Timmons and Bray: Age, growth, and sexual maturity of Rhmobatos productus 
355 
agreement ± 3 bands (2 guitarfish). Only bands in 
total agreement (from 43 guitarfish) were used in 
the final analysis. 
The linear model best represented growth of com- 
bined sexes of guitarfish because the coefficient of 
determination was 0.90 for the linear regression, and 
0.81 for the nonlinear von Bertalanffy curve (Fig. 7: 
Table 1). For females only, the linear regression and 
the von Bertalanffy curve produced similar values 
Estimated age (yr) 
Figure 7 
Growth of the guitarfish predicted by a linear regression 
(dotted line). The solid line represents a von Bertalanffy 
growth curve for the data. Equations are those used to cal- 
culate the regression. 
for the coefficient of determination (r 2 =0.95, and 
r 2 =0.94, respectively). For males, the linear regres- 
sion also appeared to be a better predictor (r 2 =0.78) 
than the von Bertalanffy curve (r 2 =0.70; Table 1). 
Residuals for both the linear regression and the von 
Bertalanffy model (females and males) clustered 
evenly about both sides of the prediction lines. There 
was no reason to suggest any violation of homo- 
scedasticity in either model. 
If it is necessary to predict the total length of a 
specimen from fish markets using only the tail re- 
gion of the guitarfish, the following equation is sug- 
gested (Fig. 8): 
Table 1 
A comparison of linear regression parameters and von Bertalanffy parameters for male (n= 24) and female (n=19) guitarfish and 
for combined sexes (n= 43). 
Linear regression parameters 
von Bertalanffy parameters 
Y-intercept 
Slope 
r 2 
k 
t 0 
r 2 
Female 
34.02 
8.29 
0.95 
594 
0.016 
-3.80 
0.94 
Male 
47.00 
6.32 
0.78 
142 
0.095 
-3.942 
0.70 
Female and male 
43.33 
6.90 
0.90 
228 
0.047 
-4.030 
0.81 
