Simpfendorfer: Demographic analysis of Rhizopnonodon taylori 



981 



Table 2 



Life history table (or Rhizopnonodon taylori based on various estimates of female natural mortality (M). /?„ = net reproducdtive 

 rate; G = generation time: r = intrinsic rate of population increase; ty, = population doubling time; F, = level of fishing beyond 

 which the population can not replace itself. 



Scenario Source of Af 



A 

 B 

 C 

 D 

 E 

 F 

 G 

 H 



Hoenig(1983) 

 Pauly (1980) 

 Jensen (1996) (age) 

 Jensen (1996) (growth) 

 Jensen ( 1996) (Pauly ) 

 Gunderson (1980) 

 Gunderson and Dygert ( 1988) 

 Catch curve (female) 



M 



Rn 



0.60 

 1.34 

 1.65 

 1.52 

 1.62 

 1.49 

 0.70 

 0.56 



1..547 

 0.200 

 0.096 

 0.130 

 0.103 

 0.140 

 1.123 

 1.758 



2.233 

 1.442 

 1.301 

 1.353 

 1.312 

 1.366 

 2.063 

 2.304 



0.212 

 -0.869 

 -1.297 

 -1.119 

 -1.256 

 -1.078 

 0.057 

 0.271 



t. 



3.273 

 -0.798 

 -0.534 

 -0.619 

 -0.552 

 -0.643 

 12.102 



2.554 



0.140 

 -0.600 

 -0.910 

 -0.780 

 -0.880 

 -0.750 

 0.038 

 0.179 



ture, AAFC) the values of r at different levels of F 

 and AAFC were calculated. F after AAFC was 

 assumed to be constant across all ages. 



Results 



Mortality 



The calculation of M from the various life history 

 relations produced estimates ranging from 0.6 to 

 1.65 (Table 2). All the methods based on von 

 Bertalanffy growth parameters gave results 

 greater than one. Only the Hoenig (1983) and 

 Ciunderson and Dygert (1988) methods yielded 

 estimates less than 1. 



The length and age frequency data for R. taylori 

 on which the age analysis of Simpfendorfer ( 1993 ) 

 was based are shown in Figure 2. Estimates of M 

 from the catch curves were 0.698 for males and 

 0.561 for females (Fig. 3). The first age class (0) 

 for both males and females was excluded from the 

 regression analysis to estimate M because it was 

 lower than, and to the left of, the peak In A^ value. 

 Fitting of quadratic functions to the data did not sig- 

 nificantly improve the fit to the data points selected 

 (male: F=3.86, P=0.188; female: F=1.35, P=0.329), 

 confirming that their inclusion did not violate the 

 assumption relating to constant mortality. 



Demographic analysis 



Life history tables were constructed to obtain demo- 

 graphic results for the eight different estimates of M 

 for an unfished Rhizoprionodon taylori population 

 (Table 2). Three of the eight (A, G, and H) gave posi- 

 tive values of r: the methods of Hoenig (1983), 



10 



S 6 



Litter size = 0.019TL- 7.919 



500 550 600 650 700 750 800 



Total length (mm) 



Figure 1 



Litter size of Rhizoprionodon taylori as a function of mater- 

 nal length. Data from Simpfendorfer ( 1993). 



Gunderson and Dygert (1988) and the catch curve 

 method. The catch curve and Hoenig (1983) meth- 

 ods produced similar demographic results, with r 

 values between 0.2 and 0.3, and with doubling times 

 of 2.5 to 3.5 years (Table 2).These were considerably 

 higher than for the Gunderson and Dygert (1988) 

 method, which gave an r value of 0.057 and a dou- 

 bling time of 12.1 years. The values of r for the re- 

 maining scenarios (B-F) were all much less than zero 

 (-0.869 to -1.297; Table 2), indicating population 

 decrease even with no fishing. 



The value of M from the catch curve method was 

 used to test the sensitivity of the life history table 

 values to changes in model structure and age para- 

 meters. There was a large difference in age-specific 

 reproductive rate calculated with a proportion of the 



