KILLAM and PARSONS: AGE AND GROWTH OF THE BLACKTIP SHARK 



The equation is 



W = a a 



exp' 



,(-(6*r) + c) 



where W = weight in kg, a = asymptotic weight 

 in kg, r = number of vertebral rings, and h and c 

 are constants. 



The von Bertalanffy (1938) growth equation 

 was used to predict a gi'owth curve for male and 

 female blacktip sharks (Ricker 1975). The equa- 

 tion is 



L, = LAI - exp 



-K(t - l„) 



), 



where L, = length at age t in years, L,c = 

 maximum theoretical length, K = the rate at 

 which the asymptote is reached, and t,) = the 

 theoretical age at zero length. The von Ber- 

 talanffy growth equation was fit to observed 

 data using a nonlinear, Statistical Analysis Sys- 

 tem method (SAS Institute, Inc. 1982). 



RESULTS 

 Centrum Analysis 



A total of 140 vertebral centra were i-ead from 

 86 female and 54 male C. li)i>batnfi. These sharks 

 ranged in size from 52.4 to 183.0 cm. The mini- 

 mum and maximum numbers of translucent 

 rings counted were and 11. Centrum radii 

 ranged from 2.8 to 10.6 mm. Initially, exact 

 agi'eement of ring counts was reached on 83"^ of 

 the readings, lo'vc differed by one ring, and 2'7c 

 by two rings. Translucent ring counts made on 

 sectioned centra were very similar to those of 

 the corresponding whole centra. The number of 

 translucent rings counted ranged fi-om 1 through 

 10. When comparisons between whole and sec- 

 tioned centra were made, exact agi'eement was 

 reached on 15 centra, 3 centra differed by one 

 ring and 2 centra differed by two rings. In addi- 

 tion, the ring structures counted on the sec- 

 tioned centra were coincidental with those en- 

 hanced by the pencil method. Subjectivity is in- 

 volved in both methods and sometimes resulted 

 in slightly different counts. 



The relationship between blacktip shark total 

 length and centrum radius was linear for both 

 sexes. Analysis of covariance (ANCOVA) in- 

 dicated no significant difference in the regi'es- 

 sion lines between sexes, so data were combined 

 into the relationship (Fig. 2) 



TL = 63.2 + {16.7)R (r = 0.9797, A^ = 130) 



where TL = total length in mm and R = centrum 

 radius in mm. 



Initial vertebral ring deposition appeared to 

 occur at or shortly after parturition. Examina- 

 tion of 15 centra from embryonic C. limbatiis, 

 which ranged in size from 48.4 to 61.8 cm, had no 

 observable translucent ring formation. These 

 sharks were collected between April and June 

 1986. If a ring had been deposited prior to birth, 

 it should have been visible in the near-term em- 

 bryos. Free-swimming juveniles captured in late 

 May and early June had a translucent ring de- 

 posited on the edge of the centrum. With subse- 

 quent gi'owth, this "birth" ring became more 

 apparent as opaque tissue was deposited distally 

 to it. 



Marginal increment data on juvenile blacktip 

 sharks (52.4-116.0 cm) of both sexes were com- 

 bined, because at this age there were no signifi- 

 cant differences in sizes (Student's f-test, P > 

 0.10). Fish captured in early February were ap- 

 proximately 9 months old and had very small 

 marginal increments (0.1-0.2 mm), indicating re- 

 cent ring deposition. Analysis of juveniles taken 

 later in the spring and summer showed that mar- 

 ginal increments increased in width (Fig. 3) to as 

 much as 1.4 mm until December when the next 

 ring formed. This analysis suggests ring deposi- 

 tion occurs during the months of December- 

 January and, in the juvenile blacktip shark, is an 

 annual event. 



Early Growth Rates 



The relatively large numbers of juvenile and 

 subadult blacktip sharks examined in this study 

 allowed the estimation of early gi'owth rates. A 

 length-frequency distribution of sharks < 120 cm 

 TL captured during the months of May-August 

 suggests three separate size classes at approxi- 

 mately 68, 93, and 111 cm (Fig. 4). A length- 

 month distribution (Fig. 5) indicated three dis- 

 tinct size classes for juvenile blacktip sharks 

 <120 cm, and these fish appeared to represent 

 three separate cohorts. Rapid growth of the 

 young sharks produced distinct separations in 

 length and weight between these age classes 

 (Table 1) and each of the sharks examined from 

 these three age classes had one, two, or three 

 translucent rings, respectively. This further 

 supports the annual nature of vertebral ring de- 

 position in juvenile blacktip sharks. 



Growth of neonatal sharks was found to be 

 rapid. Mean size of free-swimming juveniles cap- 

 tured during June was 60.5 cm (SD = 3.9, A' = 



847 



