Andrews et al.: Bomb radiocarbon and tag-recapture dating of Carcharhinus plumbeus 
463 
juvenile tiger sharks were in agreement over time with 
a hermatypic coral record from Florida, indicating there 
was no phase lag in terms of the timing of the A 14 C sig- 
nal for the early growth of vertebrae. In contrast, the 
older adult, one that lived through the period of bomb 
testing to nearly the end of the marine z3 14 C record, was 
mostly in phase with the porbeagle record as an adult, 
and deviated to match the coral record in what would 
have been the juvenile portion of the adult vertebrae. 
These findings can be logically attributed to tiger shark 
juveniles feeding on short-lived and near-surface food 
sources and adults shifting to older food sources, repre- 
sented as a phase lag that can be attributed to trophic- 
level changes (Kneebone et ah, 2008). A similar sce- 
nario was observed for the great hammerhead ( Spliyrna 
mokarran), where there was close agreement with a 
coral A 14 C record in some years and attenuation of the 
z\ 14 C signal in others (Passerotti et al., 2010). For the 
sandbar shark, carbon is derived from mixed sources 
throughout ontogeny and Z\ 14 C values range from agree- 
ment with the attenuated and phase-lagged porbeagle 
record to agreement with the elevated and timely coral 
records. In general, no time-specific correlation was ob- 
served with either record, and this finding is consistent 
with the wide-range of sandbar shark feeding habits. 
The use of vertebrae as an exclusive tool to age sand- 
bar shark has well-documented limitations. Casey et 
al. (1985) used strict age-estimation criteria and not- 
ed that ages may be underestimated owing to a large 
number of uncounted growth bands at the margin. 
The uncounted banding pattern did not fit the criteria 
formulated from observed early growth; therefore, the 
bands were not counted at the time. A subsequent tag- 
recapture study provided support for the notion that 
age was underestimated and evidence was presented 
for much slower growth and greater longevity (Casey 
and Natanson, 1992). Tag-recapture data generated 
since Casey and Natanson’s publication and presented 
herein provides an indication of an even greater longev- 
ity (33-36 years). These data were further supported by 
the bomb radiocarbon results that indicated ages were 
underestimated late in adult life. 
For age determination of smaller sharks, Casey and 
Natanson (1992) suggested that the band counts may 
have an annual periodicity until a threshold size and 
age, at which the deposition rate changes. At the time of 
the Casey and Natanson’s study, the only age validation 
was from a laboratory OTC study for sharks no larger 
than 112 cm ( Branstetter, 1988), and the conclusion was 
that periodicity of band-pair deposition changed after 
5-6 years. Branstetter (1988) argued that the Casey et 
al. (1985) criteria were limited to early growth and this 
view is supported by the bomb radiocarbon and OTC 
findings of the present study. 
Conclusion 
In light of these results, it is important to emphasize the 
need for age validation across all size (and age) classes 
for the sandbar shark. Given the potential for changes 
in the periodicity of band pair formation throughout 
ontogeny, the application of complementary age valida- 
tion methods is preferred in order to provide a verifiable 
and defensible position for the determination of impor- 
tant life history parameters for sharks and other fishes. 
Acknowledgments 
We would like to thank F. F. Snelson (Florida Program 
for Shark Research, retired) for help with project logis- 
tics. Prior work and data provided by the long-term 
tagging of J. Casey, H. Pratt, and C. Stillwell, and the 
original histological sections to which we refer, made 
longevity estimates possible. We appreciate the contribu- 
tion of tag-recapture data by N. Kohler (Apex Predators 
Program). We also thank R. Humphreys, E. DeMartini, 
R. Nichols, and three anonymous reviewers for comments 
that improved the manuscript. Moss Landing Marine 
Laboratories provided infrastructure support. T. Brown 
at Lawrence Livermore National Laboratory handled 14 C 
measurements. This work benefitted greatly from NMFS 
funding of the National Shark Research Consortium. 
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