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Fishery Bulletin 98(2) 



nursery grounds to pup, and 

 any juveniles who may have 

 returned for the summer. One 

 of the scenarios we ran is 

 extreme in that every neo- 

 nate survives (Z=0) and preg- 

 nant adults are not fished for 

 the entire time they are preg- 

 nant. But in this scenario, 

 juveniles are not protected. If 

 F is not reduced on juveniles, 

 subadults, and resting adults, 

 the model shows that the pop- 

 ulation will decrease until F 

 is reduced to 0.097. This is 

 higher than the F = 0.07 which 

 would be needed to stabilize 

 the population without any 

 protection for neonates, and 

 would almost be met by the 

 50'^ reduction in quota as sug- 

 gested by the 1996 SEW. How- 

 ever, these scenarios assume 



complete protection of protected stages from either 

 fishing or natural mortality. Thus, they probably 

 over-estimate the effectiveness of the potential man- 

 agement action. Overall, these models indicate that 

 nursery ground closures or size limits that protect 

 only neonates and juveniles, or neonates and preg- 

 nant adults, are not likely to be the ultimate solu- 

 tion. Additional measures will need to be taken to 

 protect the sandbar shark. 



Subsequent runs of the model showed that size 

 limits that protect juvenile and subadult stages will 

 not act to rebuild the population alone, despite the 

 fact that the model indicates these stages are the 

 most sensitive to survival. In these cases, F needs to 

 be reduced to 0.10 or 0.12, respectively. If the current 

 F estimate of 0.20 is correct and if a 50% reduction in 

 quota reduces F by 50%, size limits to protect either 

 stage and a reduction in quota of between 40% and 

 50% may be sufficient to stabilize the population. 



All scenarios indicate that the sandbar shark stock 

 will most likely be rebuilt through a combination 

 of management strategies. With nursery closures 

 or size limits that protect only one stage, the stock 

 will decline if fishing mortality remains the same as 

 that currently estimated. Because the model's esti- 

 mates of population growth are sensitive to survival 

 at the juvenile and subadult stages, because these 

 stages have the highest proportion of the population 

 in the stable stage distribution, and because sub- 

 adults have relatively high reproductive values, ide- 

 ally any management strategies selected should be 

 those that conserve these stages. , 



Neonate 



Juvenile 



Subadult Pregnant adult Resting adult 



Fecundity (/)  Growth from stage ( G, ) D Stage residence (P,l 



Figure 6 



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

 residence if neonates and juveniles are protected iF, .,=0; F., ^ -=0.20). 



Most of the model projections indicate that the 

 total sensitivities of juveniles and subadults are the 

 greatest. The sensitivity of population growth to 

 events during these stages suggests two things: man- 

 agement needs to focus on protecting juveniles and 

 subadults, and scientists need to collect accurate esti- 

 mates of F and M for juveniles and subadults. Possi- 

 ble conservation efforts could include minimum sizes 

 to protect immature sandbar sharks or time-area clo- 

 sures to protect both juveniles and subadults during 

 their migrations. If our model is correct, it is impor- 

 tant to take measures to protect these stages soon, 

 not only because the model shows that the population 

 abundance decreases quickly at current estimates of 

 F, but also because there is evidence of strong year 

 classes of immature sandbar sharks entering the 

 fishery ( Branstetter and Burgess^). In 1994, Sminkey 

 suggested that the 1987, 1989, and 1992 year classes 

 in Chesapeake Bay were exceptionally strong. It will 

 be easier for the fishery to recover if we have strong 

 year classes on which to build. 



This is not the first time the use of a stage-based 

 model has concluded that conservation efforts should 

 target juveniles and subadults of a long-lived species 

 more than newborns. Rates of population growth in 

 many marine species are effected more by changes 

 in survival of juvenile and subadult stages than by 

 changes in survival of other stages, or by changes in 

 fecundity (Heppell et al., 1999). For example, Grouse 

 et al. (1987) and Crowder et al. (1994) concluded 

 that population growth rate was most sensitive to 

 the survival of large juvenile loggerhead sea turtles. 



