Andrews et ai.: Radiometric age validation of Megalops atlcinticus 
397 
pies were an average age and average fork length for 
each age group. The growth coefficient for male radio- 
metric ages (£=0.19 ±0.12) was similar to the annulus-de- 
rived estimate (£=0.12) according to the margin of error 
and was driven largely by one middle-age sample (Fig. 4). 
The growth coefficient for female radiometric ages (£=0.08 
±0.04) was also similar to the annulus-derived estimate 
(£=0.10). The male asymptotic length for radiometric ages 
(L „- 1550 ±83 mm FL) encompassed the result from annu- 
lus counts (L oc =1567 mm FL). For females, the asymptotic 
length was greater (L „= 2030 ±227 mm FL) but still en- 
compassed the result from annulus counts (L TC =1818 mm 
FL). The closeness of the radiometric estimate of to the 
maximum size was probably driven by the exceptionally 
large tarpon used in our study (Fig. 5). 
The large size of the 2-year-old otolith core, coupled with 
the relatively high 226 Ra activity and high sensitivity of the 
TIMS technique, permitted age determination of smaller 
samples than previously possible (Kastelle et ah, 1994; 
Fenton and Short, 1995). In a recent radiometric aging 
study of the blue grenadier ( Macruronus novaezelandiae ), 
six pooled otolith cores were needed to attain a sample 
weight of approximately 1 g with measurable 226 Ra activ- 
ity (Fenton and Short, 1995). Although the 226 Ra activity 
for the samples in that study were a factor of 10 lower than 
the 226 Ra activities observed in our study, the technique 
used was less sensitive than TIMS and resulted in rela- 
tively high analytical uncertainties (12-21%). In a recent 
study of sablefish (Anoplopoma fimbria ), where radium ac- 
tivities were similar to the activities observed in our study, 
the margin of error was relatively low (~4%), but otolith 
cores still needed to be pooled to attain approximately 1 g 
(Kastelle et ah, 1994). The use of TIMS to determine 226 Ra 
in our study made it possible to age age-groups that were 
approximately one third of a gram and individual otolith 
cores that were less than 0. 1 g with analytical uncertain- 
ties typically less than 2%. Because of this advance, the 
greatest contribution to the aging error was no longer from 
226 Ra determination, but from the determination of 210 Pb 
by means of a-spectrometry (Andrews et ah, 1999b). In our 
study, the determination of 210 Pb activity contributed from 
77% to 90%' of the aging error. 
The radiometric aging technique is well supported as a 
valid aging tool in numerous fish aging studies (Bergstad, 
1995; Burton et ah, 1999). In our study, 226 Ra activities 
varied by approximately an order of magnitude, but the 
210 Pb activities never exceeded the activity of 226 Ra. There- 
fore, it is Highly unlikely that 210 Pb was not the result of 
ingrowth from 226 Ra incorporated during otolith formation. 
By performing a complete analysis on individual otoliths, 
we found that we could use radiometric age estimates with 
more confidence than estimates derived from pooled otolith 
samples. More recent successes with this technique (An- 
drews et ah, 1999a) suggest that it is a reliable means to 
obtaining accurate age estimates. Radiometric age deter- 
minations in our study, therefore, indicate that the inter- 
pretation of growth zones in Atlantic tarpon otoliths can be 
difficult, and in some cases can be inaccurate. In addition, 
our study provides conclusive evidence that the longevity 
of the Atlantic tarpon is greater than 50 years. 
Conclusions 
Accurate age determination of heavily harvested fish spe- 
cies is critical to the formulation of responsible man- 
agement strategies. The typical growth-zone validation 
techniques have limited applicability to long-lived fishes 
(Mace et ah, 1990; McFarlane and Beamish 1995). Under- 
estimated longevity and overfishing were factors that led 
to the decline of the Pacific ocean perch (Sebastes alutus) 
and the orange roughy (Hoplostethus atlanticus'. Beamish, 
1979; Smith et ah, 1995). Because of the highly variable 
growth zone patterns (Fig. 3) and irregular life history of 
the Atlantic tarpon, conventional aging methods appear 
to be problematic for this species. The improved radio- 
metric aging technique used in our study incorporated 
a growth-zone independent chronometer that, when judi- 
ciously applied, will enable accurate age determination 
of many threatened species. At a time when many fish 
species are suffering from fishing pressure and changing 
oceanographic conditions, new methods need to be applied 
to help ascertain appropriate management strategies. 
The radiometric aging technique, which has been success- 
fully applied to the sablefish, whose longevity has been 
validated with other techniques (Kastelle et al., 1994, 
Beamish and McFarlane 2000), promises to be a valuable 
tool for aging species with unknown or difficult-to-inter- 
pret otolith growth patterns. 
Acknowledgments 
This work was supported in part by funding from the 
Department of the Interior, U.S. Fish and Wildlife Ser- 
vice, Federal Aid for Sportfish Restoration, project number 
F-59. Measurement of radium samples with TIMS was 
performed by colleagues in the Department of Earth Sci- 
ences at University of California, Santa Cruz. We espe- 
cially thank Craig Lundstrom, Zenon Palacz, and Pete 
Holden, and the University of California, Santa Cruz. This 
work was presented at the First International Tarpon 
Symposium held at the University of Texas Marine Sci- 
ence Institute in Port Aransas, Texas, 15-16 February 
2001. We also thank Joan Holt and the organizers of the 
symposium, including Paul Swacina of Tarpon Tomorrow, 
a nonprofit organization aimed at tarpon restoration and 
conservation. Additional funding was provided by a grant 
from the National Sea Grant College Program, National 
Oceanic and Atmospheric Administration, U.S. Depart- 
ment of Commerce, under grant number NA36RG0537, 
project number R/F-148 through the California State 
Resources Agency. 
Literature cited 
Andrews, A. H., G. M. Cailliet, and K. H. Coale. 
1999a. Age and growth of the Pacific grenadier ( Coryphae - 
noides acrolepis ) with age estimate validation using an 
improved radiometric ageing technique. Can. J. Fish. 
Aquat. Sci. 56:1339-1350. 
