344 



Fishery Bulletin 94(2). 1996 



Discussion 



Demographic analysis using life history tables is a 

 useful tool for fishery managers to evaluate poten- 

 tial population changes under various conditions of 

 fishing mortality (Hoenig and Gruber, 1990). In this 

 study, the demographic analyses indicate that if fish- 

 ing mortality continues to target small sandbar 

 sharks, beginning with 8-year-old sharks, at the level 

 of fishing estimated for 1986-91 (F=0.25; Anony- 

 mous 1 ), the population will decrease by >79? per year. 

 The population of sandbar sharks along the mid-At- 

 lantic coast declined to about 15% of its previous level 

 over a 13-year span (Musick et al., 1993; Musick et 

 al. 4 ), suggesting that this demographic study may 

 be an accurate estimate of potential population 

 changes as a result of excessive fishing mortality. The 

 potential population increase rates reflect an evolu- 



tionary strategy not well adapted to sudden and se- 

 vere population depletions, particularly if applied to 

 many age classes simultaneously or persisting over 

 several years. 



Natural mortality is difficult to estimate directly 

 for any fish population; therefore we chose to use the 

 longevity relationship of Hoenig (1983) to estimate 

 this parameter. Additionally, following the example 

 of Hoff (1990), we reduced the natural mortality by 

 half to examine the "best-case" population under the 

 conditions of the vertebrally derived growth model. 

 This simulation may indicate the maximum poten- 



4 Musick,J.A.,S.Branstetter,andT.R Sminkey. 1994. Trends 

 in shark abundance from 197-1 to 1993 for the Chesapeake Bight 

 region of the U.S. mid-Atlantic coast. Contrib. SB-19, Shark 

 evaluation workshop, 14-18 March 1994, Southeast Fisheries 

 Science Center, Natl. Mar. Fish. Serv., Miami. FI, 33149. 16 p. 



