406 



Fishery Bulletin 101(2) 



Myers, 1998, Frisk, 2000). Recent assessment studies in 

 the northeast U.S. (Northeast Fisheries Science Center^), 

 suggest that the biomass of the winter skate may be below 

 threshold levels mandated by the Sustainable Fisheries 

 Act (SFA). To add insight into the life history of this species 

 and the status of the stock (Simpfendorfer, 1993; Frisk et 

 al., 2001 ), we estimated age and growth rates of L. ocellata 

 by interpreting annular counts and marginal increments 

 on vertebral centra from specimens collected in the western 

 Gulf of Maine. 



Materials and methods 



Sampling 



A total of 304 winter skates were captured by otter trawl 

 between November 1999 and May 2001 at locations that 

 ranged from 1.6 to 32 km off the coast of New Hampshire. 

 Approximate depths at these locations ranged between 9 

 and 107 m. Skates were maintained alive on board the 

 vessel until transport to the University of New Hampshire's 

 Coastal Marine Laboratory (CML). There, individual fish 

 were euthanized (0.05g/L bath of MS222). We measured 

 total length (TL in mm) as a straight line distance from the 

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

 in mm) as a straight line distance between the tips of the 

 widest portion of pectoral fins. Total wet weight (kg) was 

 also recorded. In order to differentiate between the small, 

 immature specimens of little skates (Leucoraja erinacea, 

 a congener species also found in the Gulf of Maine) and 

 winter skates, rows of teeth in the upper jaw were counted. 

 Skates whose number of teeth ranged between 72 and 110 

 per row were identified as L. ocellata and skates whose 

 number of teeth ranged between 38 and 64 per row were 

 identified as L. erinacea (Bigelow and Schroeder, 1953). To 

 reduce any uncertainty in species identification, skates 

 having between 38 and 71 teeth per row were not used in 

 this study. 



Preparation of vertebral samples 



Vertebral samples, taken from above the abdominal cavity, 

 were removed from 132 females and 98 males, labeled, 

 and stored frozen. After defrosting, three centra from each 

 specimen were freed from the vertebral column, stripped 

 of excess tissue and air dried. Large centra were cut sagit- 

 tally, while held within a vise, with a DremeF"^' tool fitted 

 with a mini-saw attachment. Smaller centra were sanded 

 with a DremeF'^' tool to replicate a sagittal cut. Processed 

 vertebrae were mounted horizontally on glass microscope 

 slides and ground with successively finer grits (#180, 

 #400, #600), of wet-dry sandpaper. Each vertebra was then 

 remounted and the other side ground to produce a thin (300 

 micrometer) "hourglass" section. 



Counts of annuli 



Vertebral sections were viewed through a compound micro- 

 scope (25-40x) with reflected light (Fig. 1). A growth ring 

 (annulus) was defined as an 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). 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 calca- 

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



Three nonconsecutive counts of annuli were made for 

 the three vertebral sections from each specimen without 

 prior knowledge of the length of the skate or previous 

 counts. If the variability between readings was more than 

 two years, that particular specimen was eliminated from 

 further analyses. Count reproducibility was estimated by 

 using the index of average percent error ( lAPE ) described 

 by Beamish and Fournier (1981): 



M/^£=1/A'^(|//?^(|a-,, -X,|/X,)jxl00, 



where N = the number of skates aged; 



R = the number of readings; 



X = the ith age determination of thejth fish; and 



X = the average calculated for thejth fish. 



An upper limit for the lAPE was arbitrarily set at 15'7( for 

 each vertebra. Vertebrae with statistically acceptable lAPE 

 indexes were used for estimation of asymptotic growth 

 rates (Brown and Gruber, 1988; Cailliet and Tanaka, 

 1990). The average of the mean counts for all three centra 

 defined the age estimate for each specimen (Casey et al., 

 1985; Wintner and Cliff, 1996). 



A von Bertalanffy growth function (VBGF) was fitted 

 to the data with the following equation (von Bertalanffy, 

 1938): 



where L, 



K 



to 



total length at time t (age in years); 

 theoretical asymptotic length; 

 Brody growth constant; and 

 theoretical age at zero length. 



^ Northeast Fisheries Science Center. 1999. 30th northeast 

 regional stock assessment workshop, 477 p. Northeast Fisher- 

 ies Science Center, 166 Water Street Woods Hole, MA 02543- 

 1026. 



Growth in length data were analyzed by using FISHPARM, 

 a computer program for parameter estimation of nonlinear 

 models with Marquardt's ( 1963) algorithm for least-square 

 estimation of nonlinear parameters (Prager et al., 1987). 



Marginal increment analyses 



The annual periodicity of band pair formation was inves- 

 tigated by using marginal increment analyses (MIA). 

 Because the annuli in older specimens were closer together, 

 marginal increments were calculated from five specimens 

 per month whose centra contained either four or five 

 annuli. For MIA determination, the distance of the final 

 opaque band and the penultimate opaque band from the 

 centrum edge were measured with an ocular micrometer. 



