NOTE Beerkircher et aL: A Monte Carlo demographic analysis of Carcharhinus falciformis 



173 



because of the lack of validated age and growth informa- 

 tion and the uncertainty in the estimates of longevity and 

 fecundity for the silky shark. It is difficult to determine 

 how representative the von Bertalanffy parameters (Bon- 

 fil et al, 1993), which were derived from silky sharks in 

 the Campeche Bank in the southern Gulf of Mexico, are 

 for silky shark populations from the entire southeastern 

 U.S. Atlantic. Although tagging data (Kohler et al, 1998) 

 indicate movement of silky sharks between the Gulf of 

 Mexico and the U.S. Atlantic coast, the small number of 

 tagged and recaptured individuals makes firm conclusions 

 about whether these are the same or separate stocks very 

 difficult. The age and growth results of Bonfil et al. ( 1993) 

 from the southern Gulf of Mexico differed from a study on 

 silky sharks by Branstetter ( 1987 ) in the northern Gulf It 

 is possible, however, that these variations may be a result 

 of methodological or sampling differences because Brans- 

 tetter's (1987) data came from pelagic longline operations 

 and had fewer specimens over 250 cm TL, compared to Bon- 

 fil et al.'s (1993) data which came mainly from specimens 

 caught in gillnet fisheries. Even if migration of silky sharks 

 occurs between the northern Gulf of Mexico, southern Gulf 

 of Mexico, and southeastern U.S. Atlantic coastal region, 

 the populations of sharks in these areas might have signifi- 

 cant life history differences. Given the absence of age and 

 growth studies from Atlantic silky sharks, the Gulf of Mex- 

 ico studies must be regarded as the best available age and 

 growth information for this species. Similarly, fecundity 

 and longevity estimates are relatively uncertain. Especially 

 important in fecundity estimates is information regarding 

 age-specific birth rate and the possibility of reproductive 

 senescence, which can affect demographic results. 



As with other demographic analyses, theoretical esti- 

 mates of natural mortality (M) were used in our study. It 

 has been shown that the output of demographic models of 

 shark populations is particularly sensitive to changes in 

 M, especially during the first few years of life (Sminkey 



and Musick, 1996; Liu and Chen, 1999; Cortes, 2002). 

 Moreover, results may vary considerably in analyses that 

 use different methods of M estimation (Simpfendorfer, 

 1999). In recognition of these limitations, three different 

 methods were used here to estimate M. The methods of 

 Chen and Watanabe (1989) and Jensen (1996) are based 

 on a relationship between M and growth rates. Because 

 the age and growth information for silky sharks is un- 

 validated, the von Bertalanffy growth parameters used in 

 our study may not adequately describe individuals from 

 the population analyzed. Finally, the method of Peterson 

 and Wroblewski (1984) involves the assumption that M 

 is due primarily to predation. Although this may be true 

 for the younger ages of large coastal sharks like the silky 

 shark, it may not be true for the larger, older sharks. Ad- 

 ditionally, wet weight was used instead of dry weight, as 

 prescribed in the original equation of Peterson and Wro- 

 blewski (1984), because the estimates of M obtained by 

 using wet weight seem more believable for sharks (Cortes, 

 2002). Despite the difficulties associated with all three 

 methods of estimating natural mortality, these methods, 

 together with catch curves and tag-recapture studies, are 

 the only methods available for estimating M. The use of 

 Monte Carlo simulation to randomly vary the estimates of 

 natural mortality can account for some of the uncertainty 

 associated with these methods (Cortes, 1999; 2002). 



Conclusions from this analysis of the silky shark popu- 

 lation off the southeastern U.S. coast are similar to those 

 for other large shark species. Even under scenarios as- 

 suming natural mortality only, the population would grow 

 at a fairly slow rate, and even moderate levels of fishing 

 mortality might produce population declines. Inclusion 

 of age-dependent patterns of fishing mortality into the 

 analyses, however, appear to substantially affect model 

 results. Several key elements that need to be clarified to 

 obtain a more conclusive analysis for silky sharks include 

 validated age and growth information, life history param- 



