Andrews et al.: Bomb radiocarbon and tag-recapture dating of Carcharhinus plumbeus 
455 
formation of vertebrae from organic carbon sources. Use 
of measurements from known-age individuals, in rela- 
tion to measurements made in adult vertebrae, ruled out 
the possibility of reworked vertebral carbon throughout 
the life of the shark. This procedure enabled age valida- 
tion for porbeagle and shortfin mako (Campana et al., 
2002; Ardizzone et al., 2006). By contrast, a study of the 
white sharks ( Carcharodon carcharias) of the eastern 
North Pacific Ocean indicated that aspects of life histo- 
ry, such as large-scale movements and feeding below the 
ocean mixed layer, can lead to mixed h 14 C results that 
confound attempts to validate age (Kerr et al., 2006). 
The sandbar shark (Carcharhinus plumbeus ) is a 
cosmopolitan species of subtropical and tropical seas 
and was the cornerstone large coastal shark taken in 
the western North Atlantic (WNA) and Gulf of Mexico 
(GOM) bottom longline shark fisheries from the early 
1980s until 2008 when they were allotted to a research- 
only fishery (NMFS, 2008). Modeling regional popula- 
tion dynamics has led to conclusions that the population 
is in need of rebuilding (Brewster-Geisz and Miller, 
1999; Cortes, 1999). The most recent stock assessment 
of the large coastal shark complex of this region re- 
vealed that sandbar sharks are currently overfished, 
and overfishing is occurring (NMFS, 2006). Because 
fishing authorities set management measures using 
stock assessment models that increasingly and neces- 
sarily rely on age data (Cailliet and Andrews, 2008), 
even greater importance must be placed on age valida- 
tion as a requirement for stock assessments (Payne, 
2006). Validated age data enable stock assessment sci- 
entists 1) to understand and monitor long-term changes 
in population age-structure; 2) to determine the timing 
of important life history events (e.g., age at first matu- 
rity); 3) to measure vital rates (e.g., growth and natural 
mortality); and 4) to monitor fishing mortality rates and 
their long-term effects on the population. 
Some validated age and growth information exists 
for sandbar shark, but more comprehensive information 
on its biological development is needed. Age has been 
validated for juvenile sandbar sharks in Hawaii by us- 
ing marginal increment analysis and oxytetracycline 
(OTC) marking (Romine et al., 2006), and for adults 
up to approximately 17 years in Australia with the use 
of tag-recapture data (McAuley et al., 2006). However, 
studies geographically removed from the WNA are of 
limited use for that region. In the WNA-GOM region, 
one laboratory study validated growth up to 112 cm 
(Branstetter, 1987). Estimates of age, growth, and lon- 
gevity were primarily “unvalidated” by using vertebral 
centra and observations of growth in tag-recapture pro- 
grams (Casey et al., 1985; Casey and Natanson, 1992; 
Sminkey and Musick, 1995; Merson and Pratt, 2001). 
Use of tag-recapture data to determine growth char- 
acteristics can be complicated because revisions may 
be needed as additional recaptures continue to provide 
new data over time ( cf. growth parameters presented 
in Casey and Natanson [1992] with those in Casey et 
al. [1985]). In addition, maximum size (L max ) is typi- 
cally underestimated in tag-recapture studies, although 
the tag-recapture method offers several advantages, 
including a useful verification of younger age classes, 
estimates of longevity, and valid measures of age and 
growth when used in concert with OTC-marked growth 
bands. Therefore, use of both bomb radiocarbon and tag- 
recapture dating methods can produce a series of age 
and growth determinations that can facilitate accurate 
growth modeling throughout ontogeny of a species. The 
sandbar shark is not known to move into deep water; 
thus it is a good candidate for bomb radiocarbon dating 
because complications from greatly depleted 14 C sources 
with depth are unlikely (i.e., Kerr et al., 2006). It was 
hypothesized that an application of bomb radiocarbon 
dating would 1) provide independent estimates of age 
that either corroborate or refute age estimates from 
counting growth band-pairs; and 2) provide a minimum 
longevity for sandbar shark. It was further hypoth- 
esized that additional tag-recapture age and growth 
data from OTC-injected sandbar sharks would be in 
agreement with the bomb radiocarbon age data. 
Materials and methods 
Bomb radiocarbon dating 
Sandbar shark vertebrae, collected from the WNA and 
stored frozen, with capture years ranging from 1965 to 
1985 were obtained from 1) the Apex Predators Program 
of the National Marine Fisheries Service (NMFS; n- 4); 
and 2) the Florida Program for Shark Research (n=l) 
for bomb radiocarbon analyses (Table 1). Successful 
application of bomb radiocarbon dating requires that 
structures used to determine age come from sharks 
that were alive during some portion, or all, of the period 
of rapid increase in 4 14 C from atmospheric bomb test- 
ing (-1955 to 1970 for the marine environment). Five 
sharks collected between 1965 and 1981 were selected 
for analysis on the basis of estimated age from sex spe- 
cific growth curves (Casey et al., 1985) and collection 
dates, to estimate birth year (Table 1). Age estimates 
from growth band counts for four of these sharks were 
made before our study by using histological techniques 
described elsewhere (Casey et al., 1985). Contiguous 
vertebrae were used for the 14 C analyses. 
Vertebrae from the five individual sharks were 
sampled for 14 C analysis by using accelerator mass 
spectrometry (AMS). A section of the vertebral cen- 
trum was removed from the corpus calcareum of each 
vertebra along the sagittal plane. Sections were cut 
thicker than typically used for age estimates (2-3 
mm) to ensure that there was adequate material to 
meet minimum sample size requirements. Sections 
were mounted on glass microscope slides with fine- 
meshed, double-stick nylon tape. A New Wave® (Elec- 
tro Scientific Industries, Fremont, CA) micromilling 
machine with a 0.3-mm diameter bit (Brassier®, Sa- 
vannah, GA) was used to drill a series of overlapping 
holes around the circumference of the targeted growth 
band pair (one opaque and one translucent band; sensu 
