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Fishery Bulletin 94(2). 1996 



the Chesapeake Bay is the result of a marked de- 

 crease in the large coastal shark population. In this 

 case juvenile survival is probably less dependent 

 upon predation and may be age-independent (M=0. 10 

 for all ages). Assuming this natural mortality level, 

 the population increases at 6.4%/yr (Table 1). 



The life history parameters generated by the two 

 growth models with the best estimates for natural 

 mortality, suggest similar population increase poten- 

 tial but on two different time scales. The intrinsic 

 rates of population increase are 0.062 and 0.034 for 

 the growth models of Sminkey and Musick (1995) 

 and Casey and Natanson ( 1992), but the generation 

 times are 20.37 and 39.05 years, respectively, and 

 population doubling times are 11.2 and 20.4 years, 

 respectively. The life history tables with the verte- 

 bral growth model agree well with the results of Hoff 

 ( 1990), although he used an age-fecundity relationship. 



The effects of fishing mortality on the intrinsic rate 

 of increase with both growth models demonstrate the 

 detrimental effect of exploitation on immature fish 

 (Table 3). At fishing mortality levels >0.10, the popu- 

 lation will decline. The ages used for these estimates 

 (8 and 15) correspond to the mean carcass size in the 

 1986-91 large coastal shark fishery and are based 

 upon the most recent data available. At the currently 

 estimated fishing mortality level (0.25) these popu- 

 lations are not viable and will eventually collapse. 

 The recent analyses of Musick et al. (1993) and 

 Musick et al. 4 report that the adult coastal stock has 

 already been reduced to only 15"% of its abundance 

 in 1980-81. Clearly the sandbar shark, with a slow 

 growth and low net reproductive rate typical of most 

 elasmobranchs, cannot withstand even a low rate of 

 fishing mortality on immature individuals. 



Cailliet ( 1992) used demographic analyses to exam- 

 ine population growth in the leopard shark, Triakis 

 semifasciata, including fishing mortality to begin at 

 several ages. The population would replace itself (r=0) 

 if fishing began at age 4 at the estimated rate (F =0.084 ) 

 and would slowly increase (0.4— 6.4%/yr) if fishing did 

 not begin until later ages (5, 10, 15, 21 years). How- 

 ever, it was demonstrated that if fishing mortality 

 doubled (F=0.168), the age at which F starts must 

 be >12 to produce a growing population. Although 

 females first mature at age 17, Cailliet ( 1992) con- 

 cluded that a size limit of 110 cm TL (approximately 

 13 years) would ensure population replacement at 

 this moderately low fishing level. 



A demographic analysis of the Pacific angel shark, 

 Squatina californica, examined the effects of low fish- 

 ing mortality rates (F=0.05, 0.10, 0.20, 0.22) simu- 

 lated to begin at age 10 yr (Cailliet et al., 1992). This 

 age was the estimated age of entry into the fishery 

 as well as the approximate age of first maturity. 



Cailliet et al. (1992) concluded that the population 

 would grow very slowly (0.4%/yr) at F=0.20, would 

 decrease at higher F, and that a size limit above the 

 size of first maturity should be imposed in the absence 

 of better estimates of natural and fishing mortality. 



Demographic analyses of the Atlantic sharpnose 

 shark, Rhizoprionodon terranovae, classified as a 

 "small coastal" species in the Fishery Management 

 Plan (FMP) for sharks of the Atlantic Ocean, indi- 

 cated that, in the absence of fishing mortality, the 

 population would increase by only about 4.5%/yr 

 (Cortes, 1995). This best estimate is considerably 

 below the rate derived for "small coastal" sharks of 

 the Atlantic Ocean (e r =1.91; 91%/yr) (Parrack 5 ). Fur- 

 thermore, if fishing mortality continues at the esti- 

 mated rate for 1986-90 (F=0.43) (Anonymous 1 ), a size 

 limit of about 97 cm TL (nearly 6 years old) would 

 have to be imposed just to allow population replace- 

 ment (Cortes, 1995). Cortes ( 1995) demonstrates that 

 extreme modification of the reproductive and survival 

 parameters was required to attain the population 

 increase rate derived by Parrack 5 and concluded that 

 management of the Atlantic sharpnose under the 

 FMP was based on unrealistic biological parameters. 



This demographic analysis of sandbar sharks pro- 

 vides additional life history information on accept- 

 able levels of exploitation. If the current FMP rec- 

 ommendation of F=0.25 for MSY (Anonymous 1 ) is 

 implemented in an unrestricted fishery, the sandbar 

 shark population will not recover. This level of fish- 

 ing would be acceptable for a healthy population if a 

 minimum size limit of 135 cm precaudal length or 

 about 23 kg carcass weight (size at first maturity) 

 were imposed and juvenile survival remained high. 

 But, with the current severe depletion of the sand- 

 bar shark population of the western North Atlantic, 

 far more restrictive fishing mortality levels must be 

 implemented to allow the population to rebuild it- 

 self. Such a conservative approach would also pro- 

 vide a buffer against natural perturbations during 

 the crucial recovery phase. 



Acknowledgments 



This research formed part of a dissertation submit- 

 ted by the senior author to the School of Marine Sci- 

 ence, The College of William and Man, in partial 

 fulfillment of the requirements for the degree of Doc- 

 tor of Philosophy. We thank S. Branstetter for his 



Parrack, M. I, 1990. A preliminary study of shark exploita- 

 tion during 1986 1989inthi U.S FCZ Contrib. MIA-90-493, 

 Southeast Fisheries Science Center, Natl Mar Fish Serv., Mi- 

 ami, FL 33149, 23 p 



