Fulford and Dillon: Intrapopulation variability in stable isotope data for Cynosaon nebulosus 
113 
-97 10' -92 45' -88 20' -83 55' -79 30' 
Figure 1 
Map of 10 collection sites for Spotted Seatrout (Cynoscion nebulosus) captured in 
coastal Mississippi and the western Mississippi Sound during 2007-09 for this study 
of intrapopulation variability in stable isotope values for this species. Sites are labeled 
as Bayou Caddy (BC), Dupont (DP), Bayou Portage (BP), oyster reefs (OR), Back Bay 
(BB, 2 locations), Fort Bayou (FB), Davis Bayou (DB), Graveline Bayou (GB), Marsh 
Lake (ML), and Barrier Islands (BI). Fish were collected at random at the Barrier 
Island site, and therefore fish collected at the Barrier Islands may have been collected 
adjacent to either of the 2 largest of the Barrier Islands. 
and similar values reported in the literature for the 
coastal Gulf of Mexico (Winemiller et ah, 2007). Pre- 
viously reported 8 15 N values for primary producers — 
needlegrass rush ( Juncus roemerianus Scheele), smooth 
cordgrass ( Spartina alterni flora), and edaphic algae — 
in a salt marsh within Graveline Bayou ranged from 
5.2%<? to 6.1%c (Sullivan and Moncreiff, 1990). Values 
for shoalweed ( Halodule wrightii Asch.), associated 
epiphytes, and sand microflora from Horn Island were 
reported to range from 4.4%e to 7.8 %o, and various spe- 
cies of macroalgae from the same site had 8 15 N values 
ranging from 4.7%o to 10.0%c (Moncreiff and Sullivan, 
2001). Although much remains to be learned about 
nitrogen sources and variability within Mississippi 
Sound as one travels from the mainland to the Barrier 
islands, the average 8 15 N value of 6.0 was considered a 
reasonable estimate on the basis of the data available. 
Intrapopulation variability of 8 13 C and S 15 N was calcu- 
lated separately on the basis of a comparison of isotope 
ratios for hatchery-reared Spotted Seatrout (n= 20) fed 
with a single processed food. Fish used for variability 
estimates were kept under constant conditions of tem- 
perature and salinity and fed continually from a fixed 
food source for at least 6 months before analysis. This 
food source was a commercially available pellet food 
used successfully in the culture of Spotted Seatrout 
(Blaylock 2 ), and all feed used for this study was taken 
from a single lot to assure compositional consistency. 
Stable isotope data were analyzed to quantify dif- 
ferences in both 8 13 C and S 15 N isotope values by loca- 
tion and sampling month with an analysis of variance 
(ANOVA). All post-hoc comparisons were conducted 
with a Tukey’s honestly significant difference (HSD) 
test. The relationships between both 8 13 C and 8 15 N iso- 
tope values and fish size also were fitted to a simple 
linear regression to test for the existence of a slope 
significantly greater than zero that would indicate a 
relationship between isotope enrichment and fish size. 
Both 8 13 C and S 15 N isotopes values in liver tissue were 
compared with results from white muscle and tested 
for deviation from a 1:1 relationship with a comparison 
of slopes U-test). All statistical tests were conducted 
with a type-I error rate of 0.05. 
2 Blaylock, R. 2011. Persona! commun. Thad Cochran 
Marine Aquaculture Center, Gulf Coast Research Laboratory, 
Univ. Southern Mississippi, 703 East Beach Road, Ocean 
Springs, MS 39564. 
