Cortes: Demographic analysis of Rhizopnonodon terraenovae 



61 



in S up to 50% (scenario 10) had a more distinct 

 effect on r (166% increase), t x2 (2.7 times shorter), 

 and R (50% increase), but did not affect G (Tables 2 

 and 3). Increasing age 1+ survivorship (S) by 10% 

 (scenario 11) had a similar effect on all the demo- 

 graphic parameters to increasing S o by 50% (scenario 

 10; Tables 2 and 3), whereas increasing S by 50% 

 (scenario 12) had a very profound effect on all the 

 demographic parameters, increasing r by 759%, 

 shortening t x2 by almost 9 times (similar to scenario 

 8), increasing R o by over 800% and lengthening G by 

 25% (Tables 2 and 3). 



Doubling longevity (t max ) to 20 years (scenario 13) 

 also markedly affected r (418% increase), t x2 (5 times 

 shorter), and R o (290% increase), and produced the 

 largest value of G (8.3 or a 44% increase) in all sce- 

 narios (Tables 2 and 3). 



Finally, the extreme manipulations of scenario 14 

 (reducing t mat to 3 years, doubling m x , increasing S 

 and S o to 95%, with a t max of 10 years) produced a 13- 

 fold increase in r, a value of t x2 more than 14 times 

 shorter, a 22-fold increase in R o and only a 16.3% 

 increase in G (Tables 2 and 3). 



For all simulations, population doubling time (t x2 ) 

 was lessened and generation length (G) was the de- 

 mographic parameter less sensitive to changes in 

 input biological parameter values. 



With the estimated mean fishing mortality from 

 1986 to 1989 (F=0.428) added to natural mortality 



starting at each age interval from 9 to 0, R Q and r 

 were progressively reduced as F was progressively 

 started closer to age-0 (Table 4). In scenarios 1 

 (S o =0.432) and 2 (S o =0.512), r was always negative 

 and became increasingly so as simulated fishing 

 started earlier in the life of R. terraenovae. Only by 

 using best case scenario (scenario 3) values could the 

 population be made to replace itself or grow by ma- 

 nipulating age at first capture. When fishing pres- 

 sure was applied between 6 and 5 years of age or 

 about 97 cm total length (TL) the population was able 

 to replace itself. Generation length remained the 

 same under the three scenarios but progressively 

 decreased as fishing mortality included progressively 

 earlier ages. Theoretical halving time also progres- 

 sively shortened as fishing started at younger ages, 

 whereas in the best case scenario doubling time in- 

 creased as age at first capture dropped from 9 to 6 years. 



The effect of added fishing mortality on survivor- 

 ship can be identified as a progressive decrease in 

 percentage survival as fishing starts progressively 

 earlier in the lifespan of the shark (Fig. 2). Age-spe- 

 cific reproduction also decreases significantly as fish- 

 ing mortality is applied at progressively earlier ages 

 (Fig. 3). 



When the estimated mean fishing mortality from 

 1986 to 1989 (F=0.428) was added to natural mor- 

 tality in scenarios 4 through 14 (Table 5), A , the 

 earliest age at which sharks can first be captured to 



