Abstract. The Atlantic 



sharpnose shark, Rhizoprionodon 

 terraenovae, is a small coastal spe- 

 cies caught in recreational fisher- 

 ies and as bycatch in the shrimp 

 trawl and longline fisheries in the 

 Gulf of Mexico. Demographic 

 analyses incorporating the best 

 available information on validated 

 age and growth, age at maturity 

 (t mat ), maximum age (t max ), repro- 

 ductive habits, and age-specific 

 natural mortality and fecundity 

 were performed. An initial set of 

 three life history tables based on 

 input parameters t ma =4, t max =10, 

 constant age 1+ survivorship 

 (S=0.657), and varying first year 

 survivorship (S o =0.432, scenario 1; 

 S o =0.512, scenario 2; S o =0.657, sce- 

 nario 3 or best case scenario) 

 yielded net reproductive rates (i? ) 

 ranging from 0.844 to 1.284, a gen- 

 eration length (G) of 5.8 years, and 

 instantaneous rates of population 

 change (r) ranging from -0.029 to 

 0.044. Further simulations were 

 performed to test the sensitivity of 

 the computed demographic param- 

 eter values to modifications in vari- 

 ous input biological parameter val- 

 ues (scenarios 4 through 14). Over- 

 all, manipulations of biological pa- 

 rameters m x , t maV and t max caused 

 large variations in demographic 

 parameters r, < x2 , and R , while G 

 remained relatively stable. All the 

 demographic parameters proved 

 more sensitive to changes in S than 

 to changes in S . The initial set of 

 analyses (scenarios 1 through 3) 

 was then rerun with the estimated 

 mean fishing mortality from 1986 

 to 1989 (F=0.428) added to natu- 

 ral mortality. Age 6+ sharks can 

 enter the fishery under the best 

 case scenario only to allow the 

 population to replace itself. Ages at 

 first capture (A rep ) with F=0.428 

 that would allow full population 

 replacement were also calculated 

 for scenarios 4 through 14. This 

 study indicates that management 

 of R. terraenovae under the Federal 

 Management Plan (FMP) for 

 sharks of the Atlantic Ocean is 

 based on unrealistic biological 

 characteristics for this species. 



Demographic analysis of the 

 Atlantic sharpnose shark, 

 Rhizoprionodon terraenovae, 

 in the Gulf of Mexico 



Enric Cortes 



Center for Shark Research 



Mote Marine Laboratory 



1600 Thompson Parkway, Sarasota. Florida 34236 



Manuscript accepted 31 May 1994. 

 Fishery Bulletin 93:57-66 (1995). 



The Atlantic sharpnose shark, Rhi- 

 zoprionodon terraenovae, is an 

 abundant coastal carcharhinid spe- 

 cies found in shelf waters of the 

 western North Atlantic and Gulf of 

 Mexico (Compagno, 1984), reported 

 to reach a maximum size of approxi- 

 mately 110 cm total length (Com- 

 pagno, 1984 1. Although not targeted 

 by any U.S. commercial fisheries, in 

 the Gulf of Mexico it is caught in 

 recreational fisheries and discarded 

 as bycatch in the shrimp trawl fish- 

 ery (NMFS, 1993) and shark and 

 reef fish longline fisheries (person, 

 obs. ). However, age at first entry in 

 the various fisheries is unknown. R. 

 terraenovae is grouped under the 

 "small coastal" species category in the 

 Federal Management Plan (FMP) for 

 sharks of the Atlantic Ocean, which 

 determined that this species group 

 was not overfished, based on a stock 

 assessment resulting in an estimate 

 of finite rate of population increase 

 (e r ) of 1.91. Thus, no quotas or size 

 limits exist for this species despite its 

 importance in several fisheries. 



Biological and life history charac- 

 teristics of R. terraenovae in the 

 Gulf of Mexico are now well docu- 

 mented (Parsons, 1983, 1985; 

 Branstetter, 1986, 1987). However, 

 this information has not yet been 

 applied to analyses of the popula- 

 tion dynamics of this species, nor 

 have the results of such analyses 

 been published. Furthermore, as is 

 the case with most shark species, 



sufficient information necessary for 

 stock assessment is lacking (Hoff, 

 1990). Because long-term records of 

 catch and effort or the age composi- 

 tion by species are not available, 

 traditional surplus production mod- 

 els or more elaborate age-structured 

 methods of stock assessment have 

 seldom been used for sharks. Ow- 

 ing to the paucity of fisheries data, 

 several investigators have used de- 

 mographic analysis to gain insight 

 into the population dynamics and 

 exploitation rates of shark re- 

 sources. This type of analysis has 

 been utilized to construct life his- 

 tory tables or Leslie matrices 

 (Caughley, 1977; Krebs, 1985), 

 which are summaries of age-specific 

 mortality and fertility rates operat- 

 ing on a population with the as- 

 sumption of a stable age distribu- 

 tion. This technique allows estima- 

 tion of parameters important to the 

 dynamics of any given population. 

 Thus, Hoenig and Gruber (1990), 

 Cailliet (1992), and Cailliet et al. 

 ( 1992) produced estimates of popula- 

 tion dynamics by applying a demo- 

 graphic analysis of the lemon shark, 

 Negaprion brevirostris, the leopard 

 shark, Triakis semifasciata, and the 

 angel shark, Squat ina californica, 

 respectively. Hoff ( 1990), in addition, 

 estimated maximum sustainable 

 yield for the sandbar shark, Car- 

 charhinus plumbeus, in a modified 

 stock production model incorporat- 

 ing life history information. 



57 



