458 
Fishery Bulletin 109(4) 
Year of formation 
Figure 1 
Plot of sandbar shark ( Carcharhinus plumbeus) A U C measured from vertebrae in relation 
to the estimated year of formation (determined from growth band counts). Observations 
from individual sharks are connected with lines and labeled accordingly. For calibration 
purposes two regional hermatypic coral records and the porbeagle (Larmia nasus) A 14 C 
records were used. In addition, a LOESS curve was fitted to the porbeagle z\ 14 C data to 
provide a central distribution reference (LOESS, P=0.5, 2-parameter regression). Note that 
the sandbar shark 4 14 C values, plotted in relation to estimated age, were phase lagged by 
up to 11 years. Calibration of age was necessary for some sandbar shark age estimates 
(see Figs. 2-4). 
sharks were not in agreement with the reference Z\ 14 C 
chronologies. 
The age of three adult sandbar sharks (SB 745, SB 
749, SB 47970) was underestimated by approximate- 
ly five to 11 years in relation to the reference z\ 14 C 
chronologies and resulted in an increase in age to at 
least 20 to 26 years. The age of specimen SB 749 was 
underestimated by at least 11 years after alignment 
with the porbeagle 4 14 C reference record for a revised 
age of at least 26 years. To make the alignment, we 
had to assume that the innermost sample (year-1) of 
the vertebra was an inaccurate extraction that in- 
cluded more recent (postbomb) material (Fig. 2). This 
conclusion was supported by the measured postbomb 
A 14 C level from the innermost sample of the corpus 
calcareum (-7.8 %o, SD = 4.4) when compared to more 
recently formed samples that were clearly prebomb and 
further into the life of the shark (Table 2). The age 
of specimen SB 47970 was underestimated, requiring 
adjustment of 10 to 12 years based on a simultaneous 
alignment of the measured Zl 14 C values in relation to 
the coral and porbeagle shark 4 14 C reference chronolo- 
gies. The near peak z\ 14 C value could not have been 
formed earlier than 1965 (based on the maximum 
rise in 4 14 C from the Florida coral record), yet the 
measured prebomb 4 14 C value could be no later than 
1962 (based on the rise in 4 14 C from the porbeagle 
record; Fig. 3). With these alignment constraints, and 
with the assumption that there was no problem with 
interpreting age from the early growth between the 
two samples (five years), a median age of 25 ±2 years 
was determined for this shark. The age for specimen 
SB 745 was underestimated by at least five years to 
align with the porbeagle zi 14 C record, but this shark 
could have been older because prebomb values alone 
are not diagnostic (Fig. 4). 
OTC tag-recapture dating 
Since 1992, 173 tagged sandbar sharks have been recap- 
tured as part of the NMFS Apex Predators Program. 
Ten of these sharks were estimated to have lived more 
than 20 years (20.1-31.1 years) based on time-at-liberty 
(TAL) and the estimated age at time of release deter- 
mined from the sex-specific growth curves of Casey et 
al. (1985). These sharks were tagged at 80-183 cm fork 
length (FL) and had a TAL of 2.6-26.9 years (Table 3). 
In addition, 22 sharks with estimated lengths at tagging 
