Dippold et al.: Growth, mortality, and movement of Rachycentron canadum 
469 
Table 5 
Candidate models evaluated for estimation of survivor¬ 
ship (S) and probability of tag recovery (/) with data 
from tagging and recapture of cobia (Rachycentron 
canadum) in the Gulf of Mexico and South Atlantic 
Ocean during 1988-2014. Time-invariant parameters 
are represented by (.) and time-variant parameters are 
represented by (t). The models were evaluated by using 
Akaike’s information criterion (AIC), and Akaike weight 
(w ;) was calculated for each model. 
AICc Model 
Model 
AICc 
AAICc 
weight 
likelihood 
S(.)j\ t) 
10081.5 
0.00 
0.79 
1.00 
S(.)f(.) 
10084.2 
2.72 
0.20 
0.26 
S(t)/(t) 
10091.3 
9.82 
0.01 
0.01 
S(t)/(.) 
10109.7 
28.19 
0.00 
0.00 
nual tag-recovery rate ranged from 0.013 to 0.041 and 
averaged 0.023 (Fig. 6). Peaks in tag-recovery rates oc¬ 
curred in 1993 and from 2006 to 2007; however, the f 
parameter remained relatively constant throughout the 
duration of the tagging program (Fig. 6). 
Discussion 
In the most recent Gulf of Mexico cobia 
stock assessment (SEDAR 1 ), several re¬ 
search needs were identified that, if re¬ 
solved, would help enable reviewers to 
evaluate the appropriateness of an as¬ 
sessment model and allow the determi¬ 
nation of stock status to be made. The 
information provided in this study fills 
information gaps that exist for cobia in 
the Gulf of Mexico and South Atlantic 
Ocean that will aid in future assessment 
efforts and allow for an accurate deter¬ 
mination of stock status. The informa¬ 
tion includes an alternative approach to 
modeling the length-at-age relationship, 
a description of the annual Z and eluci¬ 
dation of the annual movement, and the 
distribution patterns of individuals. 
A primary objective of our research 
was to provide alternative length-at- 
age parameter estimates and compare 
them with estimates available in the 
published literature. Of the models we 
evaluated, the GROTAG VBGF model 
resulted in the best option for fitting 
growth increment information from 
tag-recapture studies because it explic¬ 
itly addresses variability in individual 
growth, effectively handles the presence 
of outliers, and is able to accurately evaluate mean val¬ 
ues of L„—a failing of other algorithms with tag and 
recapture information (e.g., Fabens, 1965). Our mean 
estimate of derived from the length-at-age analysis 
with the GROTAG VBGF model is lower than estimates 
reported in other studies of individual growth dynam¬ 
ics with the use of otoliths. This result could be due 
to the sex-aggregated nature of the tagging data that 
were available (our estimate was often higher than 
male-specific estimates and lower than female-specific 
estimates, Table 3). The lack of sex-specific information 
is a confounding feature of cooperative tagging pro¬ 
grams (Dippold et al., 2016) and likely has an effect on 
the estimated length-at-age parameters. Another pos¬ 
sible explanation for the lower L„ reported here is that 
anglers typically keep cobia of legal length (>84 cm or 
33 in) and tagged individuals were generally smaller 
or close to the minimum length limit, both of which 
may bias our length-at-age parameter estimates. We 
also found that the mean k in this study was greater 
than that of many other published estimates (Table 
3). Because the VBGF parameters are strongly and 
negatively correlated, the higher k estimate may be in 
part caused by the lower estimate of L„. Despite the 
sexual dimorphism in length-at-age between males and 
females, both the SEDAR assessments of cobia in the 
Gulf of Mexico and Atlantic Ocean used a sex-combined 
3-parameter VBGF to describe the length-at-age rela¬ 
tionship. Therefore, although in the tagging program 
data that we evaluated, sex was not recorded, and the 
lack of sex determination is a disadvantage of coop- 
1990 1995 2000 2005 2010 2015 
Year 
Figure 6 
The estimated probability of tag recovery (/) from the best supported 
tag-recovery mortality model fit to data from tagging and recapture of 
cobia (Rachycentron canadum) in the Gulf of Mexico and South Atlantic 
during 1988-2014. Error bars indicate 95% confidence intervals. 
