244 



Fishery Bulletin 98(2) 



Nursery ground closures and size limits 



Results of this scenario show that the protection 

 of neonates and juveniles from all fishing mortality 

 slowed the decline (r=-0.058) but could not stabilize 

 the population. When projected forward 20 years, 

 the population abundance is 49% of the initial abun- 

 dance (Table 1). In order to stabilize this model, F 

 had to be reduced on subadults and adults to 0.109. 

 Thus, this closure provides a 54% increase in the F 



-0.10 



-0.20 



required to maintain a sustainable population over 

 that required in the absence of nursery closures and 

 a 12% increase in F over the extreme option mod- 

 eled above (Table 4). Protecting neonates and juve- 

 niles through nursery ground closures or size limits 

 would require a 46% reduction in F over those levels 

 currently estimated to be operating in the fishery. 



Juveniles have the highest proportion of individu- 

 als in the stable stage distribution ( Table 2 ). Pregnant 

 adults and resting adults have the highest reproduc- 

 tive values (Table 3). Again, the 

 model shows the highest sensitivi- 

 ties to the juvenile and subadults 

 stages (Fig. 6). 



Protecting either juveniles or sub- 

 adults alone still leads to a declin- 

 ing population. When F2=0, after 

 20 years the population is 38% that 

 of the initial population (Table 1). 

 When F3=0, the population at 20 

 years is 50% that of the initial 

 population (Table 1). Further runs 

 indicated that the population is sta- 



Instantaneous rate of fishing mortality 



Figure 3 



The relation between the intrinsic rate of increase (r) and fishing mortality IF). 

 F,.r,i,cai '^ reached at 0.071. If F is less than F^r,r,ra/' ^^^ population will increase. If 

 F is greater than f, .„(„.„/. the population will decrease. 



0.03 



Fishing mortality 



Figure 4 



Isoclines showing the intrinsic rate of increase (r) at different rates of fishing iF) 

 and natural mortality (A/l. The population will increase if r is greater than zero. 

 The population will decrease if r is less than zero. 



bilizedifF. 



3,4,5 



0.101 (whenF2=0) 



or if F245 = 0.120 (when 



^3=0). 



Quota reductions of 50% and 40%, 

 respectively, are required to achieve 

 these critical levels of F. In both 

 cases the stable stage distribution is 

 achieved within 24 years. The stable 

 stage distribution proportions and 

 reproductive values of each stage 

 are listed in Tables 2 and 3. Figure 

 7 shows the sensitivity of the model 

 to fecundity, growth, and stage res- 

 idence when F.2 - 0. These sensitivi- 

 ties were approximately the same 

 when F3 = 0. As in the other models, 

 juveniles and subadults have the 

 highest sensitivity. 



Discussion 



The model projects that the sand- 

 bar shark population is unlikely to 

 increase unless F is reduced below 

 F CRITIC M.- '^^^ value calculated 

 here is less than the F critical value 

 of 0.10 that Sminkey and Musick 

 (1996) predicted by using a life 

 table. Both Sminkey and Musick's 

 (1996) and Cortes's (1999) results 

 and those presented here indicate 

 that current estimates of F are 



