Frazier et al.: Growth rates of Sphyrna tiburo estimated from tag-recapture data 
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Materials and methods 
Data collection 
Data sets from long-term mark-and-recapture studies 
were available from 2 regions (the northeastern GOM and 
estuarine waters of the Atlantic coast of the southeastern 
United States), corresponding to the areas within each 
region where published, region-specific age and growth 
studies occurred. Mark-recapture data were provided for 2 
surveys conducted in the GOM. Mote Marine Laboratory 
provided data from fishery-independent gill-net surveys 
conducted from 1993 through 2006 in the eastern GOM 
(primarily from Yankeetown to Charlotte Harbor, Flor- 
ida). Detailed descriptions of the survey methods used 
by Mote Marine Laboratory can be found in Hueter and 
Tyminski’ and in Hueter and Tyminski (2007). Data for 
the northeastern GOM were obtained for the period from 
2003 through 2014 from the National Marine Fisheries 
Service from its Gulf of Mexico Shark Pupping and Nurs- 
ery (GULFSPAN) survey. A fishery-independent survey 
conducted with gill nets made of multiple stretched-mesh 
panels, the GULFSPAN survey is used to assess popula- 
tions of juvenile sharks from Cat Island, Mississippi, to 
Anclote Key, Florida, from April through October each 
year. Additional details about the GULFSPAN survey can 
be found in Bethea et al. (2015). 
For the population in the Atlantic region, bonnethead 
mark-recapture data collected from 1998 through 2019 
were available from the South Carolina Department of 
Natural Resources as part of its Cooperative Atlantic 
States Shark Pupping and Nursery (COASTSPAN) sur- 
vey. The COASTSPAN survey is conducted in estuarine 
waters along the coast of South Carolina from Saint 
Helena Sound to Bulls Bay during April-September each 
year. Detailed descriptions of COASTSPAN survey meth- 
ods can be found in Ulrich et al. (2007). 
For all surveys, upon capture, the precaudal length, fork 
length (FL), total length, and stretch total length of each 
shark were measured in a straight line along the axis of 
the body to the nearest half centimeter (for sharks from 
the GOM) or nearest millimeter (for sharks from the 
Atlantic region). If healthy, sharks were tagged externally 
either with a nylon dart tag (142-mm tag, Hallprint Fish 
Tags*, Hindmarsh Valley, Australia) or a FT-1-94 or T-bar 
Anchor Tag (Floy Tag Inc., Seattle, WA), at the base of the 
first dorsal fin (for both populations: GOM and Atlantic 
region), or with a 3.5-cm rototag (Dalton ID Systems Ltd., 
Henley-on-Thames, UK), inserted through the cartilage 
of the leading edge of the first dorsal fin (for the popula- 
tion in the Atlantic region only). A limited number of indi- 
viduals in the Atlantic region were double tagged with a 
” Hueter, R. E., and J. P. Tyminski. 2002. U.S. shark nursery 
research overview, Center for Shark Research, Mote Marine 
Laboratory 1991-2001. Mote Mar. Lab. Tech. Rep. 816, 31 p. 
[Available from website.] 
Mention of trade names or commercial companies is for identi- 
fication purposes only and does not imply endorsement by the 
National Marine Fisheries Service, NOAA. 
rototag and a 12-mm, 125-kHz internal passive integrated 
transponder (Biomark Inc., Boise, ID) at the base of the 
first dorsal fin prior to release. 
Upon recapture during a survey, the date of capture, 
tag number, capture location (latitude and longitude), and 
all aforementioned length measurements were recorded 
before a shark was rereleased. If a bonnethead was recap- 
tured by a commercial or recreational fisherman (not 
during a survey), lengths, tag number, date of capture, and 
general capture location were requested. A subset of bon- 
netheads recaptured in the Atlantic region were sacrificed 
as part of age validation research (Frazier et al., 2014). 
Effects of tagging on growth 
Recaptured bonnetheads sacrificed and aged for age- 
based modeling of growth in a previous study (Frazier 
et al., 2014) were used in this study to test if there was 
any effect of tagging and tag type on growth. The residu- 
als at recapture (i.e., at sacrifice, calculated as expected 
FL at age minus observed length at estimated age) used 
for analysis were from use of the final VBGF in Frazier 
et al. (2014). Residuals at initial capture (i.e., at tagging) 
were determined by using FL at initial tagging and esti- 
mated age at initial tagging (calculated as estimated age 
at sacrifice minus time at liberty) to remodel the data set. 
The change in residuals at initial capture and in resid- 
uals at recapture of sacrificed bonnetheads were plotted 
against time at liberty. If tagging or tag type had a nega- 
tive effect on growth, most data points would be less than 
zero with the slope of the trendline significantly differ- 
ent from zero, indicating slower than predicted growth 
in tagged sharks. 
Modeling growth with tag-recapture data 
Growth increment data were modeled by using the Francis 
(1988b) method (i.e., by using the GROTAG model). In the 
event that a single individual was recaptured multiple 
times, only data from the initiai capture and final recap- 
ture were used in analyses to give equal weight to each fish 
and to maximize time at liberty (Welsford and Lyle, 2005). 
Data for all recaptured sharks (excluding data as previ- 
ously mentioned for sharks recaptured multiple times) 
were used in the model regardless of time at liberty, neg- 
ative growth, or potential outliers because the GROTAG 
model can use these data to inform several calculated 
parameters. Growth has been found to be significantly dif- 
ferent between sexes in previous growth studies (Parsons, 
1993; Carlson and Parsons, 1997; Lombardi-Carlson et al., 
2003; Frazier et al., 2014); therefore, sex-specific growth 
was modeled for both regions. 
The GROTAG model, which includes an implementa- 
tion of a maximum likelihood approach, was used to fit 
the VBGF (von Bertalanffy, 1938) to data for change in FL 
from initial capture to final recapture (AL) and for change 
in time at liberty (AT). The GROTAG model is a reparame- 
terization of the Fabens growth model (Fabens, 1965) that 
incorporates seasonal growth. Mean annual growth (g), 
