238 



Fishery Bulletin 98(2) 



two particular management options, nursery ground 

 closures and size limits. We exercised the general 

 model framework to address four fundamental ques- 

 tions. What is the intrinsic rate of increase of sandbar 

 shark populations under current patterns of exploi- 

 tation? What is the sustainable level of fishing mor- 

 tahty '•P critical'''^ What is the effect of ehminating 

 fishing mortality on early stages, either through nurs- 

 ery ground closures or through the introduction of 

 size limits? For each question, we provide the results, 

 predictions, and interpretation of sensitivity analyses 

 to indicate the reliability of our conclusions. 



Materials and methods 



Life history of sandbar sharks 



The first step in developing a stage-based model is 

 to review the life history of a species to identify 

 appropriate stages. Results of tagging and age and 

 growth studies (Springer, 1960; Casey et al., 1985; 

 Casey and Natanson, 1992; Sminkey and Musick, 

 1995, 1996) indicate that sandbar sharks are a long- 

 lived species with low fecundity. These studies also 

 indicate that females, males, and juveniles segregate 

 by water depth and distance from shore. However, 

 estimates of key vital rates are inconsistent. The 

 generally accepted estimate of mortality and fecun- 

 dity schedules indicates that sandbar sharks mature 

 between 12 and 15 years of age and live to around 30 

 years of age (Casey et al., 1985; Sminkey and Musick, 

 1996 ). Another estimate suggests that sandbar sharks 

 may not mature until age 29 and may live past 50 

 (Casey and Natanson, 1992). In our model we used an 

 age at maturity of 15 years. From the biological func- 

 tion and the migration pathways determined by these 

 studies, we identified five stages in the life history 

 of the sandbar shark: neonates, juveniles, subadults, 

 pregnant adults, and resting adults (Fig. 1). 



Neonates are young-of-the-year sharks. Sandbar 

 shark neonates are bom fully developed at a fork length 

 (FL) between 43 and 52 cm (Castro, 1983; Branstetter 

 and Burgess^). They remain in this stage for one year 

 before becoming juveniles. Juveniles are the first stage 

 to show a seasonal pattern of movement. In the winter, 

 juveniles migrate to warmer waters, often to the edge 

 of the Gulf Stream off North Carolina. In the summer, 

 juveniles return to their nursery grounds. They con- 

 tinue this seasonal migration until they are between 6 

 and 10 years old (Casey et al., 1985; Branstetter and 

 BurgessM. In contrast, subadults, while still not yet 

 mature, follow the adult migration pattern. This migra- 

 tion pattern consists of swimming along the Atlantic 

 coast of the United States as far nor:th as New England 



in the summer and traveling south to warmer waters 

 in the winter (Castro, 1983). In this model, individu- 

 als remain in the subadults stage for 8 years, at which 

 point they may be 15 years of age, and then join the 

 reproductive population. 



Fifty percent of female sandbar sharks are mature 

 at about 150 cm FL (Springer, 1960; Casey et al., 

 1985; Sminkey and Musick, 1996) or 12 to 15 years of 

 age depending on the von Bertalanffy model. Female 

 sandbar sharks give birth at an average of 8 or 9 

 pups once every other year (Springer, 1960; Smin- 

 key and Musick, 1996). Larger sharks do not appear 

 to give birth to a greater number of pups (Sminkey 

 and Musick, 1996). Gestation lasts between 9 and 

 12 months (Castro, 1983). Pregnant females pup in 

 shallow bays and estuaries along the east coast of 

 North America, including Chesapeake Bay (Smin- 

 key and Musick, 1996), Delaware Bay (Pratt and 

 Merson^) and the waters off the coast of South Caro- 



•* Pratt Jr., H. L, and R. R. Merson. 1996. Report of the 1996 

 apex predators investigation: sandbar .shark nursery grounds 

 project. Apex predators investigation. Northeast Fisheries Sci- 

 ence Center, Natl. Mar., Fish. Serv., NOAA, Narragansett, HI. 



