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Fishery Bulletin 103(1) 



example, the thorny skate reaches total lengths of over 

 100 cm in the Gulf of Maine (Collette and Klein, 2002), 

 whereas specimens captured off the Labrador coast do 

 not reach total lengths >72 cm (Templeman, 1987). 

 Although no directed fisheries for this species exists in 

 the Gulf of Maine, this skate meets the minimum 1*4 

 pound-cut pectoral-fin size sought after by wing proces- 

 sors (Sosebee, 2000; NEFMC 1 - 2 ). Unfortunately, because 

 landings are not reported by species, the proportion of 

 thorny skates to the total wing market is unknown. Re- 

 cent assessment studies in the northeast United States 

 (NEFSC 3 ) indicate that the biomass of thorny skates 

 is declining, and is below threshold levels mandated 

 by the Sustainable Fisheries Act (SFA; NMFS 4 ). Thus, 

 owing to the recent commercial interest in this species 

 and the concomitant decline in population size, obtain- 

 ing life history information for this species has become 

 more important. In order to provide insight into the 

 biology of this species and to determine the stock status 

 (Simpfendorfer, 1993; Frisk et al., 2001), our objectives 

 were to estimate age and growth rates of A. radiata 

 based on banding patterns in vertebral centra from 

 specimens collected in the western Gulf of Maine. 



Marine Laboratory (CML). There, individual fish were 

 euthanized (0.3g/L bath of MS222). Total length (TL in 

 cm) was measured as a straight line distance from the 

 tip of the rostrum to the end of the tail, and disc width 

 (DW in cm) as a straight line distance between the tips 

 of the widest portion of pectoral fins. Total wet weight 

 (kg) was also recorded. 



Preparation of vertebral samples 



Vertebral samples, taken from above the abdominal 

 cavity, were removed from 320 thorny skates ( 154 females 

 and 166 males), labeled, and stored frozen. After having 

 been thawed, three centra from each specimen were 

 removed from the vertebral column, stripped of excess 

 tissue and air dried. Large centra were cut sagittally 

 with a Dremel™ tool fitted with a mini saw attachment 

 while held with a vice-like device. Smaller centra were 

 sanded with a Dremel™ tool to replicate a sagittal cut. 

 Processed vertebrae were mounted horizontally on glass 

 microscope slides and ground with successively finer-grit 

 (no. 180, no. 400, no. 600) wet-dry sandpaper. Each ver- 

 tebra was then remounted and the other side ground to 

 produce a thin (300-micrometer) hourglass section. 



Materials and methods 

 Sampling 



Thorny skates were captured by otter trawl in an approx- 

 imate 900 square mile area centered at 42°50'N and 

 70°15'W in the Gulf of Maine between June 2001 and 

 May 2002. These locations varied from 30 to 40 km off 

 the coast of New Hampshire. Approximate depths at 

 this location ranged between 100 and 120 m. This area 

 was chosen for two reasons: 1) these waters support 

 the vast majority of commercial fishing in New Hamp- 

 shire and can be easily accessed during normal fishing 

 operations; and 2) because of rolling closures within 

 the Gulf of Maine, an experimental fishing permit was 

 granted to us by the National Marine Fisheries Service 

 (NMFS) to collect thorny skates in this location during 

 the months of April, May, and June, when these waters 

 are closed to commercial fishing. Although our sam- 

 pling was conducted in a small portion of the species' 

 range, the sizes of thorny rays collected corresponded to 

 those collected during the NEFSC bottom trawl surveys 

 conducted throughout the Gulf of Maine and Georges 

 Bank (NEFMC 1 ; NEFSC 3 ). From this information, it is 

 unlikely that differences in other biological parameters 

 exist. 



Skates were maintained alive on board the vessel un- 

 til arrival at the LTniversity of New Hampshire's Coastal 



3 NEFSC (Northeast Fisheries Science Center). 1999. 30th 

 Northeast regional stock assessment workshop. NEFSC, 

 166 Water Street, Woods Hole, MA 02543-1026. 



4 NMFS (National Marine Fisheries Service). 2002. Annual 

 report to Congress on the status of U.S. fisheries 2001, 142 

 p. NMFS, NOAA, Silver Spring, MD 20910. 



Band counts 



Vertebral sections were digitally photographed with a 

 Canon Powershot S40 attached to a Leica S8PO dis- 

 secting microscope and reflected light. Magnification 

 depended on the size of the section and varied from 4x 

 to 12x (Fig. 1). A growth ring (band count) was defined 

 as one opaque and translucent band pair that traversed 

 the intermedialia and that clearly extended into the 

 corpus calcareum (Casey et al., 1985; Brown and Gruber, 

 1988; Sulikowski et al., 2003). The birth mark (age zero) 

 was defined as the first distinct mark distal to the focus 

 that coincided with a change in the angle of the corpus 

 calcareum (Casey et al., 1985; Wintner and Cliff, 1996; 

 Sulikowski et al, 2003). 



Precision and bias 



Nonconsecutive band counts were made independently 

 by two readers for each specimen used in the study with- 

 out prior knowledge of the skate's length or of previous 

 counts. A Tukey test was used to test for differences 

 between ages. Age determination bias between read- 

 ers was assessed through the use of an age-bias plot. 

 This type of graph displays band counts of one reader 

 against a second reader in reference to an equivalence 

 line. Specifically, reader 2 is represented as mean age 

 and 95% confidence intervals corresponding to each 

 of the age classes estimated by reader 1 (Campana et 

 al., 1995). Divergence from the equivalence line, where 

 reader 1 = reader 2, would indicate a systematic dif- 

 ference between readers. Precision estimates of each 

 reader were calculated by using the coefficient of varia- 

 tion (CV) as described by Chang (1982) and Campana 

 et al. (1995). 



