Andrews et al.: Radiometric age validation of Megalops atlanticus 
393 
Figure 1 
Expected 210 Pb: 226 Ra ingrowth curves and observed 210 Pb: 226 Ra activity ratios for male and female 
Atlantic tarpon ( Megalops atlanticus) individuals and age groups. Expected ingrowth curves repre- 
sent initial uptake ratios (R 0 ) of 0.0, 0.1, and 0.2. Expected 210 Pb: 226 Ra ingrowth during early oto- 
lith growth (until core-age) is based on a linear mass-growth model. After core growth, the secular 
equilibrium model for 210 Pb: 226 Ra resumes and continues to unity. Data points are annulus-derived 
ages (average age for age groups) plotted against measured 210 Pb: 226 Ra activity ratios. Vertical 
bars represent analytical uncertainty of 210 Pb and Z26 Ra measurements. Horizontal bars represent 
the range of annulus-derived ages for age groups. The upper limit of the 210 Pb: 226 Ra ratio for the 
oldest female exceeded 1.0; therefore, high radiometric age is mathematically undefined (Eq. 2). 
Table 3 
Comparison of annulus-derived ages and radiometric ages for Megalops atlanticus. The mean age of each group is based on annu- 
lus-derived age estimates. The radiometric age range is based on low and high activity ratios from analytical uncertainty calcula- 
tions. The radiometric age calculations are based on the measured ratio of 21u Pb: 226 Ra. 
Age-group 
or single 
otolith 
age (yr) 
Mean 
annulus- 
derived 
age (yr) 
Radiometric 
age range 
(yr) 
Radiometric 
age 
(yr) 
Age-group 
or single 
otolith 
age (yr) 
Mean 
annulus- 
derived 
age (yr) 
Radiometric 
age range 
(yr) 
Radiometric 
age 
(yr) 
Male 
Female 
2-4 
3.0 
1. 1-1.2 
1.1 
3-4 
3.4 
0.9-1. 1 
1.0 
13 
13 
19.6-28.7 
23.7 
15-24 
22.6 
14.6-16.4 
15.5 
17 
17 
22.2-35.3 
28.0 
35 
35 
15.3-19.3 
17.2 
18-21 
19.0 
11.5-12.7 
12.1 
36 
36 
25.5-35.9 
30.2 
31-36 
33.8 
22.4- 27.0 
24.6 
37 
37 
21.5-27.8 
24.5 
32 
32 
31.0-56.2 
41.0 
44 
44 
31.2-42.6 
36.3 
32 
32 
31.8 - 40.8 
35.9 
48-50 
48.8 
24.8-29.4 
27.0 
55 
55 
50.6-undefined 7 
78.0 
1 Upper radiometric age limit for the 78-yr female is undefined because the 210 Pb: 226 Ra ratio exceeded 1.0. 
