800 



Abstract-The bycatch of Australia's 

 northern prawn fishery (NPF) com- 

 prises 56 elasmobranch species ( 16 

 famihes). The impact of this fishery on 

 the sustainability of these species has 

 not been addressed. We obtained esti- 

 mates of catch rates and the within-net 

 survival of elasmobranchs. Carcha- 

 rhinus tilstoni, C. dussumieri, Rhyn- 

 chobatus djiddensis, and Himantura 

 toshi represented 65% of the bycatch. 

 For most species, >50'7f of individuals 

 in the bycatch were immature, and 

 some species recruited to the fishery at 

 birth. For all species combined, 66% of 

 individuals in the bycatch died in the 

 trawl net. 



The relative sustainability of elasmo- 

 branchs caught as bycatch was exam- 

 ined by ranking species with respect 

 to their susceptibility to capture and 

 mortality due to prawn trawling and 

 with respect to their capacity to recover 

 once the population was depleted. The 

 species that were least likely to be sus- 

 tainable were four species of pristids, 

 Dasyatis brevicaudata. and Himantura 

 jenkinsii. These are bottom-associated 

 batoids that feed on benthic organisms 

 and are highly susceptible to capture in 

 prawn trawls. The recovery capacity of 

 these species was also low according to 

 our criteria. Our results provide a valu- 

 able first step towards ensuring the 

 sustainability of elasmobranchs that 

 are caught as bycatch by highlighting 

 species for management and research. 

 The effectiveness of turtle excluder 

 devices (TEDs) in reducing elasmo- 

 branch bycatch varied greatly among 

 species but was generally not very 

 effective because most of the captured 

 species were small. 



Sustainability of elasmobranchs caught as bycatch 

 in a tropical prawn (shrimp) trawl fishery 



llona C. Stobutzki 



Margaret J. Miller 



Don S. Heales 



David T. Brewer 



CSIRO Marine Research 



PO Box 120 



Cleveland 



Queensland 4163, Australia 



E mail address ilona stobut2ki@,csiro-au 



Manuscript accepted 29 May 2002. 

 Fish. Bull. 100:800-821 12002). 



Worldwide, there is increasing concern 

 over the capture of elasmobranchs 

 (sharks and rays) as bycatch. The 

 global landings of elasmobranchs are 

 currently 760,000 metric tons (t) but 

 a similar amount is part of unreported 

 bycatch (Stevens et al., 2000). This 

 bycatch is unmanaged in most fisher- 

 ies and elasmobranchs are less able to 

 sustain their populations under fishing 

 regimes designed to sustain the target 

 teleosts or invertebrates (Heuter, 

 1998). Some species have declined 

 significantly because they are captured 

 as bycatch, e.g. the common iDipturus 

 batis) and barndoor iD. laevis) skates 

 (Brander, 1981; Casey and Myers, 

 1998). Despite these prevailing fishery 

 practices, there have been few evalu- 

 ations of the ability of elasmobranch 

 species to sustain population levels 

 (Walker and Hislop, 1998). 



In general, an evaluation of the sus- 

 tainability of any bycatch species is 

 hampered by a lack of information. This 

 is particularly so for elasmobranchs. 

 Elasmobranch bycatch is often not 

 recorded (Bonfil, 1994), or when it is 

 recorded, the species composition is un- 

 known. There is also limited biological 

 information on most bycatch species, 

 such as age at maturity, growth rate, 

 and fecundity. This lack of information 

 hampers the use of conventional stock 

 assessment methods to determine the 

 population status of these species. 



Australia has a highly diverse elasmo- 

 branch fauna; almost half of the species 

 are endemic (Last and Stevens, 1994). 

 In northern Australian waters, elas- 

 mobranchs are impacted by a range of 

 fisheries. Gillnet, longline, and dropline 



fisheries target shark species. Sharks, 

 rays, and sawfish are also caught as 

 bycatch in dropline and gillnet fisheries 

 that target teleosts and in trawl fisher- 

 ies that target teleosts or prawns. The 

 current levels of elasmobranch bycatch 

 are unknown for most of these fisheries. 

 However, we know that the retained 

 elasmobranch bycatch has increased 

 because of the rising value of elasmo- 

 branch products, such as fins. 



The largest fishery in northern Aus- 

 tralia is the northern prawn fishery 

 (NPF), which covers an area over 

 1,000,000 km- of ocean (Fig. 1) (Mc- 

 Loughlin et al., 1997). In the NPF, elas- 

 mobranchs contribute about 4"r of the 

 total bycatch weight (Stobutzki et al., 

 2001b). Prior to 2001, NPF trawlers 

 were allowed to retain shark products 

 but were restricted with respect to the 

 amount on board at any one time. Man- 

 agement required fishermen to record 

 retained bycatch in trawler logbooks, 

 but the records were not validated. In 

 1999, 4177 kg of fillet, trunk, and whole 

 shark and 1531 fins were recorded 

 (Sharp et al.M. The compulsory use of 

 turtle excluder devices (TEDs) in NPF 

 trawls, beginning with the year 2000, 

 have excluded some elasmobranchs 

 from the bycatch (Brewer et al., 1998). 

 However species-specific exclusion has 

 not been examined. 



Sharp, A., J. Malcolm, and J Bishop. 

 2000. Northern prawn fishery and Kim- 

 berley prawn fishery data summary 1999, 

 Final report to Australian Fisheries Man- 

 agement Authority, Canberra, Australia. 

 AFMA, PO Box 7051, Canberra BC ACT 

 2610, Australia. 



