimp tissues had low 
C/N ratios of 3. 3-3.7 that were consistent with 
a mostly protein composition with little lipid 
content, and consequently no corrections were 
made to the carbon isotope data for lipid contri- 
butions (Fry and Allen, 2003; Post et al., 2007). 
Mean values are given with standard errors 
of the mean (SE), unless otherwise stated. 
Statistical comparisons among multiple means 
were made by using Fisher’s least significant 
difference method with significant differences 
indicated when P<0.05. Cluster analysis was 
done with the program Statgraphics Plus vers. 
5.1 (Statpoint Technologies, Warrenton, VA). 
Results 
Analysis of CNS isotope values for 969 offshore 
brown shrimp showed that isotope values of 
larger animals generally converged to a narrow 
range that was considered representative of 
offshore resident animals (Fig. 2). The number 
of samples was not equal for large and small 
shrimp (Fig. 2) because of the irregular avail- 
ability of samples, but the pattern of conver- 
gence to much narrower isotope ranges for large 
animals was the expected pattern and the same 
as that observed in previous extensive studies of 
shrimp and fish in the northern Gulf of Mexico 
(Fry, 1981, 1983). In many cases, smaller shrimp 
had these same convergent isotope values likely 
due to early migration from estuaries and rapid 
growth on offshore diets (Fry, 1981). Divergent 
isotope values were more interesting, especially 
for S 15 N, where small animals had values both 
above and below the values for larger shrimp 
15 -! 
0 20 30 40 80 100 
Wet mass (g) 
Figure 2 
Stable isotope carbon, nitrogen, and sulfur (CNS) compositions 
(8 13 C, 5 15 N, 5 34 S; in units of%e) of brown shrimp (Farfantepenaeus 
aztecus) collected offshore in the Gulf of Mexico in summers of 
2005 and 2006. 
