350 
Fishery Bulletin 108(3) 
Discussion 
Although a larger sample size would have been prefer- 
able in this study, the results of the bomb radiocarbon 
assays support the hypothesis of annual growth band 
deposition in S. mokarran vertebrae. Additionally, the 
similarity in the timing of increase and peak in A 14 C 
between the great hammerhead shark samples and 
both reference chronologies indicates that the aging 
techniques employed in this study produce ages accu- 
rate to within a few years. It is generally accepted that 
the timing of the initial increase in 4 14 C in relation to 
prebomb values is the most accurate dated marker for 
age validation (Campana et ah, 2008). Although there 
were no prebomb samples in our study (all z\ 14 C values 
were above zero), the close alignment of values between 
S. mokarran samples and those from the coral chronol- 
ogy during the period of increase indicates that our ages 
were assigned correctly. If the specimens analyzed in 
this study had been under-aged, the entire great ham- 
merhead shark chronology would have been shifted to 
the right in relation to the coral chronology, and over- 
aging would have caused the reverse to be true. No such 
shifting was apparent. 
Differences in both the magnitude and timing of ra- 
diocarbon chronologies between vertebral samples and 
those from carbonate sources have been noted in pre- 
vious age studies of elasmobranchs. The difference in 
the magnitude of A u C values is largely attributable to 
the different carbon sources in carbonate (DIC uptake) 
compared to cartilaginous (dietary uptake) systems 
(Fry, 1988), but also to environmental factors such as 
habitat depth and the mixing rates of waters (Williams 
et al., 1987). This difference has been demonstrated in 
porbeagle (Campana et ah, 2002), shortfin mako ( Isurus 
oxyrinchus ) (Ardizzone et al., 2006), and white sharks 
(Carcharadon carcharias) (Kerr et al., 2006), as well as 
in two species of skates (McPhie and Campana, 2009), 
and can also be caused by the age of the carbon in prey 
items found at different depths, which can produce a 
delay in the radiocarbon chronology. In the case of S. 
mokarran, however, overall values of 4 14 C followed those 
of Florida coral very closely, indicating little difference 
in timing of uptake between coral and vertebrae. The 
similarity in values of 4 14 C is likely due to similarity in 
habitat for both the coral and the shark; S. mokarran 
are reef-associated for much of their lives (Compagno, 
1984) and feed on reef-associated prey (Stevens and 
Lyle, 1989), which may assimilate carbon more quickly 
because of the well-mixed shallow habitat. Kneebone 
et al. (2008) found that young tiger sharks ( Galeocerdo 
cuvieri) exhibit similar patterns in 4 14 C uptake, attrib- 
uting the pattern to a diet of small teleosts during the 
time that these sharks inhabit shallow nursery grounds. 
Campana et al. (2006) also found similar results in 
spines of spiny dogfish ( Squalus acanthias), in which 
carbon uptake into fin spines mirrored that of DIC 
uptake in otolith chronologies from the same region. 
Despite the apparent similarities between values of 
4 14 C in S. mokarran and Florida coral, there were some 
differences, such as the slight left shift in the first two 
samples from SM-112 and the depletion of 4 14 C in the 
penultimate (1970) sample from SM-112, in relation to 
the rest of the chronology. The first two samples from 
SM-112, corresponding to formative years of 1961 and 
1962, respectively, fell slightly left of the coral curve. 
Although a phase-shift to the right can be explained 
as a diet- or habitat-induced delay in carbon uptake, 
a shift to the left could indicate a slight over-aging of 
SM-112 of only 1-2 years, or the shift could be the re- 
sult of inclusion of material from more recently formed 
bands in the sample. In addition, the A 14 C in the 1970 
sample from SM-112 was depleted in comparison to 
the rest of the chronology and approached values more 
like those of porbeagle as opposed to coral. This singu- 
lar deviation could again be the result of an error in 
micromilling or could be the start of a more depleted 
trajectory for SM-112, reflecting an ontogenetic shift in 
habitat and diet. Although reef-associated for much of 
their lives, S. mokarran also undertake oceanic migra- 
tions through deeper water habitats (Compagno, 1984) 
that tend to be depleted in 4 14 C. Consumption of prey 
from these habitats would result in depleted values of 
A 14 C in the vertebrae, as demonstrated in porbeagle 
and other deepwater sharks (Campana et al., 2002). 
Another possibility for this depletion in 4 14 C is a shift 
in age of prey taken by SM-112; owing to its size this 
shark may have taken larger (and possibly older) prey. 
Obtaining additional 4 14 C samples from both sharks 
would certainly clarify these results. 
This study confirms the longevity of great hammer- 
head sharks to an age of at least 42 years, although 
maximum reported lengths indicate that they may live 
well beyond this age. Further study on the life history 
of S. mokarran is needed to identify factors affecting 
individual patterns in 4 14 C assimilation. 
Acknowledgments 
We thank observers of the National Marine Fisheries 
Service, NOAA, for obtaining samples from the directed 
shark longline fishery. W. Joyce provided technical assis- 
tance in sample processing. Funding was provided by 
the Southeast Fisheries Science Center, National Marine 
Fisheries Service, National Oceanic and Atmospheric 
Administration. 
Literature cited 
Abercrombie, D. L., S. C. Clarke, and M. S. Shivji. 
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Ardizzone, D., G. M. Cailliet, L. J. Natanson, A. H. Andrews, L. 
A. Kerr, and T. A. Brown. 
2006. Application of bomb radiocarbon chronologies to 
shortfin mako ( Isurus oxyrinchus ) age validation. En- 
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