Simpfendorfer; Growth rates of Carcharhinus obscurus 



817 



16 



12 - 



D" 





■^ CM 



Length difference (cm) 



Figure 5 



Differences between release and recapture lengths for 38 juvenile Carcharhinus obscu- 

 rus at liberty less than 30 days off southwestern Australia. 



growth data from tagged yellowfin tuna (Thunnus alba- 

 cares) up to 100 cm long. Because the current study used 

 only data from individuals up to five years old, which is 

 a short period in relation to the estimated longevity of 

 this species (45 years, Natanson et al., 1995), there was no 

 information in the data on the rate at which the length 

 reaches its asymptote (equivalent to the rate of decrease 

 in the growth rate with increasing age ). As a result the von 

 Bertalanffy growth function cannot adequately describe 

 the data. If recapture data were available for individuals 

 from older age classes, it is likely that a decrease in growth 

 rate with increasing age may have been obsei-ved and thus 

 a von Bertalanffy growth function (or the Francis (1988) 

 model with g„*gJ may have provided the best fit to the 

 data. 



The Gulland and Holt ( 1959) method relies on a decrease 

 in gi-owth rates with increasing age to estimate both L^,, 

 (:c-intercept) and K (slope). Thus, because the tag-recap- 

 ture data for C. obscurus did not contain this informa- 

 tion, the method was not able to estimate von Bertalanffy 

 growth parameters. The inability to produce results, while 

 other methods did, illustrates that the Gulland and Holt 

 (1959) method fails in some situations where the periods 

 at liberty are short in relation to the maximum age and 

 where the recaptured individuals cover only a small pro- 

 portion of the age classes present in the population. The 

 lack of information in the data on the decrease in growth 

 rate with increasing age also caused problems for the esti- 

 mation of parameters in the Fabens (1965) method. In 

 all cases, except for the noninjected group, this method 

 provided substantially lower values of L . than expected 

 from observations of the maximum length of C. obscurus 



in Western Australian waters (280 cm FL, Simpfendorfer 

 et al, unpubl. data') and from previous growth studies 

 (Natanson et al., 1995; Natanson and Kohler. 1996). Sim- 

 ilarly, values of K were substantially higher than litera- 

 ture values (0.034 to 0.062, Natanson et al., 1995; C.047, 

 Natanson and Kohler, 1996) for each of the groups except 

 the noninjected animals. The von Bertalanffy growth para- 

 meters estimated for the noninjected group by the Fabens 

 (1965) method were similar to those estimated by Natan- 

 son et al. (1995) and Natanson and Kohler (1996). 



1 Simpfendorfer, C. A.. R. McAuley, J. Chidlow, R. Lenanton. and 

 N. Hall. 1999. Biology and stock assessment of Western Aus- 

 tralia's commercially important shark species. Unpubl. report. 

 Western Australian Marine Research Laboratories, PO Box 20, 

 North Beach, Western Australia 6020. Australia. 



