Fulford and Dillon Intrapopulation variability in stable isotope data for Cynoscion nebu/osus 
115 
16.5 
16.0 
A 
15.5 
15.0 
14.5 
14.0 
13.5 
13 0 
12.5 
12.0 
11.5 
-24 -23 -22 -21 -20 -19 
1 
A Back Bay 
O Bayou Caddy 
■ Daws Bayou 
Fort Bayou 
9 Dupont 
A Graveline Bayou 
O Oyster reefs 
□ Marsh Lake 
■ Bayou Portage 
<7> Barrier Islands 
-18 -17 
16.5 
16 0 
B 
15.5 - 
15.0 - 
14.5 - 
2 14.0 - 
13.5 
13.0 
12.5 
12.0 - 
11.5 — 
-25 
1 1 1 1 1 1 1 
-24 -23 -22 -21 -20 -19 -18 
Figure 2 
Bivariate plot showing mean (±standard deviation (SD|) carbon ( 5 1 3 C ) 
and nitrogen (5 15 N) stable isotope values per mil for Spotted Seatrout 
( Cynoscion nebulosus) by (A) collection site and (B) collection month. All 
samples were collected in 2007, 2008, or 2009 off the coast of Mississippi. 
Symbols in the top panel indicate the 10 sites where fish were collected 
(see Fig. 1 for locations); black=bayou sites, white=open-bay sites, and 
gray=the offshore site. The asterisk (*) indicates a significantly different 
group (a=0.05). 
ticeable bias. The relationship between lipid-corrected 
5 13 C data from liver samples and data for muscle dis- 
played a relationship significantly different from 1:1 
U-test, slope=0.25, PcO.QOl; Fig. 4A) with a shift from 
positive to negative bias in liver 8 13 C values as muscle 
5 13 C values increased. However, although 5 15 N values 
also showed a linear trend between liver and muscle 
samples with a slope that differed significantly from 
one (5 15 N slope=0.50, PcO.OOl), liver tissue showed 
more consistent depletion bias than muscle tissue (Fig. 
4B). This bias increased with overall enrichment of 
5 15 N in muscle tissue. 
The resampling procedure was performed on data 
from 3 sites: Bayou Portage, Bayou Caddy, and the Bar- 
rier Islands. These sites had the largest sample siz- 
es and showed some contrast between nearshore and 
open-water sites. Intersample variability was related to 
pooled resample size ( n ; Fig. 5). A high mean absolute 
residual value ( >0 . 1 ) was observed for n < 3 but dropped 
rapidly to a value <0.05 for an n> 5. This pattern was 
consistent across sites despite a large difference in re- 
sidual values at smaller pooled resample sizes. In gen- 
eral, bias in mean isotopic ratio was minimal at any 
pooled resample size greater than n= 5. 
