Cortes: Demographic analysis of Rhizopnonodon terraenovae 



63 



Table 5 



Simulations of the Gulf of Mexico population of Rhizoprionodon terraenovae under several scenarios (4 through 14) using differ- 

 ent input biological parameter values and with fishing mortality (F=0.428) as in Table 4. The best case scenario (BC; top row) is 

 included in italics to facilitate comparison. All other symbols are as defined in Table 1; A is the age at which sharks can first 

 enter the fishery and still allow the population to replace itself. 



' Under this scenario, sharks can enter the fishery at age 6 and above with a fishing mortality rate of F=0.428 (F=0 for all sharks 



ages 5 and below) added to the natural mortality rate and still allow the population to replace itself (r>0). 

 2 "2" indicates baseline age-specific natality values have been doubled. 



allow for full population replacement (r >0) given the 

 fishing mortality, became progressively smaller as 

 the value of r increased (see Table 2 for reference). 

 Increasing S by 10% (scenario 9) allowed for an age 

 at first capture of 5 years, while doubling m x (sce- 

 nario 4), reducing t t to 3 years of age (scenario 5), 

 increasing S by 50% (scenario 10), or increasing S 

 by 10% (scenario 11) all had the same effect of allow- 

 ing for an age at first capture of 4 years (90 cm TL) 

 compared with 6 years (99 cm TL) under the best 

 case scenario. Reducing t by 2 years (scenario 6) 

 or increasing £ to 20 years (scenario 13) both al- 

 lowed for an A of 3 years (82 cm TL), while reduc- 

 ing t mat by 1 year and doubling m x (scenario 7) al- 

 lowed an A of 2 years. Under the most extreme 

 manipulations, which included reducing t mat by 2 

 years and doubling m x (scenario 8), increasing S by 

 50% (scenario 12), and reducing t mat to 3 years, dou- 

 bling m x , and setting S and S o at 95% (scenario 14), 

 an age at first capture of 1 year (55 cm TL; scenarios 

 8 and 12) and of years (32 cm TL) could be applied 

 in a given year. 



Discussion 



These demographic analyses using the best available 

 information indicate that the Gulf of Mexico popula- 

 tion ofR. terraenovae may be very vulnerable to fish- 



ing pressure. Results showed that, based on known 

 life history parameters, the population's intrinsic rate 

 of increase was, at best, only r=0.044, equating to a 

 finite rate of e r =1.045, which is much lower than the 

 rate estimated for "small coastal" species in the stock 

 assessment used to develop the FMP for sharks of 

 the Atlantic Ocean (e r =1.91). Furthermore, compa- 

 rable rates to the FMP values were only obtained 

 after extreme manipulations of the input life history 

 parameters, which diverged too widely from observed 

 life history parameters to be realistic. For example, 

 one of the possible scenarios that would yield an es- 

 timate of e r of 1.91 implies that age at maturity has 

 to be decreased from 4 to 3 years, fertility doubled, 

 and survivorship increased by almost 50% relative 

 to the most optimistic initial scenario, i.e. the best 

 case scenario, resulting in estimates of 29.6 for R o , 

 6.7 for G, and 1.1 for t x2 . This means that, in the 

 absence of fishing, the population would almost 

 double every year. 



Rhizoprionodon terraenovae is the main species 

 caught in the Texas recreational shark fishery and 

 is also caught by the headboat and other recreational 

 fisheries in the Gulf of Mexico. More importantly, it 

 represents a significant bycatch in the shrimp trawl 

 fishery operating in the Gulf of Mexico and to a lesser 

 extent in the longline reef fish and shark fisheries, 

 and in the gillnet fishery in the same area. The lack 

 of data on the age and size at which individuals of R. 



