Gerard and Muhling: Variation in the isotopic signatures of juvenile Lut/anus griseus 
101 
1 -| 
0 . 5 - 
0 - 
- 0 . 5 - 
-1 - 
- 1 . 5 - 
-2 - 
- 2 . 5 - 
-3 - 
- 3 . 5 - 
2002 
P 
4 
° . A a a 
+ A 
A A 
A 
• Biscayne Bay 
A Dry Tortugas 
+ Florida Bay 
O Lower Keys 
■ Ten Thousand Is 
-12 -10 
-6 
-4 
-2 
0 
1 
0.5 
0 
- 0.5 
-1 
- 1.5 
-2 
- 2.5 
-3 
- 3.5 
2003 
-12 -10 
• • 
i°^b° 
• Biscayne Bay 
+ Florida Bay 
O Lower Keys 
■ Ten Thousand Is 
-6 -4 
8 13 C 
0 
Figure 2 
Scatter plot depicting spatial separation of juvenile gray snapper 
(Lutjanus griseus) stable isotope otolith signatures (5 18 0 and 
d 13 C) collected in 2002 and 2003 among five southern Florida 
regions: Ten Thousand Islands, Florida Bay, Biscayne Bay, 
Lower Keys and Dry Tortugas. 
and at Sprigger Bank, results for 2002 were also 
distinct from those for 2005. Where interannual 
differences were present, they were usually most 
discernible with respect to carbon, and less so for 
oxygen (Figs. 3 and 4). The annual mean SL of 
fish used in this analysis ranged from 135 mm 
(±2.6 mm standard error [SE] ) in 2002 to 169 mm 
(±3.4 mm SE) in 2001 at Old Dan Bank, and from 
156 mm (±13.5 mm SE) in 2001 to 160 mm (±16.1 
mm SE) in 2005 at Sprigger Bank. However, the 
linear correlation between fish standard length 
and carbon isotope signature for samples included 
in temporal analyses was very weak, and only 
marginally significant (correlation coefficient 
r 2 =0.03, P=0.04). 
Discussion 
Otoliths can be used to separate fish by region 
on the basis of isotopic composition. This study 
revealed spatial separation for carbon and oxygen 
isotopes between five geographical regions in 
southern Florida. In any marine ecosystem, tem- 
perature and salinity interact to influence the 
concentrations of oxygen isotopes. Carbon and 
oxygen isotopic concentrations typically increase 
with salinity and decrease with increasing tem- 
perature (Elsdon and Gillanders, 2002). 
Oxygen isotopes in the otolith are deposited in 
equilibrium with the carbon isotopic composition 
of the ambient water in which the fish resides 
(Campana, 1999). Carbon isotopes in the oto- 
lith, however, are deposited in disequilibrium 
to ambient carbon isotopic composition of the 
water mass. This disequilibrium is explained as 
an effect of metabolism and therefore provides 
information on the position of a fish in the food 
chain and on changes in metabolic rate over the 
life of a fish (Gauldie, 1996; Dufour et al., 2005; 
Huxham et al., 2007). The regional variation in 
the otolith isotopic composition of gray snapper 
indicates the regional differences in the environ- 
mental conditions and nutrient sources available 
to these fish throughout their early life. Regional 
isotopic differences provide a unique chemical 
signature that can be used to identify where 
individual adult gray snapper spend their time 
as juveniles. 
Results show spatial separation of stable oxy- 
gen and carbon isotopic ratios among collec- 
tion sites within a region. Significant differences in 
carbon and oxygen isotopic composition among sites 
(approximately 10 km apart) indicate high site fidelity 
and a high classification accuracy for specific sites at 
this scale. Classification success with the leave-one-out 
cross validation procedure for a single observation in a 
region was approximately 80%, thereby allowing for the 
possibility classifying an unknown fish with reasonable 
certainty to a region. The spatial stable isotopic varia- 
tion, coupled with a successful cross validation result, 
provides a means of substantiating the hypothesis that 
juvenile gray snapper exhibit strong site fidelity, and 
provides a specific isotopic tag to determine whether 
Florida Bay is a significant source of adults on the 
reef. 
Elemental composition of otoliths has been shown 
to be valuable for identifying the relative contribution 
of juveniles from different nursery habitats to adult 
