Simpfendorfer: Demographic analysis of Rhizoprionodon taylori 



985 



cially age at maturity) would decrease uncertainty. 

 However, the length-frequency data supplied by 

 Simpfendorfer ( 1993) support the hypothesis that/?. 

 taylori mature at age one and produce their first lit- 

 ter at age two. 



Increasing age at first capture for R. taylori in- 

 creased the level of fishing mortality that the popu- 

 lation could sustain. Allowing individuals to remain 

 unfished until theyhad produced one litter (AAFC—2) 

 made fishing mortality levels up to 0.67 sustainable, 

 whereas any level of fishing mortality was sustain- 

 able if individuals were allowed to produce two lit- 

 ters (AAFC=3). In a practical sense, AAFC restric- 

 tions are implemented as size restrictions. On the 

 basis of the growth curves given by Simpfendorfer 

 (1993), size of female R. taylori at the end of each of 

 the first three years is 55. 67, and 71 cm total length 

 (TL), respectively. The fact that size changes only 

 slightly after the first two years makes the use of 

 age restrictions unworkable for older individuals. 



The gillnet fisherj' in which R. taylori is most of- 

 ten captured in northern Australian waters uses 

 mesh sizes no smaller than 10 cm (stretched mesh). 

 Gill nets of this size rarely catch animals less than 

 60 cm TL. The age at first capture in this fishery is 

 therefore approximately one year. Although R. taylori 

 less than one year of age are caught by trawlers, this 

 fact was noted by the author only once in observa- 

 tions of research trawls between 1986 and 1992 in 

 the Townsville region. On the basis that age at first 

 capture of i?. taylori in northern Australia is prob- 

 ably one year, demographic analysis indicates that 

 the level of fishing mortality (F^) that the population 

 can sustain is approximately 0.27. However, Ricker 

 (19751 showed that the level of fishing mortality 

 needed to achieve maximum sustainable yield (MSY) 

 is r/2. Thus for R. taylori F^gY = 0.135. 



Sustainability of fisheries 



The comparison of demographic assessments of short- 

 lived, fast growing, early maturing elasmobranch 

 species such as R. taylori and S. tiburo, to those of 

 longer-lived, slow growing, late maturing species 

 indicates that they are likely to be able to sustain 

 higher levels of fishing pressure than the latter spe- 

 cies. For example, Sminkey and Musick (1996) re- 

 ported that Carcharhinus plumbeus was most likely 

 to be sustainable when F^ < 0.1, or F^ = 0.25 if a mini- 

 mum size of 178 cm was used. Similarly, Cailliet 

 (1992) reported that for Triakis semifasciata. F^. - 

 0.084 did not reduce the population, but that fishing 

 mortality at double this level was sustainable only if 

 animals 12 years and older were caught. This con- 

 trasts markedly with the results for R. taylori that 



the population may be sustainable at f^=0.18, or F^ = 

 0.27 if age at first capture was one year. It is there- 

 fore possible that at least some species of elasmo- 

 branchs, in particular the shorter-lived, fast grow- 

 ing, early maturing species, are able to sustain lim- 

 ited commercial levels of fishing pressure. The ap- 

 prehensive conclusion of Holden (1974) regarding 

 sustainable fishing of elasmobranch stocks may need 

 to be revised for selected species. However, the real 

 challenge is to manage the development and regula- 

 tion of such elasmobranch fisheries effectively. 



Acknowledgments 



This research was based on work carried out at James 

 Cook University, with funding from the University 

 and the Great Barrier Reef Marine Park Authority. 

 Norm Hall provided valuable guidance on the aspects 

 of the analysis and commented upon the manuscript. 

 Tony Hart and two anonymous reviewers also pro- 

 vided valuable comments on the manuscript. 



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