56 
Fishery Bulletin 11 7(1-2) 
Table 3 
The von Bertalanffy growth function parameters calculated by using tag-recapture data in models for 
sandbar sharks (Carcharhinus plumbeus) tagged and recaptured in the western North Atlantic Ocean: 
mean asymptotic fork length (L„), growth constant ( k ), and theoretical age at a length of zero (f 0 ). 
Study 
Model 
L„ 
k 
to 
72 
This study 
Gulland and Holt (1959) 
188.4 
0.079 
-3.678 
149 
This study 
GROTAG (Francis, 1988) 
178.6 
0.094 
-3.300 
149 
Casey and Natanson (1992) 
Fabens(1965) 
186.0 
0.046 
-6.450 
33 
300 n 
O CM 1031 
□ CM 1027 
0 -1-1-i--r-1-I 
0 10 20 30 40 50 
Age (years) 
Figure 3 
The von Bertalanffy growth curves generated in this study com¬ 
pared with previously published curves for the sandbar shark 
(Carcharhinus plumbeus). Estimated ages at recapture of oxy- 
tetracycline-injected sharks are included. The open triangles in¬ 
dicate ages validated with bomb radiocarbon dating for 4 speci¬ 
mens from Andrews et al. (2011). The open square and circle 
indicate the known ages used in this study for sandbar sharks 
tagged in 1996 (CM1027) and 1998 (CM 1031) in the western 
North Atlantic Ocean. The single asterisk (*) indicates a growth 
curve based on vertebral band-pair counts, and the double as¬ 
terisks (**) indicate growth curves based on the use of tag-re¬ 
capture data. 
Modeling of tag-recapture data is provided as an 
alternate method of age determination. The growth 
curves derived from modeling tag-recapture data are 
verified by the close relationship of the estimated ages 
of OTC-marked individuals to the ages estimated for 
similarly sized fish by the tag-recapture growth curves. 
Both of the tag-recapture-derived models in this study 
output similar growth estimates to age 20 and pre¬ 
dict higher k values than growth estimates derived by 
Casey and Natanson (1992), but tag-recapture models 
from both studies underestimate L„ (maximum 
observed size: 251 cm FL; NMFS 2 ). We applied 
the size at 50% maturity calculated by Bare- 
more and Hale (2012), 154.9 and 151.6 cm FL 
for females and males, respectively, to the pa¬ 
rameters we calculated by using the GROTAG 
(Francis, 1988) and Gulland and Holt (1959) 
models: results indicate that female and male 
sandbar sharks reach 50% maturity at an age 
of approximately 18 and 17 years, respectively. 
These estimates of age at 50% maturity are 
higher than those predicted by counting ver¬ 
tebral band pairs (12 and 13 years for females 
and males, respectively; Casey et al., 1985) 
and lower than that predicted by using the 
previous tag-recapture data (nearly 30 years; 
Casey and Natanson, 1992). 
Our findings corroborate those of other 
studies on sandbar sharks (Casey et al., 1985; 
Casey and Natanson, 1992; Andrews et al., 
2011) as well as on a multitude of other shark 
and ray species (Harry, 2018) and support the 
use of modeling with tag-recapture data as an alter¬ 
native method of age determination. These results 
reinforce the notion that band-pair deposition 
may misrepresent the age of individuals, pos¬ 
sibly affecting management of the sandbar 
shark because underestimation of age leads to 
underestimation of reproductive potential and 
overestimation of yield. 
Acknowledgments 
We would like to thank G. Skomal, H. Pratt, 
and J. Casey for starting the OTC program. 
N. Kohler was instrumental in safe handling of sharks 
during OTC injections. We also thank J. Carlson and 
the observer program staff of the Panama City Lab, 
National Marine Fisheries Service, for processing and 
sending samples. Samples could not have been ob- 
2 NMFS (National Marine Fisheries Service). 2018. Un- 
publ. data. Panama City Laboratory, Southeast Fish. Sci. 
Cent., Natl. Mar. Fish. Serv., NOAA. 3500 Delwood Beach 
Rd., Panama City, FL 32408. 
