810 



Fishery Bulletin 100(4) 



and mortality. The next 19 species had a rank of 1.15, also 

 low. Carcharhinus tilstoni, C. macloti, Sphyrna lewini, Prio- 

 nace glauca. C. brevipinna. and Aetomyli'iis nichofii had the 

 highest ranks on this axis (>1.92l, indicating that they were 

 the least susceptible to capture and mortality. 



On the recovery axis (Appendix 2) Aetomyleus vespertil- 

 io, Dasyatis brevicaudatus, Pristis clavate, and P. pectinata 

 had the lowest ranks, indicating that they had the lowest 

 capacity to recover Gymnura australis. H. toshi. Hemigale- 

 us microstoma, and R. taylori had the highest ranks on this 

 axis and therefore the highest capacity to recover. 



When the ranks of the species on the two axes were plot- 

 ted (Fig. 2), Dasyatis brevicaudatus, P. pectina. P. da rata. 

 P. microdon, P. zijsron, and Himantura jenkinsii ranked 

 the lowest on the combination of the two axes, indicating 

 that they were the least likely to be able to survive cap- 

 ture as bycatch. The species Eusphyr-na blochii, H. toshi, 

 C. macloti, and C. tilstoni ranked the highest on the two 

 axes, indicating that they were the most likely to be able 

 to survive capture as bycatch. 



The impact of turtle exclusion devices on 

 elasmobranch bycatch 



Both sharks and rays taken as bycatch were significantly 

 smaller in nets with a codend fitted with a TED (Table 9). 

 The length frequency of the sharks and rays caught in the 

 nets with TEDs showed a lower proportion of the larger 

 individuals (Fig. 3). Where individual species were exam- 

 ined, there was a decrease in the size of C. dussumieri and 

 R. djiddensis caught in the net with a TED (Table 9). There 

 was no significant difference in size for H. microstoma, A. 

 annotata, and H. toshi (Table 9). However, significantly 

 larger individuals o{ Rhizoprionodon acutus were caught 

 in the net with a TED (Table 9). 



Discussion 



ing the impact of trawling in this region on elasmobranchs 

 are the catch rates and survival of species. 



Current catch rates 



Although the bycatch was highly diverse, four species 

 dominated the catch of the present study (C tilstoni, C. 

 dussumieri, R. djiddensis. and H. toshi. Table 4), occurring 

 in 14—20'^^ of trawls, so that one individual was seen at 

 least every seven trawls. However, most species (75%) con- 

 tributed <1% of the catch and had low catch rates (Table 

 4). However, even low catch rates can result in a large 

 overall take of individuals. The fishery recorded 18.314 

 days of fishing in 1999 (Sharp et al.M and if each day con- 

 sisted of four trawls (Bishop and Sterling, 1999), 73,256 

 trawls (with two nets) would have been undertaken in the 

 year. Hence for a species occurring in 1% of trawls. 733 

 individuals would have been caught in the year 



There are no long-term catch data available that can 

 be examined for changes in catch rates of elasmobranch 

 species. Although shark byproducts are recorded in NPF 

 logbooks, the data are of limited value because they are not 

 validated and not species-specific. Pender et al.^ surveyed 

 the bycatch in Northern Territory waters of the NPF during 

 the 1980s. Rhynchobatids (71% of the elasmobranch catch), 

 carcharhinids {\2'7<) and dasyatids (IKO dominated the 

 catch (Pender et al.^). All species recorded by Pender et al.^ 

 were recorded in our study. Direct comparisons of the catch 

 rates of Pender et al.'* with those of our study were not pos- 

 sible because of differences in gear, season, and region. 



Most elasmobranchs caught in bycatch are small (<1000 

 mm). For some species, this means that most individuals 

 have not bred before capture (Table 6) and therefore the 

 fishery will have a greater adverse impact on the species. 

 At least eight species were caught at sizes close to their 

 known birth size (Table 6). This finding suggests that pup- 

 ping may occur in the area of the fishery. Whether these 

 species have restricted pupping grounds is unknown. 



Of the elasmobranch species known to inhabit this region. 

 71% were taken as bycatch in the NPF The highly diverse 

 bycatch is characteristic of tropical prawn trawl fisheries 

 (Hall, 1999). Two critical pieces of information for assess- 



Within-net survival 



Our estimates of within-net survival are the first for elas- 

 mobranchs in prawn trawls. The results suggest that most 



