Fulford and Dillon: Intrapopulation variability in stable isotope data for Cynoscion nebulosus 
119 
level as fish begin to migrate offshore, although spatial 
variability in 8 15 N may also contribute to the observed 
differences. There was much overlap between sites, but 
the separation of small embayments from the Barrier 
Islands was detectable in both carbon and nitrogen, and 
therefore the seasonal-spatial pattern seems robust for 
within-site variability and occurs at a temporal scale 
larger than that for the tissue isotopic turnover rate. 
These results also are consistent with isotopic val- 
ues of Spotted Seatrout in Graveline Bayou reported by 
Sullivan and Moncreiff (1990), who found that Spotted 
Seatrout had a mean 8 13 C value of -20.7 and a mean 
5 15 N ratio of 11.8 — results that are very similar to the 
values reported here for Spotted Seatrout caught in 
Graveline Bayou. Our data, however, also indicate that 
these values are the middle of the overall range for 
8 13 C and near the bottom of the range for 8 15 N across 
all sites in Mississippi. A more comprehensive analysis 
of Spotted Seatrout position in the coastal food web 
will require sampling over a broad spatial and tempo- 
ral scale. Still, ecological patterns, such as spatial and 
temporal differences in SI values, are detectable, and 
therefore stable isotope data can be used for under- 
standing trophic relationships in this species. 
Individual variability in SI composition within a 
population can confound ecological patterns particu- 
larly for omnivores (Quevedo et ah, 2009; Sweeting 
et ah, 2005). Population variability can be caused by 
individual physiological differences (i.e., differences in 
isotopic fractionation or growth and tissue turnover 
rates); individual differences in behavior, prey avail- 
ability or prey preference; and changes in environmen- 
tal conditions that may influence individual movement, 
metabolism, growth, and diet (Barnes et ah, 2008). Our 
analysis shows that individual variability measured in 
the laboratory with a diet of a single commercial feed 
is quite low (standard deviation=0.24) in comparison 
with that derived from field studies (median standard 
deviation=1.5), showing that ecologically relevant iso- 
topic variability in Spotted Seatrout is detectable in 
field populations. 
Detection of population-level differences through 
space and time can be confounded by this individual 
variability within population-level samples. Pooling 
of individuals into a common sample for analysis is, 
therefore, an often used method for reduction of indi- 
vidual variability to increase the focus on population- 
level questions. How much pooling is enough to reduce 
individual variation? Our resampling experiment in- 
dicates that the effectiveness of pooling is maximized 
for a subsample size of 5-6 individual fish. Smaller 
subsamples introduced potential bias due to individual 
variation, and larger subsamples reduced sampling ef- 
ficiency. In general, we advocate pooling only for inves- 
tigations of interpopulation differences or for changes 
in a single population through time. Individual vari- 
ability has been found to be important in detection of 
behavior patterns (Sweeting et ah, 2005) and in com- 
munity-level studies (Bolnick et ah, 2003; Matthews 
and Mazumder, 2004). 
Conclusions 
Understanding the trophic role of omnivores, such as 
Spotted Seatrout, is a critical part of estuarine eco- 
system-based management, and SI analysis is a valu- 
able tool for trophic studies of this species. Diet data 
are needed for identification of important prey species, 
but such data can be biased by small sample sizes and 
do not provide information on differences in total as- 
similation across prey species. Stable isotope analysis 
is an important complementary tool that allows the 
characterization of trophic pathways at broader spatial 
and temporal scales. However, variability in SI values 
within a population can be caused by factors unrelated 
to diet shifts, and these mitigating factors must be ac- 
counted for and incorporated into trophic studies to 
achieve meaningful results. 
Our findings substantiate the utility of stable iso- 
tope analysis for quantification of the trophic role of 
Spotted Seatrout in an estuarine system and demon- 
strate that this trophic role changes spatially and tem- 
porally in predictable ways that can be incorporated 
into ecosystem studies. The trophic role of Spotted 
Seatrout is not static, it is dependent on factors, such 
as habitat, life stage, and fish size that are not inde- 
pendent of one another. Spotted Seatrout are known to 
display minimal movement before sexual maturation 
(Hendon et al., 2002), and this behavior indicates that 
the observed patterns in trophic role are related more 
to alterations in migration patterns as fish grow rather 
than to opportunistic movement not related to ontog- 
eny. Such distinctions are important for understanding 
the role of a fish population within an ecosystem, and 
this research will directly benefit the more comprehen- 
sive study that is needed to understand the impact of 
habitat on Spotted Seatrout production. More broadly, 
patterns in both intra- and interpopulation variability 
in SI values need to be well understood for SI analysis 
to provide an unbiased measure of trophic connectivity 
within estuarine ecosystems. Studies such as this one 
that explicitly measure these variables in a target eco- 
system are extremely valuable for the application of SI 
data to ecosystem management. 
Acknowledgments 
This study was made possible with the time and efforts 
of personnel at the University of Southern Mississippi 
Center for Fisheries Research and Development. We 
thank W. Dempster and J. Dietrich for their help in col- 
lecting fish and E. Myers and C. Messer for their help 
in processing samples for analysis. This project was 
funded by a grant from the Mississippi-Alabama Sea 
Grant Consortium and the Northern Gulf Institute. 
