Brewster-Geisz and Miller; Management of Carcharhinus plumbeus 



243 



tion (Table 2). In this scenario, 

 the proportion of neonates, 

 pregnant aduhs, and resting 

 adults in the population has 

 increased compared with the 

 proportion of stages in previous 

 scenarios. The pattern in repro- 

 ductive values remains largely 

 unchanged (Table 3). As with 

 the baseline scenario, the elas- 

 ticities show the model to be the 

 most sensitive to changes at the 

 juvenile and subadult stages. 



In summary, these model runs 

 suggest that to stabilize and 

 increase the sandbar shark pop- 

 ulation, F needs to be reduced 

 below 0.07 (Z<0. 17 ). An F of this 

 magnitude requires more than 

 the full 50'7r quota reduction to 

 be implemented. 



Protecting neonates 

 and pregnant adults: 

 an extreme example 



Results show that, even after protection of neonates 

 and pregnant females, at current levels of F the pop- 

 ulation still decreases rapidly (r=-0.079). The popu- 

 lation, projected 20 years forward, is only 38% of the 

 initial abundance (Table 1). This percentage com- 

 pares with reductions to 139f of initial abundance 

 and ;-=-0. 124 in the base run, where F^ 345 = 0.2. In 

 order to have a stable population ir-O) under this 

 scenario, we needed to decrease F to 0.097. Without 

 protecting neonates and pregnant females, F must 

 be reduced to 0.07. Thus, protecting neonates and 

 pregnant females provides a 37% increase in the F 

 required to maintain a sustainable population (Table 

 4). However, it must be emphasized that although 



Neonate Juvenile Subadult Pregnant adult Resting adult 



I Fecundity i/'i H Growth from stage iG, i D Stage residence iP, i 



Figure 2 



The proportional sensitivities (elasticity I of each stage to fecundity, growth, and stage 

 residence under current fishing conditions (F=0.20). 



this option does provide some protection, implemen- 

 tation would still require a 52% reduction in F over 

 those levels currently estimated to be operating in 

 the fishery. This reduction is in contrast to the 64% 

 reduction in F required to reach sustainable rates of 

 exploitation in the absence of this protection. 



Juveniles had the highest proportion of individuals 

 in the stable stage distribution (Table 2). Pregnant 

 females and resting adults have the highest repro- 

 ductive values (Table 3). Similar to the previous sce- 

 nario, projections show the highest overall sensitivity 

 to transitions involving the abundance of subadults 

 (Fig. 5). However, in contrast with earlier simulations, 

 projections show additional substantial sensitivity to 

 transitions into the resting adult stage (Fig. 5). 



