676 



Fishery Bulletin 100(4) 



MW 



MIR = MW/PBW 



AC 



:K 



Figure 3 



An agG-2+ Mustelus canis vertebra showing the calculation of the mar- 

 ginal increment ratio, MW = margin width, PBW = previous band width, 

 MIR = marginal increment ratio, AC = angle change. 



growth during the winter months. This study followed the 

 criteria found in Casey et al. (1985), who defined an "an- 

 nulus" as a mark that appears as an opaque band in the 

 intermedialia and continues as an opaque band into the 

 corpus calcareuni. 



A random sample of twenty vertebrae from each of ten 

 10-cm size classes (33-132 cm TL) was read independent- 

 ly by two readers, and a chi-square test was used to test 

 for systematic differences between the ages. The number 

 of observations above the main diagonal of a contingency 

 table of reader one's and reader two's ages was compared 

 with the number of observations below the main diagonal 

 to determine if this ratio was significantly different from 

 1:1. The percent agreement (PA), i.e. the percentage of ver- 

 tebrae in each length group that were assigned the same 

 ages by both readers, was determined to test for precision 

 between the two readers. 



Marginal increment analysis verified the annual nature 

 of the narrow opaque growth bands. The distance from the 

 last opaque band to the edge of the margin was measured 

 and divided by the width of the last growth band on the 

 vertebra to determine the marginal increment ratio (MIR, 

 Fig. 3). The margin width was divided by the distance to 

 the angle change or birthmark for age-1 animals. For age- 

 animals, the distance from the angle change to the edge 

 of the vertebrae was measured and divided by the distance 

 from the focus to the angle change. The mean MIR for each 

 month was plotted for juvenile-size animals to determine 

 if there was a yearly pattern in margin width. 



The length-at-age data were used to generate a von Ber- 

 talanffy growth curve for males and females by using the 

 computer program SigmaPlot (SPSS Inc., 2000) and the 



Marquardt-Levenberg algorithm to estimate curve-fitting 

 parameters (Press et al., 1986; Marquardt, 1963; Nash, 

 1979; Shrager, 1970, 1972). 



To determine if there was a period of the year when 

 smooth dogfish were growing at a faster rate, the mean to- 

 tal length of age-0 and age-1 smooth dogfish was plotted for 

 each month. For this procedure, we used data from several 

 years and we assumed that every year class followed the 

 same general growth pattern during the first two years of 

 life. The mean monthly length of age-0 animals determined 

 for the 1997 and 1998 cohorts was also calculated. Because 

 we did not collect free-swimming age-0 M. canis during 

 May, the largest estimate of birth length (40 cm) was used 

 to minimize the possibility of creating an artificially large 

 growth increment during the summer months. 



Results 



Vertebrae were collected from 918 smooth dogfish ranging 

 in size from 33 to 132 cm TL. The relationship between TL 

 and VR for males and females was not significantly differ- 

 ent (ANCOVA, P<0.05); therefore the data for both sexes 

 were combined. The statistically significant relationship 

 (P<0.001) between TL and VR was positive and curvilin- 

 ear (Fig. 4): 



TL = -0.477(W?)-2 + 17.06 (VR) + 0.807 



(;!=833, r2=0.97,P<0.001]. 



Of the original 918 vertebral samples, 894 animals were 

 aged and vertebrae from 24 animals were found to be an- 



