NOTE Beerkircher et al : A Monte Carlo demographic analysis of Carcharhinus falaformis 



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using density-independent matrix simulation models. 

 These r values place the silky shark toward the middle 

 of the range of productivity values calculated for a mix of 

 species (Smith et al., 1998; Cortes, 2002). 



The pessimistic results of the model that incorporated 

 Z calculated from the catch curve are consistent with the 

 stock assessments that indicate large coastal sharks are 

 overfished and suggest, in particular, that exploitation 

 rates in the pelagic longline fishery may not be sustain- 

 able for silky sharks. However, incorporating gear selec- 

 tivity with the modified catch curve method altered the 

 results, yielding one of the highest rates of population 

 growth of any of the scenarios. If the assumptions about 

 the relationship between the catch curve and selectivity 

 are reasonable, the results indicate that incorporation of 

 gear selectivity into demographic analysis is important. 



The importance of considering selectivity as explored by 

 the fishing mortality scenarios was less clear. Scenario 3 

 indicated that a recently estimated level of fishing mortal- 

 ity would produce declines in the population, whereas sce- 

 nario 6 indicated that the fishing mortality level produc- 

 ing maximum sustainable yield would result in a slightly 

 increasing population. When exponentially decreasing 

 F-at-age was incorporated (scenarios 4, 5, 7, and 8). the re- 

 sults differed — but much less so than those of scenarios 1 

 and 2 — from those obtained in the constant F scenarios (3 

 and 6). All the fishing mortality scenarios (3-8) used mean 

 values of F estimated through density-dependent model- 

 ing and were used here only as comparative base values. 



Stock assessments of northwestern Atlantic shark 

 stocks are based on species groups rather than individual 

 species. These assessments use data from fisheries of vari- 

 ous gear types, which may not correctly represent the life 

 history or the pattern of exploitation of an individual spe- 

 cies. In that respect, Carcharhinus falciformis may be an 



excellent example. Off the coast of the southeastern Unit- 

 ed States, pelagic longlines may be the dominant cause 

 of silky shark fishing mortality, whereas bottom longlines 

 rarely capture, and drift gillnets do not capture, this spe- 

 cies (Parrack et al., 1993; Trent et al., 1997). Examination 

 of length frequencies from the pelagic longline fishery 

 obsei-ver database suggests that age 8-1- silky sharks are 

 rarely caught. Therefore, the assumption of constant fish- 

 ing mortality throughout all age groups for this species 

 is probably invalid, at least for the pelagic fishery off the 

 southeastern United States. If individual species manage- 

 ment of silky sharks is ever to become a reality, the pat- 

 terns of selectivity of longline gear are very important. 



It seems logical that the larger the shark, the higher the 

 probability it can bite through the monofilament gangions 

 used by U.S. pelagic longliners and escape. Because size 

 is correlated with age, there must be a functional rela- 

 tionship that describes the probability of retention and 

 capture for a given age. Accurate elucidation of such gear 

 selectivity patterns requires knowledge of the actual size- 

 at-age characteristics of the population that can only be 

 derived from sampling studies in which methods that are 

 not size-selective are used, or that are at least less size- 

 selective than longline gear. These types of comparative 

 data are almost wholly lacking for sharks in the pelagic 

 environment. Indirect methods of estimating selectivity 

 for shark species by comparing catches from gillnets of 

 various mesh sizes have shown promise (Kirkwood and 

 Walker, 1986; Simpfendorfer and Unsworth, 1998). Previ- 

 ous indirect studies of selectivity in hook and line gear 

 (Cortez-Zaragoza et al., 1989; Ralston, 1990) cannot be 

 applied in this case because selectivity in those studies 

 was described as a function of hook size rather than the 

 actual breaking strength of the line, which is the specu- 

 lated mechanism of selection in the present study. Walker 



