166 



Fishery Bulletin 103(1) 



al. (2003) suggested that rareness of large in- 

 dividuals was most likely responsible for their 

 over estimation of L x . Similarly, in a study of the 

 blue shark (Prionace glauca) in the northwest 

 Atlantic, Skomal and Natanson (2003) suggested 

 that earlier studies on the same species contained 

 artificially inflated L x estimates and lower growth 

 rates because of the lack of maximum-size fish. 

 The rareness of large specimens in our study may 

 have been due to these larger individuals being 

 able to avoid the fishing gear or may indicate that 

 mortality, natural or fishing induced, prevents 

 them from attaining these lengths. Exploratory 

 manipulation of our data indicated that inclusion 

 of maximum observed sizes (i.e., thorny skates 

 over 103 cm TL) produced divergent results with 

 regard to von Bertalanffy parameters. For ex- 

 ample, the addition of maximum-size fish, using 

 20 years as the maximum age (Templeman, 1984) 

 and 105 cm as the maximum total length (from 

 the present study), reduced the combined sex L x 

 from 124 cm TL to 116 cm TL. However, the same 

 effect was not documented when adding hatching 

 size (age zero) fish (note: because no documented 

 size for thorny skates exists within the Gulf of 

 Maine, the authors used known hatching sizes for 

 a similar congener species, the winter skate [Leucoraja 

 ocellata]). Be that as it may, the reasonable fit of the 

 thorny skate data (Table 2) to the VBGC (Fig. 5) for 

 A. radiata, along with a comparison with other batoid 

 studies (Table 3), indicates that this is an appropriate 

 model for this skate species. 



Growth rates were similar for both sexes of thorny 

 skate (£ = 0.13 for females and 6 = 0.11 for males) and 

 commensurate with other skate species of a similar 

 size. The oldest age obtained for male and female 

 thorny skates was 16 years (Table 1). These data are 

 in agreement with the assumption that larger batoids, 

 such as A. radiata, R. pullopunctata (Walmsley-Hart et 

 al„ 1999), and L. ocellata (Sulikowski et al., 2003) are 

 longer lived and slower growing than smaller species. 

 For instance, R. erinacea, which reaches a total length 

 of 52 cm, has been aged to 8 years and found to have 



a corresponding k value of 0.352 (Johnson, 1979; War- 

 ing, 1984). 



The reduction in biomass of the thorny skate below 

 threshold levels mandated by the SFA necessitates the 

 development of management measures to rebuild these 

 stocks in accordance with the Magnuson-Stevens Fish- 

 ery Conservation and Management Act. However, the 

 development and implementation of a successful fisher- 

 ies management plan for the species require in-depth 

 analyses of appropriate biological information. More- 

 over, accurate stock assessment data for skates is dif- 

 ficult to collect in the northeast U.S. because individual 

 species are rarely differentiated in landings information 

 (NEFMC 1 2 ). As a result, fluctuations in stock size will 

 continue to be difficult to detect and successful imple- 

 mentation of fisheries management plans will remain 

 problematic. This article is the first in a series aimed 

 at providing life history data for the management of 

 thorny skates indigenous to the Gulf of Maine. The 

 basic age and growth parameters for the thorny skate 

 provided in the present study support the hypothesis 

 that A. radiata, like other elasmobranchs, require con- 

 servative management because of their slow growth rate 

 and susceptibility to over-exploitation (Brander, 1981; 

 Kusher et al., 1992; Zeiner and Wolf, 1993; Frisk et al., 

 2001; Sulikowski et al., 2003). 



Acknowledgments 



Collection of skates was conducted on the FV Mystique 

 Lady. We thank Noel Carlson for maintenance of the fish 

 at the U.N.H. Coastal Marine Laboratory and Matt Ayer 

 for his help in digitizing the vertebrae samples. This 



