Stobutzki et a\ Sustainability of elasmobranchs caught as bycatch in a tropical prawn trawl fishery 



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data were available, their survival was higher in trawls. 

 The depth range of the species was wide and their catch 

 rates during the day were the same as or higher than at 

 night. This range provides partial refuge from the night- 

 time commercial trawling. The data available suggest that 

 their recovery capacity is higher than that of most elasmo- 

 branch species (Appendix 2). Individuals of most of these 

 species are likely to have bred before capture and they are 

 smaller. These species were common in the bycatch, and 

 estimates of their removal rate (which was low) and their 

 mortality index (average) were therefore easy to deter- 

 mine. However, all species had low annual fecundities. 



This assessment of the elasmobranch bycatch is an im- 

 portant first step in ensuring their sustainability because 

 it provides a focus for future research and management. 

 The current ranking is constrained, however, by the avail- 

 able data and by the assumptions outlined in the "Meth- 

 ods" section. The effect of the lack of species-specific infor- 

 mation on the ranks should be taken into account because 

 it may reduce the rank of some species. The application 

 of our assessment has highlighted important information 

 gaps, which should be the focus of research, particularly 

 for the species that are least likely to be sustainable. 



It is also important that the assessment of the sustain- 

 ability of elasmobranch species is extended to include the 

 impact of other fisheries in the region. There are, for in- 

 stance, fisheries targeting sharks, as well as other fisheries 

 that capture elasmobranchs as bycatch. Because elasmo- 

 branch species may have a wide distribution range, their 

 populations could be impacted by several fisheries, which 



might create an unsustainable status for the population 

 overall. For example, the pristids are likely to be impacted 

 by the inshore and estuarine gillnet fisheries in this region. 

 The results of our analysis, it is to be hoped, will help in 

 the management of elasmobranch species and in earmark- 

 ing the least sustainable of these species. Future manage- 

 ment may include the use of exclusion devices (TEDs and 

 BRDs), closures, or further limits on retaining shark prod- 

 ucts. The compulsory introduction of TEDs and BRDs into 

 the NPF in 2000 is likely to affect catch rates of elasmo- 

 branchs. The TEDs have the potential to exclude large in- 

 dividuals. However, the majority of elasmobranchs caught 

 are <1000 mm (Fig. 3) and may escape through TEDs. The 

 effectiveness of TEDs will depend on their configuration 

 (particularly the width between the bars) and the size and 

 shape of the bycatch species. Rhynchohatus djiddensis, a 

 large, broad species, appeared to be excluded well by TEDs 

 (Table 9). In comparison, the smaller rays and small, slim 

 sharks were not excluded well (Fig. 3, Table 9). With the 

 introduction of TEDs to the fishery, species-specific exclu- 

 sion rates should be monitored so that these can be taken 

 into account in assessing the sustainability of a species. 

 Juveniles of many elasmobranch species are still likely 

 to be captured and their capture could potentially have 

 a large impact on their respective populations. The TEDs 

 may also be ineffective for species, such as the pristids, 

 that may tangle their saw in the net or the TED. Species 

 and the life stages of species, for which exclusion devices 

 are not effective, may require different management strat- 

 egies, such as marine protected areas. 



