172 



Fisher/ Bulletin 101(1) 



(1992) incorporated hook selectivity into an age-struc- 

 tured model, but he assumed that all recruited members 

 of a stock had an equal value of catchability with longlinc 

 gear. We believe that an assumption of equal catchability 

 is incorrect for silky sharks, and sharks in general, hooked 

 on monofilament pelagic longline gear. 



Determining actual patterns of selectivity and result- 

 ing estimates of fishing mortality-at-age, however, was 

 beyond the scope of our analysis, which was to suggest 

 that patterns of gear selectivity can affect results from 

 demographic models. We chose a simple modification of 

 a traditional catch curve and an intuitive, although arbi- 

 trary, assumption of selectivity patterns for silky sharks on 

 pelagic longlines to explore this effect. Arbitrarily derived 



selectivity schedules were employed previously to examine 

 the potential effects of gear selection in yield-per-recruit 

 models (Goodyear, 1996). Clearly this approach, as ap- 

 plied here, has limitations. For example, the curve fitted to 

 catch-at-age data used to calculate Z values estimated zero 

 total mortality at ages 12-)-, whereas the catch-at-age data 

 showed a few captures of ages 12-i- (Fig. 1). No mortality 

 is assumed for individual animals that escape the gear, 

 although it is possible that some animals may die. Despite 

 shortcomings in the methods to estimate F-at-age values 

 used in our study, better estimates of fishing mortality will 

 require more robust, comparative selectivity data. 



Like many other studies of shark populations, the re- 

 sults reported in our study must be considered preliminary 



