978 



Abstract.— Natural mortality was es- 

 timated for Rhizoprionodon taylori by 

 using seven indirect methods based on 

 relationships between mortality and 

 life history parameters and one direct 

 method, catch curve analysis. Esti- 

 mated values from indirect methods 

 were 0.60 to 1.65. whereas catch curves 

 produced values of 0.56 for females and 

 0.70 for males. Demographic analysis 

 was undertaken by using standard life 

 history table techniques. Life history 

 tables where each of the seven indirect 

 estimates of natural mortality for R. 

 taylori produced intrinsic rates of natu- 

 ral increase from -1.297 to 0.212 for an 

 unfished population, and only two of 

 the seven produced positive population 

 growth. The implications of these re- 

 sults are discussed in relation to the 

 accuracy of indirect methods for esti- 

 mating natural mortality for R. taylori 

 and other species of shark. The catch 

 curve method was considered the best 

 estimate of natural mortality and gave 

 an intrinsic rate of increase of 0.27 and 

 a doubling tmie of 2.55 years. The re- 

 sults of life history table analysis with 

 the estimate of natural mortality from 

 the catch curve analysis indicated that 

 a R. taylori population could sustain 

 fishing mortality up to 0.18 if applied 

 evenly over all age classes, or 0.67 if 

 age at first capture was two years. The 

 implications of this study are discussed 

 in relation to sustainability of elasmo- 

 branch stocks, particularly short-lived, 

 fast growing, early maturing species. 



Mortality estimates and demographic analysis 

 for the Australian sharpnose shark, 

 Rhizoprionodon taylori, 

 from northern Australia 



Colin A. Simpfendorfer 



Western Australian Marine Research Laboratones 



PO Box 20 



North Beach, Western Australia 



6020 Australia 



Present address: Center for Shark Research 

 Mote Marine Laboratory 

 1600 Ken Thompson Parkway 

 Sarasota, Florida 34236 



E-mail address colins 5 mole org 



Manuscript accepted 5 October 1998. 

 Fish. Bull. 97: 978-986 ( 1999). 



In a paper entitled "Problems in the 

 rational exploitation of elasmo- 

 branch populations and some sug- 

 gested solutions," Holden (1974, p. 

 137) concluded that "elasmobranch 

 stocks offer very limited opportuni- 

 ties for long-term exploitation." 

 Holden's conclusion was based on 

 the fact that elasmobranch life his- 

 tory traits, such as slow growth, 

 long lifespan, late age at maturity, 

 and low fecundity, make popula- 

 tions very susceptible to recruit- 

 ment overfishing. Holden's hypoth- 

 esis has been supported by evidence 

 from a number of shark stocks that 

 have been rapidly overfished. These 

 stocks include the spiny dogfish off 

 Scotland and Norway (Squalus 

 acanthias; Holden, 1968, 1977), the 

 soupfin shark off California (Galeo- 

 rhinus galeus; Ripley, 1946), the 

 basking shark of the Irish Sea iCeto- 

 rhiniis maximus; Parker and Stott, 

 1965), the porbeagle of the western 

 Atlantic (Lamna nasus; Casey et al., 

 1978), and the sandbar and dusky 

 sharks of the western North Atlan- 

 tic (Musick et al., 1993). The ma- 

 jority of these examples are long- 

 lived, slow growing, late maturing, 

 temperate-water species. This 

 group of commonly cited examples 

 does not, however, represent the full 

 range of elasmobranch life history 



traits and as such represents a bi- 

 ased set of data on which to base 

 conclusions about sustainability of 

 elasmobranch stocks. In particular, 

 the examples lack representatives 

 from tropical areas, especially 

 short-lived, rapidly growing, early 

 maturing species. With adequate 

 information on species with a wide 

 range of life histories, a more accu- 

 rate assessment of the ability of 

 elasmobranch stocks to sustain fish- 

 ing pressure should be possible. 



In recent years the use of demo- 

 graphic analysis (i.e. life history 

 tables) has become popular in the 

 assessment of elasmobranch popu- 

 lations and their ability to be sus- 

 tainably fished (e.g. Hoenig and 

 Gruber, 1990; Cailliet, 1992; CaiUiet 

 et al., 1992; Cortes, 1995; Cortes 

 and Parsons, 1996; Sminkey and 

 Musick, 1996; Au and Smith, 1997; 

 Cortes, 1998). This style of analysis 

 requires only knowledge of the life 

 history traits of a species. Because 

 demographic models are static rep- 

 resentations of populations with a 

 stable age structure, they have limi- 

 tations in respect to density-depen- 

 dent responses (e.g. density-depen- 

 dent natural mortality I and dynamic 

 processes (e.g. fishing and variable 

 reciTjitment) that can be included in 

 dynamic models. The latter types of 



II 



