342 



Fishery Bulletin 94(2], 1996 



Materials and methods 



Based on size at maturity (Springer, 1960; Sminkey, 

 1994) and von Bertalanffy growth equations for sand- 

 bar sharks determined by Sminkey and Musick 

 (1995), age at 100% maturity was determined to be 

 15 years. Casey and Natanson ( 1992 ) determined age 

 at maturity to be ca. 29 years on the basis of a von 

 Bertalanffy growth equation derived from tag and re- 

 capture information and a similar size at maturity. For 

 this demographic analysis, 15 and 29 years therefore 

 were used in separate trials as conservative estimates 

 of the age at which 100^ of females were mature. 



The age-specific natality was determined from a 

 reexamination of the data from 50 pregnant sand- 

 bar sharks collected from 1974 to 1986 (Colvocoresses 

 and Musick 2 ) and from 3 additional females collected 

 during 1990-92. Results similar to those reported 

 by Colvocoresses and Musick 2 were obtained. The re- 

 lationship between maternal size and number of pups 

 was very weak (Fig. 1; /- 2 =0.25), with the average 

 number of pups per litter equal to 8.4 (ra=53, SD=2.3). 

 Sandbar sharks produce a litter once every 2 years 

 (Branstetter 3 ; Musick, unpubl. data) with a sex ra- 

 tio not significantly different from 1:1. Thus, age- 

 specific natality was fixed at 2. 1 female pups per year 

 beginning with age of maturity. 



:! Branstetter. S. 1994. Gulf and South Atlantic Fisheries De- 

 velopment Foundation. Tampa, FL 3.3609. Personal commun. 



The probable maximum age for sandbar sharks 

 differs between the von Bertalanffy growth equation 

 derived from vertebral data (Sminkey and Musick, 

 1995) and the growth equation from tag and recap- 

 ture data (Casey and Natanson, 1992). The latter 

 study reported tagged sandbars estimated to be 22 

 (155 cm fork length, FL), 32 (157 cm FL), and over 

 40 years old ( 185 cm FL) at recapture; the 22-year- 

 old was determined to be immature. The first 2 of 

 these ages were estimated on the basis of length at 

 release, a vertebrally derived growth equation (sexes 

 combined) of Casey and Natanson ( 1992), and on time 

 at liberty. Regarding the 40+ year old shark, Casey 

 and Natanson (1992) stated "since length at first 

 maturity is 150 cm FL, it is reasonable to assume 

 that this individual was over 30 yr old at tagging 

 and over 40 yr at recapture." Casey and Natanson 

 ( 1992) then suggested that sandbar sharks may live 

 in excess of 50 years. Sminkey and Musick (1995) 

 reported that the oldest individual examined (175 

 cm FL) was 24 years old (determined by vertebral 

 counts). On the basis of vertebral data of Sminkey 

 and Musick ( 1995), it seems reasonable to consider 

 the maximum age for sandbar sharks to be about 30 

 years. For the life history tables, 30 and 60 years 

 were considered as maximum ages in separate trials. 



To examine the effects of fishing mortality (F) on 

 the demography of the sandbar shark, the survivor- 

 ship function was modified in several trials to in- 

 clude fishing mortality. Values of F included 0.25, 



