140 



Fishery Bulletin 94( 



1996 



40 



35 



30 



25 



20 



15 



10 



10 



Age (yr) 



Figure 4 



Differences between mean observed and mean back calculated 

 lengths at age for the blacktip shark, C. limbatus, aged 3,5,7,9, and 

 10 years. 



Negaprion brevirostris and had an APE index of 3.4% 

 (n=55). Their high level of reproducibility is prob- 

 ably a result of counting monthly circuli, where the 

 higher frequency and number of these rings inher- 

 ently results in a low APE. The statistical differences 

 among our three ring-enhancing meth- 

 ods, assessed by comparing APE indices 

 and goodness of fit of the growth curves, 

 were small, and it is was difficult to de- 

 cide which of the three produced both the 

 most accurate and the most reproducible 

 results. The UT method, although hav- 

 ing the second best reproducibility, was 

 chosen because of its best goodness of fit. 



indicated whether the prebirth marks were 

 present in all vertebrae examined. 



In 83% of the vertebrae, the band immedi- 

 ately after the change in angle was a nar- 

 row, translucent one. Bass et al. ( 1973) and 

 Dudley and Cliff (1993b) sampled pregnant 

 females with well-developed embryos be- 

 tween September and February, suggesting 

 that birth takes place in summer. If the as- 

 sumption made earlier, that the change in 

 angle is formed at birth is correct, then the 

 translucent band is associated with summer 

 growth. This is contrary to most other age- 

 ing studies using vertebral growth rings. 

 Branstetter (1987a) noted that in C. lim- 

 batus, band calcification occurred in the 

 northern hemisphere summer. He also found 

 that "the first growth band, bordered by the 

 birth annulus and the first postnatal annu- 

 lus, represented approximately 6 months 

 growth." Killam and Parsons (1989) found 

 that translucent ring deposition occurred 

 during December and January but referred 

 to a translucent "birth" ring, although the 

 sharks are born in May and June, the north- 

 ern hemisphere summer. In the only other 

 study of this nature in southern Africa, Walter and 

 Ebert (1991) found that in C. brachyurus opaque 

 bands are deposited in spring or summer. 



Brown and Gruber (1988) found that the change 

 in angle in Negaprion brevirostris was not laid down 



Centrum analyses 



A prebirth mark, a translucent band, was 

 found in only 20% of the vertebrae. 

 Branstetter (1987a) found that at least 

 one growth ring formed in C. lint bat us 

 during embryonic development, but 

 Killam and Parsons (1989) found none. 

 Prebirth marks have also been found in 

 Carcharhinus plumbeus (Casey et al., 

 1985), C. leucas (Branstetter and Stiles, 

 1987), C. brevipinna (Branstetter, 1987a), 

 and C. brachyurus (Walter and Ebert, 

 1991). None of these authors, however, 



200 



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140 



1 L'l ) 



1 [ 10 



Age (yr) 



Figure 5 



Von Bertalanffy growth curve for the blacktip shark, C. limbatus, sexes 

 combined. Age estimates were obtained by examination of the unstained 

 vertebrae with transmitted light. 



