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



123 



Lawler's ( 1976) L m is unrealistically high when com- 

 pared with the maximum reported size from the lit- 

 erature (308 cm FL [Springer, I960]) and with those 

 derived in both Hoenigs ( 1979) study and the present 

 one. Schwartz (1983) also used vertebral bands in 

 an attempt to age the dusky shark. In general, his 

 back-calculated size-at-age data (Schwartz, 1983; 

 Table 4) are lower than those in the present study. 

 His estimates of size at birth are much lower than 

 reported sizes at birth, suggesting that prebirth 

 bands may have been counted (Casey et al., 1985). 

 Marginal increment data from the present study do 

 not support Schwartz's (1983) hypothesis of a much 

 faster growth rate with two bands per year based on 

 his marginal increment data. 



Tagging data combined with vertebral estimates 

 indicate that the dusky shark can live for at least 40 

 years. The largest reported dusky shark in the lit- 

 erature was a 308 cm FL female (Springer, 1960) 



Table 3 

 Results of comparison of the vertebral von Bertalanffy 

 growth equations for male and female dusky sharks, Carc- 

 harhinus obscurus, using Bernard's ( 1981 ) multivariate analy- 

 sis. Shown is the estimated variance-covariance matrix (S). 



K 



K 

 t„ 



131.9769 



-0.02831 

 0.0000065 



-3.16979 

 0.00077 

 0.115042 



= S 



which corresponds to 51 years based on our verte- 

 bral VBGF. Tag and recapture data on the large 

 dusky shark at liberty for 12 years add credence to 

 the vertebral longevity estimate by verifying that 

 these fish do live up to 40 years. Thus, the dusky 

 shark appears to be a long-lived, late-maturing spe- 

 cies, exhibiting a pattern typical of carcharhinids. 

 The female bull shark, C. leucas, can live over 24 

 years and does not reach maturity until at least 18 

 years of age (Branstetter and Stiles, 1987). Tagging 

 evidence has shown that the sandbar shark, C. 

 plumbeus, can reach ages of at least 40 years and 

 may not mature until 29 years of age (Casey and 

 Natanson, 1992). The lemon shark, Negaprion 

 brevirostris, which reaches at least 20 years of age 

 and does not reproduce until 11 to 13 years, also fol- 

 lows this pattern (Brown and Gruber, 1988). Prelimi- 

 nary estimates on the age of the bronze whaler, C. 

 brachyurus, indicate that males do not mature for 

 13 to 19 years and females for 19 to 20 years. Both 

 may attain ages over 30 years (Walter and Ebert, 

 1991). The blue shark, Prionace glauca, appears to 

 be atypical of most carcharhinids in having a rela- 

 tively fast growth rate and a short life-span (13-16 

 years) (Skomal, 1990). A recent recapture from an 

 Australian school shark, Galeorhinus australis, af- 

 ter 42 years at liberty indicates that the galeorhinids 

 also follow this pattern. 3 



Casey and Natanson's (1992) study on the age of 

 the sandbar shark (C. plumbeus) showed that in a 



3 Olsen, A. H. Newton 5074, South Australia. Personal commun., 

 Jan. 1994. 



350 

 300 

 250 



200 -■ 

 150 -- 

 100 

 50 



Male 



Female 



Female age at maturity 



Male age at maturity 



10 



15 



-t—  1 — 



20 25 



Age (years) 



30 



35 



40 



— I 

 45 



Figure 3 



Von Bertalanffy growth curves generated from vertebral data for male and female dusky sharks, 

 Carcharhinus obscurus. 



