Passerotti et al: Age validation of Sphyrna mokarran 
347 
low fecundity (Musick, 1999). Although not generally 
targeted in fisheries, S. mokarran are favored among 
incidentally caught species because their fins are highly 
valued due to their size and the density of their fin rays. 
In an assessment of the Hong Kong shark fin market, 
it was found that fins from hammerhead shark species 
were among the most valuable fin types in the market 
(Clarke et al., 2004; Abercrombie et al., 2005). Recently, 
concern has arisen in regard to populations of S. mo- 
karran worldwide because the International Union for 
Conservation of Nature (IUCN) assessed the species as 
endangered. 1 These circumstances illustrate the need 
for validated age estimates of S. mokarran. Here, we 
present the preliminary results of bomb radiocarbon 
analysis as a novel and accurate method of age valida- 
tion for this species. 
Materials and methods 
Vertebrae for bomb radiocarbon age validation were 
taken from two S. mokarran specimens (SM-112 and 
SM-114) caught from commercial longline vessels off 
the Georgia coast in the U.S. south Atlantic between 
2003 and 2004. Specimens were both male, measur- 
ing 300 cm and 276 cm fork length (FL), respectively. 
Although S. mokarran are frequently caught as bycatch 
in several commercial and recreational fisheries, it is 
difficult to obtain vertebral samples from individuals 
of sufficient age for the purposes of this study. Ideally, 
specimens would have vertebral tissue formed between 
1955 and 1965, the years encompassing the period of 
initial increase in 14 C (Campana et al., 2002; Piner et 
al., 2005). However, individuals living during this time 
period would at present be quite large (>300 cm FL) and 
the occurrence of specimens of this size are infrequent 
in catches available for sampling. The two specimens 
used in this study provided the only vertebral samples 
of appropriate age available to the authors at the time 
of this study. 
Vertebrae were collected either from the area under 
the dorsal fin or above the branchial chamber, stored 
on ice, and later frozen upon arrival at the labora- 
tory. Excess tissue was manually removed from thawed 
vertebrae, which were then soaked in varying concen- 
trations of sodium hypochlorite solution for 5-30 min- 
utes to remove remaining tissue. Cleaned vertebrae 
were rinsed in tap water and stored in 70% ethanol. 
Vertebral sections (1 mm thick) were prepared by a 
single longitudinal cut with paired blades separated by 
a spacer on an IsoMet low-speed diamond-bladed saw 
(Buehler, Lake Bluff, IL). Sections were immersed in 
ethanol and digitally photographed under a binocular 
microscope at 16-40 x magnification with reflected light. 
1 Camhi, M. D., S. V. Valenti, S. V. Fordham, S. L. Fowler, and 
C. Gibson. 2009. The conservation status of pelagic sharks 
and rays: report of the IUCN shark specialist group pelagic 
shark Red List workshop, 78 p. IUCN Species Survival 
Commission Shark Specialist Group, Newbury, UK. 
Age interpretation was based on visual counts of paired 
growth increments (growth bands) from images en- 
hanced for contrast with Adobe Photoshop CS2 (Adobe 
Systems, Inc., Burlington, NJ), and interpretation was 
based on the criteria of Natanson et al. (2002). 
Vertebral tissue samples (n = 10 samples; 4-9 mg 
each) were extracted from multiple growth bands in 
the corpus calcareum region of each vertebral sec- 
tion. Extractions were performed under the binocular 
microscope with 16 x magnification. Extracted samples 
were isolated as solid pieces by using a Gesswein high- 
speed hand tool (Gesswein, Bridgeport, CT) fitted with 
steel bits <1 mm in diameter. The first-formed growth 
band (corresponding to the first year of growth) was 
extracted from each vertebra; individual growth bands 
corresponding to later years were also extracted. The 
samples from both specimens corresponding to the 
most recent growth (where growth bands were very 
narrow) consisted of 6-10 pooled growth bands. The 
presumed date of sample formation (i.e. growth band 
formation) was calculated as the year the shark was 
collected minus the growth band count from the birth 
band to the mid-point of the sample. After sonification 
in Super Q water and drying, the sample was weighed 
to the nearest 0.1 mg in preparation for 14 C assay with 
accelerator mass spectrometry (AMS). AMS assays 
also provided A 13 C (%o) values, which were used to 
correct for isotopic fractionation effects. Radiocarbon 
values were subsequently reported as A 14 C, which is 
the per mil ( %c ) deviation of the sample from the radio- 
carbon concentration of 19th-century wood, corrected 
for sample decay before 1950 according to methods 
outlined by Stuiver and Polach (1977). 
To assign dates of formation to an unknown tissue 
sample, it is necessary that the A 14 C of the unknown 
sample be compared with a A 14 C chronology based on 
known-age material (a reference chronology). To match 
the water mass characteristics of S. mokarran habi- 
tat, we used a reference chronology for Florida corals 
developed by Druffel (1989). This chronology would 
be expected to show A 14 C values comparable to those 
of the great hammerhead shark because of similarity 
of habitat. However, the carbon source for vertebrae 
is metabolic in origin unlike the dissolved inorganic 
carbon (DIC) source for coral (Campana et al., 2002). 
Therefore, we also used a reference chronology devel- 
oped from known-age porbeagle ( Lamna nasus) in the 
northwest Atlantic (Campana et al., 2002). The period 
of increase in 14 C in this chronology would be expected 
to be very similar to that of great hammerhead sharks 
inhabiting the U.S. south Atlantic, although with very 
different absolute values owing to the different water 
mixing characteristics of the two regions. 
Results 
Based on annual growth band counts, the age esti- 
mate for each vertebra was 42 years for SM-112 and 
36 years for SM-114, yielding birth years of 1961 
