138 



Fishery Bulletin 94(1). 1996 



Figure 2 



Von Bertalanffy growth curves for the blacktip shark, C. limbatus. Age 

 estimates were obtained with four different methods: UT = unstained- 

 vertebra technique; BT = "bow tie" technique; CV = crystal-violet-stain- 

 ing technique; and ALL = nine-readings technique. 



ness of fit and the greatest difference between the 

 L^ value and the observed maximum length. The best 

 goodness of fit was found for the UT method, which 

 was therefore used to obtain age estimates. 



Centrum analyses 



A distinct prebirth mark, a translucent band appear- 

 ing before the birth mark, was found in 20% of the 

 vertebrae but in none of the embryonic vertebrae. In 

 83% of the vertebrae, the first band immediately after 

 the change in angle was a narrow translucent one. 



The first three growth rings were all clearly vis- 

 ible in each of the three ring-enhancing methods. 

 Difficulty was experienced in resolving bands at the 

 centrum edge in older specimens; therefore, accurate 

 marginal increment measurements could be obtained 

 only for the five specimens younger than four years, 

 and the marginal increment analysis was omitted. 

 The observed ratio of translucent to opaque last 

 bands differed significantly from the expected ratio 

 <X 2 test, P<0.001, /?=89); consequently, annual peri- 

 odicity of the growth rings could not be confirmed. 



A linear relationship was found between centrum 

 diameter and PCL (Fig. 3). As there was no signifi- 

 cant difference between the sexes (Student's f-test, 

 P>0.2), the data were combined. The intercept of the 

 linear regression was very close to zero (-0.039 cm), 

 and we felt that any correction, such as the Fraser- 



Lee method (Branstetter, 1987a; Carlander, 1969), 

 was unnecessary. 



Mean back-calculated lengths were lower than 

 observed values, except at age zero, where back-cal- 

 culated birth size was 42 cm and the observed value 

 was 41 cm (Table 2). Lee's phenomenon, a tendency 

 for back-calculated lengths of older fish in the ear- 

 lier years of life to be systematically lower than those 

 of younger fish at the same age (Carlander, 1969; 

 Smith, 1983 ), was apparent ( Fig. 4 ). For example, in 

 a shark aged 10 years, there was a 33.6 cm differ- 

 ence between observed and back-calculated length 

 at age four; whereas an animal aged five had only a 

 21.6 cm difference at age four. 



Age and growth estimates 



The smallest female (86 cm) was two years, the larg- 

 est (181 cm) was eight years old (Fig. 5). The two 

 smallest mature females, one of which was pregnant, 

 measured 156 cm and were both seven years old. The 

 smallest (113 cm) and largest (182 cm) males were 

 two and nine years old, respectively. The smallest 

 mature male ( 150 cm) was six years old. The oldest 

 female (179 cm) was 11 years and the oldest male 

 ( 179 cm) was 10 years old. Size at birth was calcu- 

 lated to be 43 cm. 



Growth rates calculated from mean observed 

 lengths were lower, especially for immature sharks. 



