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Fishery Bulletin 99(3) 
were analyzed whole because they were similar in size to 
cores from adult fish. Because 2-yr cores were large and 
the activity of 226 Ra was relatively high (~10 to 100 times 
higher than usual; Andrews et al., 1999b), single otoliths 
were analyzed with the radiometric aging technique. 
Coring the adult otoliths required establishing a target 
size and weight that closely approximated that of an oto- 
lith from a 2-year-old fish. Dimensions and weights of 
otoliths from four juvenile fish aged 2 years were record- 
ed and averaged; the resultant dimensions were approxi- 
mately 12 mm long by 6 mm high by 1 mm thick and a 
weight of 0.1 g. Each otolith from each adult age group 
was sculpted into this shape and weight by being hand- 
ground on a Buehler Ecomet® III lapping wheel. All sam- 
ples were cleaned of any adhering contamination by fol- 
lowing specific procedures described elsewhere (Andrews 
et al., 1999b). These clean samples were placed in acid- 
cleaned 100-mL Teflon® PFA Griffin beakers and dried at 
85°C for 48 h. 
A detailed protocol describing sample preparation, chro- 
matographic separation of 226 Ra from barium and calci- 
um, and analysis of 226 Ra using thermal ionization mass 
spectrometry (TIMS) is described elsewhere (Andrews et 
al., 1999b). Only an overview of the 226 Ra procedures is 
given here with details on the determination of 210 Pb ac- 
tivity. Because the levels of 226 Ra and 210 Pb typically found 
in otoliths were extremely low (from femtograms [10~ 15 g] 
for 226 Ra and attograms [10~ 18 g] for 210 Pb) and because 
of the great potential for contamination from calcium, bar- 
ium, and lead, trace-metal clean procedures and equip- 
ment were used throughout sample preparation, separa- 
tion, and analysis. All acids used were double distilled 
(GFS Chemicals®) and dilutions were made with Milli- 
pore® filtered Milh-Q water (18 MQ/cm). 
To determine 226 Ra activity with thermal ionization 
mass spectrometry (TIMS), the sample must be clean of 
naturally occurring organics (such as otolin). Organic resi- 
dues elevate background counts in the 226 Ra region and 
increase the analytical uncertainty during TIMS analy- 
sis. Dried and weighed samples were dissolved in beakers 
on hot plates at 90°C by adding 8N HN0 3 in 1-2 mL ali- 
quots. Alternation between 8N HN0 3 and 6N HC1, with 
an aqua regia transition, several times resulted in com- 
plete sample dissolution. The dried sample, after dissolu- 
tion, formed a yellowish foam. To further reduce any re- 
maining organics, and to put the residue into the chloride 
form required for the 210 Pb activity determination proce- 
dure, the samples were redissolved in 1 mL 6N HC1 and 
taken to dryness five times at ~90°C. A whitish residue in- 
dicated that a sufficient amount of the organics had been 
removed. These samples were used to determine 210 Pb ac- 
tivity prior to TIMS analysis. 
Determination of 2:c Pb Activity 
To determine 210 Pb activity in the otolith samples, the 
alpha-decay of 210 Po was used as a daughter proxy for 
210 Pb. To ensure that activity of 210 Po was due solely to 
ingrowth from 210 Pb, the time elapsed from capture to 
210 Pb determination was greater than 2 yr. Samples pre- 
pared for 210 Po analysis were spiked with 208 Po, a yield 
tracer. The amount of 208 Po added was estimated on the 
basis of observed 226 Ra levels in otoliths of juvenile tarpon. 
This amount was adjusted to five times the expected 21C) Po 
activity in the otolith sample to reduce error in the 210 Pb 
activity determination. The spiked samples were redis- 
solved in approximately 50 mL of 0.5N HC1 on a hot plate 
at 90°C covered with a watch glass. The 210 Po and 208 Po- 
tracer were autodeposited for 4 hours onto a silver plan- 
chet (Flynn, 1968). The activities of these isotopes were 
determined by using alpha-spectrometry on the plated 
samples. Quantification of the 210 Po was made by subtract- 
ing a detector blank and reagent counts from each peak 
region-of-interest, by multiplying the 2 iop o; 208 p 0 coun t 
ratio by the known 208 Po activity, and by correcting for 
decay back to the time of plating. To attain sufficient 
counts, samples were counted for 21-23 d. The solution 
remaining after polonium plating was dried and saved for 
226 Ra analyses. 
Determination of 226 Ra Activity 
To prepare the samples for 226 Ra activity determination 
with TIMS, each sample was spiked with 228 Ra, a yield 
tracer, and a newly developed ion-exchange separation 
technique was used to isolate radium from calcium and 
barium (Andrews et al., 1999b). The final samples were 
processed by using TIMS and the measured ratios of 
226 Ra: 228 Ra were used to calculate 226 Ra activity. The anal- 
ysis of unspiked otolith samples indicated that 228 Ra was 
not present in measurable quantities in otoliths of juve- 
nile fish, and no adjustment was necessary for the mea- 
sured 226 Ra: 228 Ra ratio in spiked samples. 
Radiometric age determination 
To assess the feasibility of applying the radiometric aging 
technique to Atlantic tarpon, uptake of 210 Pb and 226 Ra 
was assessed in otoliths from juvenile fish. Because the 
age of juvenile Atlantic tarpon is better constrained than 
that of adults, age was determined by using 210 Pb: 226 Ra 
disequilibria in whole otoliths from juvenile fish. For the 
juvenile otolith samples, the age determined would be 
higher than expected if a significant amount of exogenous 
210 Pb was incorporated into the otolith. 
Age was estimated from the measured 210 Pb and 226 Ra 
activities (Eqs. 1 and 2). Because the activities were mea- 
sured from the same sample, the calculation was indepen- 
dent of sample mass. For adult samples, where estimated 
age was greater than that of the 2-year-old core, radiomet- 
ric age was calculated as follows with an equation derived 
from Smith et al. (1991) to compensate for the ingrowth 
gradient of 210 Pb: 226 Ra in the otolith core, 
