170 



Fishery Bulletin 101(1) 



intervals for the parameters i?y, G, and r. Finite 

 rates of population increase or decrease (e'^) and 

 approximate percentile confidence intervals were 

 then calculated based on the r value output. All 

 simulations were run by using Microsoft Excel 

 spreadsheet software. 



Results 



The three methods used to estimate natural mor- 

 tality yielded annual survivorship (S) values rang- 

 ing from 0.657 to 0.904 (Table 2). The method of 

 Peterson and Wroblewski produced S values of 

 0.760 (age 01 to 0.904 (age 22), slightly higher than 

 the values produced by the method of Chen and 

 Watanabe (0.657 to 0.874, ages 0-11; 0.890, ages 

 12-h). The method of Jensen produced the lowest S 

 value (0.851 ) for ages 7 and above, and the highest 

 survivorship in the first 4 years of life. 



The catch-at-age histogram indicated that few 

 silky sharks over the age of 7 years were caught 

 in the southeastern pelagic longline fishery (Fig. 

 I). The catch curve estimated a Z value of 0.329 (7-^'=0.838, 

 Fig. 2). The Z-at-age values calculated from the modified 

 catch curve method predicted values of Z = for ages 12 

 and above (scenario 2; Table 3). Values of F-at-age used in 

 scenarios 3-8 (Table 3) were as high as 0.287 (scenario 4, 

 age 0). 



The results of the base scenario (Table 4) indicated that 

 the silky shark population, in absence of fishing mortal- 

 ity, would increase at a median rate of 4.9 'vi/yr. The ap- 

 proximate percentile lower confidence limit also showed 

 positive population growth (e''=1.027). Incorporating the 

 Z obtained from the catch curve into the model (scenario 1 ) 

 resulted in the greatest rate of population decline of any 

 of the scenarios considered (e'^=0.895). However, the use of 

 the Z-at-age values from the modified catch curve method 

 (scenario 2) produced a positive population increase of 

 1.8 Vf/yr 



With estimated F for large coastal sharks in 1997 ap- 

 plied to all age groups (scenario 3), the population would 

 decline at a rate of 4.4 '7t/yr. Application of exponentially 

 decreasing fishing mortality resulted in a more optimistic 

 median e^ value when the mean F was confined to ages 

 0-12 ( scenario 4 ), but when mean F was applied to all ages 

 (scenario 5), the output was more pessimistic than that 

 for the constant F = 0.093 model. With F estimated for 

 maximum sustainable yield (scenario 6), the silky shark 

 population would slowly increase at a rate of 1.5 '/c/yr. 

 Incorporation of variable F-at-age in scenarios 7 and 8 re- 

 sulted in e"" values slightly higher and lower, respectively, 

 than the constant F scenario. 



Discussion 



6 8 10 12 

 Age class (years) 



14 16 18 20 



Figure 1 



Estimated silky shark [Carcharhinus falciformis^ catch-at-age from 

 pelagic longlines off the southeastern U.S. coast, 1992-98. 



The base .scenario results given in our study match well 

 those from other demographic models for silky sharks. 

 Using a demographic model that incorporated density- 



dependence. Smith et al. ( 1998) found an r value of 0.043 

 for silky sharks. Cortes (2002) found an average r value 

 of 0.055 for silky sharks from the southern Gulf of Mexico 



