Cortes: Demographic analysis of Rhizopnonodon terraenovae 



65 



compensatory mechanisms countering stock deple- 

 tion may include increased growth with correspond- 

 ing earlier age at maturity, earlier size at maturity, 

 higher fecundity, immigration of stocks from other 

 areas, and increased survival rates through density- 

 dependent regulation (Walker, 1992). 



The estimates of demographic parameters derived 

 under the best case scenario in this study are within 

 the range of values derived for other species of sharks 

 (Table 6). The estimate of R o ( 1.28) for/?, terraenovae 

 is almost identical to that derived by Hoenig and 

 Gruber (1990) for N. brevirostris (1.27), probably 

 owing to the similar fecundity in the two species. 

 Generation length (G) is considerably shorter in R. 

 terraenovae than in any other species because of its 

 shorter lifespan. The estimates of r and e r fall be- 

 tween those for other species. These relatively low 

 values in a fast-growing, short-lived species such as 

 R. terraenovae can be explained mainly by low fe- 

 cundity and high mortality levels as the demographic 

 analyses showed. Theoretical doubling time (t x2 ) also 

 lies between estimates for other species of sharks. 



In the light of the incomplete yet reasonable bio- 

 logical information available, it seems sound to con- 

 clude that R. terraenovae, as perhaps some of the 

 other small coastal species included in the FMP, is a 

 species vulnerable to fishing. This becomes especially 

 relevant with the recent implementation of the shark 

 FMP which has set quotas and trip limits and has 

 provided for fisheries closures for larger coastal spe- 

 cies while allowing the unrestricted capture of small 

 coastal sharks, of which R. terraenovae is the most 

 representative species. Increased fishing pressure on 

 R. terraenovae, especially on younger animals, would 

 certainly deplete the stocks of this species. 



The present study raises some serious concerns 

 about the use of surplus production models utilizing 

 very short series of catch and effort data and not tak- 



ing into account the biological characteristics of a 

 species. In the particular case of R. terraenovae, it 

 can lead to policies that in turn lead to overfishing 

 and stock depletion. It is advised that bycatch of R. 

 terraenovae in the shrimp fishery and other fisher- 

 ies in the Gulf of Mexico, as well as in the eventual 

 development of a directed fishery for this species, be 

 closely monitored in the near future. Development 

 of stock identification techniques are also needed to 

 determine stock structure and the degree of mixing 

 and immigration of this species into fishing areas. 

 Updated, more complete fisheries statistics combined 

 with improved biological information should be used 

 to re-assess the status of R. terraenovae in the small 

 coastal species group of the shark FMP for future 

 sound management actions. 



Acknowledgments 



This study was made possible by the work on age 

 and growth of R. terraenovae by S. Branstetter and 

 G. Parsons and by the studies on reproduction by G. 

 Parsons, who also kindly made available the rela- 

 tionship between fecundity and female total length. 

 I particularly thank Gregor Cailliet and an anony- 

 mous referee for their constructive review of this 

 manuscript. This work was supported by a post- 

 doctoral research fellowship from the Ministry of 

 Education and Science of Spain to the author. 



Literature cited 



Branstetter, S. 



1986. Biological parameters of the sharks of the northwest- 

 ern Gulf of Mexico in relation to their potential as a com- 

 mercial fishery resource. Ph.D. diss., Texas A&M Univ., 

 College Station, TX, 138 p. 



