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Fishery Bulletin 109(4) 
Figure 3 
Plot of sandbar shark ( Carcharhinus plumbeus) 4 14 C measured from vertebrae in relation 
to the estimated year of formation (determined from growth band counts), showing that an 
adjustment of formation dates for sandbar shark specimen SB 47970 (by an additional 11 
years) was necessary to match the porbeagle ( Lanina nasus) 4 14 C record (filled diamonds). 
Adjusted age was increased to 25 years. The assumption was made that the missing years 
were those in the late-adult years (as reflected in the outer part of corpus calcareum) and 
that early growth was well quantified. This span in z) 14 C values is perhaps the most diag- 
nostic in terms of age determination; age could not be older by more than 2 years because 
of limits to the rise in A 14 C from the Florida coral record (-1965 for the measured Z) 14 C 
level), and the prebomb sample could not have been younger by more than 1 year because of 
the limits of the porbeagle ( Lamna nasus ) 4 14 C record (-1964 for the measured 4 14 C level). 
Age for this sandbar shark was likely constrained to a range between 23 and 27 years. 
were reliable as a measure of annual growth to at least 
12 years (10 and 12 years validated in this study with 
bomb radiocarbon and OTC marking, respectively). After 
this time of band formation in the vertebrae, either 
growth-band pairs do not provide an accurate measure 
of annual growth or the criteria for counting must be 
changed to incorporate a finer growth band structure. 
Until all sizes and ages can be validated, it would not 
be possible to determine how the growth-band counting 
criteria need to change. 
Discussion 
The comparisons of measured A 14 C values from sandbar 
shark vertebrae with regional reference chronologies 
provided age determinations that exceeded age esti- 
mates from visual growth-band counts for three of the 
largest sharks in this study. Levels of A 14 C recorded in 
sandbar shark vertebrae during the rise in marine A 14 C 
and postbomb periods were unexpectedly low based on 
their estimated year of band formation determined from 
growth-band counts. This finding led us to conclude that 
ages had been underestimated for these adult sandbar 
sharks by 5 to 11 years, thereby providing explanation 
for the temporal offset and providing evidence that these 
individuals were considerably older. 
Considerable evidence shows that diet is the pri- 
mary source of carbon in the skeletal structure of 
sharks and that collagen retains its time specificity 
in respect to its deposition in vertebrae (Fry, 1988; 
Campana et ah, 2002, 2006); hence, an alternative 
explanation for the unexpectedly attenuated A 14 C val- 
ues measured in sandbar shark vertebrae could be a 
shift or mix in dietary carbon sources. To address this 
potential explanation for sandbar sharks, we turned 
to the well-documented study of white sharks from 
the eastern North Pacific Ocean (Kerr et al., 2006). 
For white sharks, the unexpectedly low A 14 C values in 
the vertebrae could not be explained as problems with 
age estimation; collection year and known-age juvenile 
samples provided temporal constraints that eliminated 
