Fulford and Dillon Intrapopulation variability in stable isotope data for Cynoscion nebulosus 
117 
was small but significant and generally supports an in- 
crease in trophic level as fish grow and move offshore. 
The increase in 5 15 N observed over the size range of 
fish in our study was close to 3%c, nearly a full tro- 
phic level. The size range of fish in this study was 
relatively wide but probably includes only fish beyond 
their first year — an assumption made on the basis of 
published growth curves (Fulford and Hendon, 2010). 
The probable age range of these fish suggests that an 
ontogenetic diet shift associated with maturation is dif- 
ficult to extrapolate from these data because the trend 
of increasing heavy isotope values is confounded by 
changes in prey availability, by increases in fish prey 
capture efficiency with seatrout size, and by the poten- 
tial variability of carbon and nitrogen sources within 
the Mississippi Sound and its subestuaries. The largest 
Spotted Seatrout were caught in open water, mainly 
offshore near the Barrier Islands, where prey fishes of 
higher trophic level, such as anchovies (e.g., Striped 
Anchovy [Anchoa hepsetus ]) that have a mean trophic 
