Smmkey and Musick. Demographic analysis of Carcharhmus plumbeus 



345 



tial for population growth in the absence of fishing 

 mortality. The annual population increase rate of 

 nearly 12% is modest when compared with teleost 

 reproductive potential (Hoff, 1990) and probably is 

 not actually attainable in sandbar shark populations. 



The Casey and Natanson ( 1992 ) growth model as- 

 sumes the maximum age attained is 60 years, re- 

 sulting in an estimate of M = 0.07. However, if natu- 

 ral mortality was 0. 10, the life history table predicts 

 a population decrease rate of 0.1% in the absence of 

 any increased neonate mortality or fishing mortal- 

 ity (Table 2). This indicates a nonviable population 

 under natural mortality levels, particularly if neo- 

 nate and juvenile mortality were higher. Therefore, 

 if sandbar sharks are assumed to grow according to 

 this model, it seems reasonable to accept the lower 

 estimate of natural mortality. 



There is great uncertainty regarding age-specific 

 natural mortality during the first two years of life 

 when juvenile sandbar sharks are vulnerable to pre- 

 dation by large coastal sharks. The trials with in- 

 creased mortality during these years demonstrate 

 the sensitivity to natural mortality estimates in the 

 life history table. If age at maturity is 15 years and 

 mortality is >0.10, the population increase rate is 

 considerably reduced (2.1%-5.1%/yr), suggesting that 



the population may be near equilibrium (r=0.0) un- 

 der these conditions (Table 2). However, Musick et 

 al. ( 1993) and Musick et al. 4 suggest that the appar- 

 ent stable abundance of juvenile sandbar sharks in 



