KILLAM and PARSONS: AGE AND GROWTH OF THE BLACKTIP SHARK 



was more similar to otiier species of pelagic fish 

 such as the blue shark, dolphin, and tuna. 



Age-Weight 



While gi'owth in length was fit with the von 

 Bertalanffy gi-owth equation, a logistic equation 

 provided a significant fit to age-weight data. In a 

 similar manner, Parsons (1987) reported that 

 Sphynia tihuro age-weight data were best fit 

 with a logistic equation. Both the von Berta- 

 lanffy and logistic growth equation imply that 

 the increase in length and weight of C li))ibatHS 

 is asymptotic. Ricker (1979) cited contrasting 

 opinions on the feasibility of asymptotic growth 

 for fishes, and stated that usually a few older 

 individuals in a fish population may be consider- 

 ably larger than the asymptote, particularly in 

 terms of weight. Carcharliiniis li)ubatiis >10 

 years old probably gi'ow very little in length 

 each year. The results of this study suggest that 

 C. li))ibatus tends toward a W,, and appear 

 to grow very little in weight at older ages. 



Age and Growth Estimates 



The von Bertalanffy growth equation closely 

 described the gi'owth of C li)>ibati(s. Estimated 

 size at birth was approximately 53.0 cm which 

 corresponds closely to that of observed data. 

 Maximum theoretical length from the von Berta- 

 lanffy growth equation was 195.0 cm for females 

 and 166.5 cm for males, similar to the maximum 

 length of females and males collected during this 

 study, 183.3 and 165.0 cm, respectively. Maxi- 

 mum lengths reported for C. limbatus in the 

 Gulf of Mexico were 191.0 and 175.0 cm for 

 females and males, respectively (Clark and von 

 Schmidt 1965); within the 95% confidence inter- 

 vals predicted for L^. (Table 2). Branstetter 

 (1987) estimated L,. at 176.0 cm for blacktip 

 sharks captured in the northwestern Gulf of 

 Mexico although his estimates were with sexes 

 combined (34 females, 13 males). Because 

 growth curves differ between males and 

 females, this underestimates Ly, for females 

 and overestimates L^, for males. This may 

 also influence estimated ages of maturity for the 

 sexes; for example, males included in the growth 

 curve would slow the rate at which the curve 

 approaches a particular size and thus result in an 

 older estimated age at maturity for females. 



A positive linear relationship between shark 

 length and centrum radius has been established 

 in many shark species. In /. oxyrinclius (Pratt 



and Casey 1983) and G. cuvieri (Branstetter et 

 al. 1987), a curvilinear relationship may be more 

 applicable to the data. In either situation, this 

 allows back-calculation of length at time of ring 

 deposition. Back-calculated sizes were smaller at 

 each age class than sizes from observed and pre- 

 dicted data (Table 3, Figs. 6, 7). Ring deposition 

 occurs during the winter months of December- 

 January; however, 83.4% of the blacktips were 

 captured during May-September; thus the in- 

 crease in size between time of ring deposition 

 and time of capture produced the above dispar- 

 ity. This situation is less evident in older C. 

 Umbatus with decreased annual growth rates. 



Growth rates estimated for adolescent and 

 mature C. linibatus were 9-10 and 3-4 cm 

 TL/yr, respectively. These rates were similar to 

 those found by Branstetter (1987a) in the north- 

 western Gulf of Mexico, although he reported 

 that lengths at age for female and male C. Um- 

 batus were similar. This study found a signifi- 

 cantly larger total length of females at age 7 or 

 greater. Age at maturity for blacktips captured 

 in the Tampa Bay area and in the northwestern 

 Gulf of Mexico were similar for males (4-5 years) 

 but differed among females. Females reach ma- 

 turity in 6-7 years near Tampa Bay, and 7-8 

 years in the northwest Gulf of Mexico. Similarity 

 of life history parameters for C. Umbatus cap- 

 tured in the Tampa Bay area (Killam 1987), in 

 the east central Gulf of Mexico (Springer 1940; 

 Clark and von Schmidt 1965), and in the north- 

 ern Gulf (Branstetter 1981, 1987a) suggest a con- 

 tinuous population of this species in the Gulf of 

 Mexico. 



ACKNOWLEDGMENTS 



We would like to express our appreciation to 

 the many people who volunteered their time for 

 this project. First, we would like to thank the 

 commercial fishermen, B. Spaeth, J. Sage, D. 

 Lester, and D. Garrison; their help was gi-eatly 

 appreciated. Special thanks are also due to many 

 students from the USF Marine Science Depart- 

 ment, employees of Florida Department of Na- 

 tural Resources, and others who made this pro- 

 ject possible, especially L. Bingler, T. Barber, 

 D. Barber, D. Crewz, C. Dohner, B. Hoffman, 

 B. McMichael, M. Mitchell, E. Peebles, and G. 

 Tolley. We would like to thank J. Briggs, N. 

 Blake, and J. Torres for their advice and en- 

 couragement, and the Clearwater Power Squad- 

 ron for their financial contribution, which was 

 very helpful in the completion of this project. 



855 



