Simpfendorfer; Demographic analysis of Rhizoprionodon taylori 



979 



models require abundance data (e.g. CPUE time-se- 

 ries data or fishery-independent sui-veys) that are not 

 available for many elasmobranch populations. In cases 

 where abundance data are not available, demographic 

 models may represent the best available technique for 

 the analysis of stock dynamics. 



The Australian sharpnose shark, Rhizoprionodon 

 taylori. is an ideal example of a small, short-lived, 

 fast growing, early maturing tropical species ( Simpf- 

 endorfer, 1992, 1993). It is endemic in the inner con- 

 tinental shelf waters of northern Australia between the 

 Northwest Shelf and southern Queensland (Last and 

 Stevens, 1994). It is captured throughout much of its 

 range as bycatch in commercial fishing operations, 

 including gillnet fisheries for barramundi, mackerel, 

 and shark, and prawn trawl fisheries (Simpfendorfer 

 and Milward, 1993; Last and Stevens, 1994). This 

 paper reports the results of mortality estimation and 

 demographic analysis for R. taylori. Mortality was 

 estimated in two ways. First, a number of relations 

 between mortality and life history parameters from 

 the literature were used to evaluate the appropri- 

 ateness of these methods for estimating natural 

 mortality in sharks. Second, catch curve analysis was 

 conducted to produce a direct estimate of mortality. 

 The results of the demographic analyses are used to 

 comment on the probable sustainability of short- 

 lived, fast growing, early maturing, tropical elasmo- 

 branch populations. 



Materials and methods 



Estimation of mortality 



Two approaches were taken to estimate mortality in 

 R. taylori. The first was to employ relationships be- 



tween life history parameters and natural mortality 

 (M) or total mortality (Z) from the literature. Seven 

 relationships were chosen (Table 1) to investigate 

 variability in their results. All seven were based 

 heavily on data from teleost fish, although most in- 

 cluded some data from elasmobranchs. The most 

 widely used relationship in elasmobranch studies is 

 that of Hoenig ( 1983), which uses a linear function 

 to estimate total mortality from maximum age. Al- 

 though this method estimates total mortality, it was 

 assumed to represent natural mortality for/?, taylori 

 because there was little or no fishing for this species 

 in the study area (Simpfendorfer, unpubl. data). The 

 method of Pauly (1980) uses two parameters of the 

 von Bertalanffy growth curve (L^^ and K) and aver- 

 age temperature to estimate natural mortality. 

 Jensen (1996) reanalyzed Pauly's (1980) data and 

 found that natural mortality could be estimated with 

 the same level of accuracy based only on the value of 

 K. In the same paper Jensen (1996) also gave two 

 other relationships for estimating natural mortality 

 based on life history theory, one based on K and the 

 other on age at maturity. The final two relationships 

 selected used maximum female gonadosomatic in- 

 dex (GSI) as an indicator of reproductive effort to 

 estimate natural mortality. The method of Gunderson 

 ( 1980) was based on only 10 species; Gunderson and 

 Dygert (1988) expanded this to 20 species. Data for 

 calculation of M were taken from Simpfendorfer ( 1992; 

 GSI ) and Simpfendorfer ( 1993; von Bertalanffy param- 

 eters, maximum age, and age at maturity). 



The second approach to the estimation of mortal- 

 ity was catch curve analysis (e.g. Ricker, 1975; Vetter, 

 1987). Age data for the catch curves were taken from 

 Simpfendorfer ( 1993). Ages were converted to whole 

 years and the natural log of the number of individuals 

 (In N) was plotted against age for males and females 



