Frazier et al.: Growth rates of Sphyna tiburo estimated from tag-recapture data 
-e— GROTAG model 
-& Age-based model 
100 120 
nl 
40 60 80 
L 
20 
-e- GROTAG model 
-& Age-based model 
80 100 120 
60 
40 
Growth rate (mm FL/year) 
20 
-4- Atlantic region 
-~ GOM 
So 
=) 
i 
g 
=) 
60 
20 40 
0 
550 650 
Fork length (mm) 
Figure 3 
Growth rates of male bonnetheads (Sphyrna tiburo) estimated with the opti- 
mal GROTAG models, which are based on tag-recapture data, and mean 
growth rates calculated with the age-based (Francis, 1988a) von Bertalanffy 
growth function, with associated 95% confidence intervals (dotted lines). 
Growth rates are shown for bonnetheads tagged (A) in the northeastern Gulf 
of Mexico (GOM) and (B) in the estuarine waters of the Atlantic coast of the 
southeastern United States (Atlantic region). Also provided are (C) estimates 
of growth rates from the optimal GROTAG model for each of the 2 regions. 
Age-based models were generated by using original length-at-age data from 
Lombardi et al. (2007) for sharks in the GOM and from Frazier et al. (2014) 
for sharks in the Atlantic region. Data used in the GROTAG models are from 
bonnetheads tagged and recaptured in the GOM during 1993-2006 and in the 
Atlantic region during 1998-2019. Error bars indicate 95% confidence inter- 
vals generated by bootstrapping (with 5000 iterations). FL=fork length. 
339 
and differences in these growth rates 
may be an artifact of sample size given 
the observed 95% CIs. Regardless, these 
variability estimates are among the 
highest published values for elasmo- 
branchs with values ranging from 0.06 
for the tope (Galeorhinus galeus) (Fran- 
cis and Mulligan, 1998) to 0.58 for the 
spotted estuary smooth-hound (Mustelus 
lenticulatus) (Francis and Francis, 1992), 
with most falling in a range of 0.15—0.40 
(e.g., Simpfendorfer, 2000; Meyer et al., 
2014; Natanson and Deacy, 2019). 
Results from the final GROTAG mod- 
els indicate a strong seasonal growth 
pattern in the population in the Atlantic 
region (u=1) but not in the population in 
the GOM (the seasonal growth parame- 
ter was not included in the final GOM 
model). Latitudinal variation in growth 
has been previously detected for bon- 
netheads in the GOM but not for those 
in the Atlantic region (Parsons, 1993; 
Carlson and Parsons, 1997; Lombardi- 
Carlson et al., 2003; Frazier et al., 2014). 
The more variable growth for sharks 
in the GOM and lack of a seasonal sig- 
nal may indicate that there are differ- 
ences within the population in growth 
along the western coast of Florida, 
where sampling for this study occurred. 
Site fidelity has been established for 
bonnetheads in both the Atlantic region 
and the GOM (Heupel et al., 2006; 
Driggers et al., 2014), and bonnetheads 
are known to make predictable tem- 
perature-driven migrations in response 
to declining water temperatures (Ulrich 
et al., 2007; Driggers et al., 2014). In the 
Atlantic region, all bonnetheads were 
tagged and recaptured within their 
established summer range. In the GOM, 
bonnetheads were also predominantly 
captured and recaptured during sum- 
mer months; however, capture and 
recapture of sharks took place at multi- 
ple locations along the coast. Because 
these sampled subpopulations likely 
have site fidelity to their area of cap- 
ture, they likely experience differential 
growth due to differences in water tem- 
peratures (Carlson and Parsons, 1999; 
Pistevos et al., 2015), in food availabil- 
measurements at tagging and recapture and consequently 
lower chances of errors in measurement. 
Results from the final GROTAG models for females indi- 
cate that growth is more variable for bonnetheads from 
the GOM (v=0.63) than for those from the Atlantic region 
(v=0.56); however, these differences are not significant 
ity or energetic value of food (Bethea et al., 2007; 
Vucic-Pestic et al., 2011), and in energetic needs during 
winter migrations necessary to find optimum water tem- 
peratures for overwintering (Carlson and Parsons, 1999; 
Hoffmayer et al., 2006). However, the lack of a detection 
of seasonal growth patterns and greater growth 
