Natanson et al.: Age and growth estimates for Carcharhinus obscurus 



300 - 

 250 



g 200 



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? 150 

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£ 100 - 



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Large females not used for 

 regression calculation 



10 15 



Radius (mm) 



20 



25 



Figure 1 



Relationship between vertebra] radius (cm) and fork length (cm) for male and female dusky sharks, 

 Carcharhinus obscurus. 



be further established owing to a lack of winter 

 samples. January was used as the month of band 

 formation for the assignment of age classes (Casey 

 et al., 1985). 



Back-calculated length at first band (80.2 cm FL 

 male; 85.8 cm FL female ) corresponded closely to the 

 known size at birth of 85-100 cm TL (Castro, 1983; 

 Compagno, 1984). The first winter band would have 

 formed after approximately six months growth (as- 

 suming January deposition and spring parturition), 

 and the following bands represented annual growth 

 (Branstetter, 1987). The oldest female in the sample 

 was 33+ years and the oldest male, 25+ years. 



The parameters of the VBGF determined from the 

 back-calculated data were similar to known life his- 

 tory characteristics except that the predicted L^ for 

 males was higher than that for females (Table 2). Those 

 samples that were neonates with no visible birthmark 

 were excluded from the VBGF analysis. Therefore, only 

 114 samples (47 male and 67 female) were included in 

 the final calculations. The t Q and if values appear simi- 

 lar between the sexes (Table 2). However, the male and 

 female growth curves are significantly different (P<0.05) 

 based on Bernard's ( 1981) multivariate analysis (Table 3). 

 The results indicated that the differences were caused by 

 the t and L x values (in order of significance). 



The reported size at maturity for the dusky shark 

 is 231 cm FL and 235 cm FL for males and females, 

 respectively. These lengths correspond to 19 years 

 for males and 21 years for females based on the ver- 

 tebral growth curves (Table 4). 



Length frequency 



Length observations from a total of 208 female and 133 

 male dusky sharks were used to calculate von 

 Bertalanffy parameters by using length-frequency 

 analysis. Samples were obtained from 1961 to 1987 for 

 the months May through November. Because of small 

 yearly sample sizes, data for all years were combined 

 by month. 



A comparison of the VBGF parameters from the 

 length-frequency analysis [LF] with those derived from 

 vertebral analysis (Table 2) shows that the L m and t 

 values from the vertebral analysis for females were 

 lower than those derived from the length-frequency 

 analysis, and that the K value for females was basi- 

 cally the same for both data sets. The length-frequency 

 analysis for males results in a lower L^ than that from 

 the vertebral analysis and in higher t and K values. 

 The VBGF differences in both sexes are not large and 

 both curves indicate late age at maturity (males: 25 yr 

 [LF], 19 yr [vertebral]; females: 16 yr [LF], 21 yr [ver- 

 tebral]) and slow growth (males: if =0.049 [LF], 0.038 

 [vertebral]; females: #=0.040 [LF], 0.039 [vertebral]) 

 (Fig. 3). The von Bertalanffy parameters for the sexes 

 combined are shown for comparison (Table 2). 



Longevity 



Tagging records from NMFS Cooperative Shark Tag- 

 ging Program show that 6,067 dusky sharks were 

 tagged and 131 recaptured between 1962 and 1992. 



