816 



Fishery Bulletin 98(4) 



As a comparison to estimates of w and s from the Fran- 

 cis (1988) method, differences in the size at release and 

 recapture for animals at Hberty less than 30 days were 

 determined. If it is assumed than these animals grew a 

 negligible amount over this period, and the animals were 

 measured perfectly by research staff at release, then the 

 mean and standard deviation of the differences should 

 provide an independent estimate of in and s. The mean dif- 

 ference in size for 38 C. obscurus at liberty less than 30 

 days was 1.17 cm FL, and the standard deviation was 2.09 

 cm FL (Fig. 5). These values are similar to those estimated 

 from the model for most of the groups 



Length-at-age for neonate releases 



A total of 241 released neonates (with open umbilical scars) 

 were recaptured, including 117 males, 120 females, four 

 of unknown sex. 92 OTC-injected animals, and 149 nonin- 

 jected animals. The linear growth rate based on all neonate 

 recaptures was 9.4 cm/year. The growth rate was higher 

 for males than for females, and higher for injected ani- 

 mals than for noninjected animals (Table 4). Comparison of 



growth rates with the homogeneity-of-slopes models (Fig. 6, 

 Table 5) indicated that there were significant differences 

 between males and females, and between injected and non- 

 injected males. However, there was no significant differ- 

 ence between injected and noninjected females (Table 5). 



Discussion 



Shark growth studies have typically used the von Ber- 

 talanffy growth function to describe how sharks grow, 

 including previous work on C. obscurus (e.g. Natanson et 

 al, 1995; Natanson and Kohler, 1996). However, in the 

 current study the results indicated that a linear growth 

 curve more accurately describes the growth of juvenile C. 

 obscurus up to five years of age. Pratt and Casey (1990) 

 suggested that for some slow growing sharks (e.g. Car- 

 charhinus plumbeus) a linear growth function might pro- 

 vide a better predictive model than the von Bertalanffy 

 growth function. Although linear growth functions have 

 been rarely used in fish growth studies, Bayliff (1988) 

 reported that this type of function provided the best fit to 



