Suhkowski et al.: Age and growth of Amblyra/a radiata 



165 



Age and growth estimates 



The von Bertalanffy growth curves (VBGC) 

 fitted to total length-at-age data (Fig. 5) pro- 

 vided a reasonable fit with a low standard error 

 for males, females, and both sexes combined 

 (Table 2). Furthermore, the VBGC parameters 

 for males, females, and the sexes combined were 

 similar, and because no differences in age-at- 

 size existed between males and females (P>0.05 

 ANOVA), those data were combined (Fig. 5). 



Discussion 



Precision estimates, the relationship between 

 TL and centrum diameter, and marginal incre- 

 ment analysis support the use of vertebral 

 centra for age analyses of thorny skates cap- 

 tured in the Gulf of Maine (Conrath et al., 

 2002; Sulikowski et al., 2003). Furthermore, 

 the 2.8% coefficient of variation indicates that 

 our aging method represents a precise approach 

 for the age assessment of A. radiata (Cam- 

 pana, 2001). Minimal width of the marginal 

 increment for thorny skates captured in August 

 and September (Fig. 4) supports the hypoth- 

 esis of annual band formation in this species. 

 Moreover, these results compare favorably to 

 cycles in marginal increments (Sulikowski et 

 al., 2003) and to annual vertebral band pat- 

 terns in other skate species (Holden and Vince, 

 1973; Waring, 1984; Natanson, 1993; Zeiner 

 and Wolfe, 1993; Walmsley-Hart et al., 1999; 

 Francis et al., 2001). However, because the 

 band counts of the largest and oldest animals 

 in the population were compressed (too small 

 for us to discern marginal increments from 

 their widths), the marginal increment analysis 

 included only juvenile skates that were s80 

 cm total length and the annular nature of the 

 growth bands was verified for only those length 

 groups. Nevertheless, we assumed that as the 

 skates grew larger and older, the annual nature 

 of growth ring deposition continued throughout 

 their lifetime (Conrath et al, 2002). 



During 42 sampling trips from June 2001 

 through May 2002 (approximately three trips 

 per month), individuals <30 cm TL were rarely 

 captured. The lack of specimens in this size 

 class and smaller size classes was most like- 

 ly due to the mesh size of the commercial trawl nets. 

 Trawl nets used by commercial fishermen in the Gulf 

 of Maine are restricted to a 6V2-inch diamond mesh- 

 size opening, which facilitates the release of most fish 

 below 30 cm TL. 



Our estimates of L r exceed the largest specimens in 

 our field collections for both females and males. Growth 

 parameters estimated from the Gompertz and logistics 

 equations also produced over-estimations of L a for the 



thorny skates in our study. Because the von Bertalanffy 

 growth curve (VBGC) is most widely used and accepted 

 in elasmobranch age and growth studies, we chose to 

 use this function to fit our data. Over estimation of L^ 

 with the VBGC has been documented in most skate 

 species studied to date (Table 3). Campana (2001) sug- 

 gested that the smallest and largest specimens are the 

 most influential in the estimation of growth. Moreover, 

 both Walmsley-Hart et al. (1999) and Sulikowski et 



