248 



Fishery Bulletin 98(2) 



icant or as observable as that for teleost fish. Sminkey 

 and Musick ( 1995) have suggested three mechanisms 

 of compensation in sharks: decreases in natural mor- 

 tality of younger sharks as the abundance of preda- 

 tory larger sharks is reduced; compensatory increases 

 in fecundity when food is more available or when uter- 

 ine mortality is reduced; and an increase in growth 

 rate and thus a decrease in natural mortality and pos- 

 sibly earlier maturity when food is abundant. In Ches- 

 apeake Bay, they found evidence of a slight increase 

 in growth rate of juvenile sharks after the popula- 

 tion had been depleted but were not able to ascertain 

 if the age of maturity had also been reduced. Late 

 age at maturity due to relatively slow growth rates 

 reduces the probability that small increases in growth 

 or increased neonate survival through density-depen- 

 dent mechanisms will compensate for fishing mortal- 

 ity (Heppell et al., 1999). 



In summary, the results when F = 0.20 for older 

 stages indicate that sandbar shark stocks are cur- 

 rently being fished above their ability to replace 

 themselves (i.e. r is negative for the best estimate 

 of F). Thus, management action (e.g. time area clo- 

 sures, reduced quota, minimum size) is needed to 

 reduce F to the level where r is zero or positive. 

 Because the model is highly sensitive to juvenile and 

 subadult survival, management actions that reduce 

 the mortality rates of these stages would likely be 

 more effective than nursery closures that protect 

 only neonates and pregnant females. 



Although our study suggests that the protection 

 of juvenile and subadult sandbar sharks may aid 

 in recovery of sandbar sharks, our method may not 

 work as well on other shark species, because life 

 history traits differ. Sandbar sharks are often con- 

 fused with other shark species such as the dusky 

 shark; therefore, whichever management strategy 

 is chosen, it should work for all large coastal shark 

 species. These problems, combined with a paucity of 

 data on pupping grounds, age at maturity, and other 

 traits, make selection of a conservation method dif- 

 ficult. The model in our study should be viewed as a 

 starting point for looking at the effect of the different 

 options available and for comparing these options 

 among the shark species involved. 



Acknowledgments 



We thank Selina Heppell, Pamela Mace, Michael 

 Fogarty, John Musick, Thomas Sminkey, Ed Houde, 

 and Steve Branstetter for comments on an earlier 

 draft of this manuscript. Support for this work comes 

 from the Chesapeake Bay Program USEPA (grant 

 CB-993080-02), the Hudson River Foundation (grant 



008/98A), and the University of Maryland Center for 

 Environmental Sciences. 



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