Simpfendorfer: Demographic analysis of Rhizopnonodon taylon 



983 



range of estimates of natural mortality for a single 

 population. Testing these values of natural mor- 

 tality for R. taylon in life history tables showed 

 that most of the methods produce values of r that 

 would result in a population decreasing even when 

 it was unfished. Such a result is biologically un- 

 reasonable. In contrast, Simpfendorfer (1999) 

 made similar comparisons of indirect estimation 

 methods for the dusky shark (Carcharhinus 

 obscurus) and found that results did not differ 

 greatly between methods. The difficulties in ap- 

 plying many of the indirect methods to R. taylori 

 may relate to its short lifespan and fast growth 

 because all but two of the methods used age or 

 growth parameters to estimate natural mortality. 

 Among the indirect methods for estimating natu- 

 ral mortality, only those of Hoenig (1983) and 

 Gunderson and Dygert (1988) yielded estimates 

 that would allow an unfished population to in- 

 crease. Although the values of natural mortality 

 from the two methods were relatively similar (0.60 

 and 0.70, respectively), they produced different 

 results when used in the life history tables to cal- 

 culate r, with the Hoenig (1983) estimate produc- 

 ing a much higher value. This difference indicates 

 that the results of demographic analysis are highly 

 sensitive to changes in natural mortality. It is un- 

 likely that the Gunderson and Dygert ( 1988 ) method 

 produced a realistic value of natural mortality be- 

 cause it is based on the assumption that GSl is an 

 index of total reproductive effort. Although this is 

 true for most fish species, it is not the case for pla- 

 cental forms (such as R. taylori) in which much of 

 the energy invested in reproduction is supplied dur- 

 ing development rather than stored in the ovary prior 

 to fertilization. Of the indirect methods used, the 

 Hoenig ( 1983 ) method is therefore likely to be the most 

 realistic estimate of natural mortality for/?, taylori. 



Fishing mortality (per year) 



Figure 4 



Contour plot of intrinsic rate of population increase (r) as a 

 function of fishing mortality (F) and age at first capture 

 (AAFC) for Rhizoprionodon taylori from northern Australia. 

 Estimates are based on a life table where natural mortality 

 was calculated by a catch curve (scenario H in Table 2). Fish- 

 ing is sustainable at values of r > 0. 



There are relatively few studies of shark popula- 

 tions in which natural mortality has been estimated 

 directly with either tagging studies (e.g. Grant et al., 

 1979; Manire and Gruber, 1993) or catch curves (e.g. 

 Cortes and Parsons, 1996). Although direct ap- 

 proaches to determining mortality would be expected 

 to produce more realistic estimates, they must also 

 meet a number of criteria in order to produce accu- 

 rate results. The assumptions for the catch curve 

 technique used in the present paper included con- 

 stant mortality, recruitment, and selectivity over all 

 age classes used in the calculations as well as ran- 

 dom collection of samples from the population. The 



