329 
National Marine 
Fisheries Service 
NOAA 
Fishery Bulletin 
e established in 1881 «= 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Abstract—Results from published age 
and growth models for bonnetheads 
(Sphyrna tiburo) indicate significant 
differences in life history between pop- 
ulations in the eastern Gulf of Mexico 
(GOM) and those in estuarine waters of 
the Atlantic coast of the southeastern 
United States (hereafter referred to as 
the Atlantic region). An age-indepen- 
dent model, GROTAG, was used with 
region-specific tag-recapture data to 
generate estimates of von Bertalanffy 
growth parameters and growth rates 
for sharks in each of these regions, and 
these estimates were compared with 
age-based life history characteristics. 
Results from the GROTAG model indi- 
cate that female bonnetheads in the 
GOM initially grew faster and attained 
a smaller maximum size than females in 
the Atlantic region. The final GROTAG 
model for females in the Atlantic region 
produced estimates of von Bertalanffy 
parameters and growth rates similar to 
those produced by the age-based growth 
model. For the population in the GOM, 
GROTAG model results indicate that 
growth rates were slower and aver- 
age maximum size and longevity were 
greater than those from age-based mod- 
els. Although models for males were 
generated with tag-recapture data, 
large 95% confidence intervals hin- 
dered comparisons. For both sexes and 
regions, calculated maximum longevity 
and age at 50% maturity are larger than 
published estimates, indicating that age 
underestimation may have occurred in 
both age and growth studies, with sig- 
nificant differences in estimates of life 
history characteristics for bonnetheads 
in the GOM. 
Manuscript submitted 1 April 2020. 
Manuscript accepted 9 October 2020. 
Fish. Bull. 118:329-345 (2020). 
Online publication date: 3 November 2020. 
doi: 10.7755/FB.118.4.3 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Growth rates of bonnetheads (Sphyrna tiburo) 
estimated from tag-recapture data 
Bryan S. Frazier (contact author)' 
Dana M. Bethea” 
Robert E. Hueter® 
Camilla T. McCandless* 
John P. Tyminski? 
William B. Driggers III° 
Email address for contact author: frazierb@dnr.sc.gov 
' Marine Resources Research Institute 
South Carolina Department of Natural Resources 
217 Fort Johnson Road 
Charleston, South Carolina 29412 
? Southeast Regional Office 
National Marine Fisheries Service, NOAA 
263 13th Avenue South 
Saint Petersburg, Florida 33701 
3 Center for Shark Research 
Mote Marine Laboratory 
1600 Ken Thompson Parkway 
Sarasota, Florida 34236 
Growth of wild fish is primarily esti- 
mated by using 3 types of models: 
age-based models in which length and 
estimated age are known; length-based 
models that use modal progressions 
in length-frequency data; and models 
that use length at capture, known time 
at liberty, and length at recapture from 
tag-recapture data to model growth 
trajectories. Length-based models are 
useful; however, they are suitable only 
for estimating growth in younger age 
classes because substantial overlap 
in lengths of fish can occur in older 
cohorts (Campana, 2001; Natanson 
et al., 2018a). Methods based on the 
use of age and tag-recapture data 
often characterize growth of fish by 
using the von Bertalanffy growth func- 
tion (VBGF) (Ricker, 1975; Francis, 
1988a). Growth of fish is most com- 
monly estimated by using age-based 
models; however, models developed 
with tag-recapture data are often used 
as alternative models for large, long- 
lived species that may not be able to 
tolerate high rates of fishing mortality 
4 Northeast Fisheries Science Center 
National Marine Fisheries Service, NOAA 
28 Tarzwell Drive 
Narragansett, Rhode Island 02882 
> Mississippi Laboratories 
Southeast Fisheries Science Center 
National Marine Fisheries Service, NOAA 
P.O. Drawer 1207 
Pascagoula, Mississippi 39567 
or for species for which age estimation 
is difficult or imprecise (Cailliet et al., 
1992; Treble et al., 2008). 
All methods of modeling growth 
have limitations and potential biases. 
Age estimation can be imprecise and 
is difficult to validate for many species 
(Beamish and McFarlane, 1983), espe- 
cially elasmobranchs (Goldman, 2004) 
and deepwater species (Rigby et al., 
2014). Validation of age estimates is 
recommended for all age and growth 
studies; however, validation of all age 
classes is difficult and rarely achieved 
(Cailliet et al., 2006). Although val- 
idation of ages has been completed 
for several species of elasmobranchs 
(Campana et al., 2002; Ardizzone et al., 
2006; Kneebone et al., 2008), there is 
a growing body of evidence that indi- 
cates that age underestimation com- 
monly occurs for long-lived species, 
such as sharks (e.g., Francis et al., 
2007; Passerotti et al., 2014; Harry, 
2018; Natanson et al., 2018b), as well 
as recent evidence that age underesti- 
mation may also occur in sharks with 
