808 



Fishery Bulletin 100(4) 



transformed (log (length + 1)) prior to analysis to normal- 

 ize the data. There were sufficient data for three species 

 of shark (Rhizoprionodon acutus, Hemigaleus microstoma, 

 and Carcharhinus dussumieri), two stingrays (Dasyatis 

 leylandi and Himantura toshi) and a shovel-nosed ray 

 (Rhynchobatus djiddensis) to examine them separately 

 with one-way ANOVAs. 



Results 



Species captured as bycatch in prawn trawls 

 of the NPF 



At least 79 species of elasmobranchs from 18 families, 

 inhabit the NPF region (Table 31. Of these, 56 species (16 

 families) have been recorded in the prawn-trawl fishery 

 bycatch (Table 3). The Carcharhinidae and Dasyatidae, 

 the most species-rich families in the region, are the also 

 the families for which the highest number of species are 

 recorded in bycatch (Table 3). There are 9 families in 

 which all species found in this region have been recorded 

 in bycatch (Table 3). 



Current catch rates 



In the research and observer surveys (1996 and 1998) 44 

 species of elasmobranchs were recorded. The highest over- 

 all catch rates were for Carcharhinus tilstoni, C. dussumi- 

 eri, R. djiddensis, and H. toshi (Table 4). These four species 

 contributed almost 65*7^ of the observed elasmobranch 

 bycatch. Carcharhinus dussumieri and C. tilstoni were 

 recorded in 20% of all trawls, i?. dijiddensis in 14'*, and 

 H. toshi in 17%. 



Size at first maturity and fecundity 



Specimens of five species of ray were examined to assess 

 size at first maturity and to provide estimates of fecun- 

 dity (Table 5). None of the species showed a change in 

 gonadosomatic index (GSl) or diameter of the largest egg, 

 both of which would clearly indicate maturity. The aver- 

 age number of embryos was low (Table 5); most species 

 had one or two, with the exception of Gymnurus austi-alis, 

 which had up to five embryos present. 



Males of most species showed an increase in GSI with 

 calcification of the claspers. The estimates of size at ma- 

 turity for the males were lower than the estimates for 

 females for four of the five species (Table 5). However, this 

 finding might have been influenced by the low numbers of 

 pregnant females sampled (Table 5). The size at maturity 

 of the males appeared to be between 44% and 79% of the 

 maximum size for the species. 



The mean size of rays caught in bycatch ranged from 182 

 mm for D. leylandi to 1117 mm for H. toshi (Table 6). The 

 mean size of sharks ranged from 541 mm for Carcharhi- 

 nus sorrah to 1643 mm for Rhina ancylostoma (Table 6). 

 For 30 species, a size at birth was available from the lit- 

 erature and, of these, eight species were caught in bycatch 

 at this -size (Table 6). 



Where an estimate of the size at first maturity (L„^) was 

 available for a species, an estimate could be made of the 

 percentage of individuals captured that were mature. In 

 species with sufficient samples sizes, the percentage of 

 mature individuals caught ranged from <1% for S. lewini, 

 to 54% for R. acutus (Table 6). Species such as D. leylandi 

 had an average size at capture not significantly different 

 from L,,,, indicating that, on average, half the individu- 

 als caught had reached maturity before capture. Species 

 such as R. acutus, with an average size less than L^, were 

 those for which the majority were unlikely to have bred 

 before capture. At the other extreme were species such as 

 G. australis. for which it was likely that the majority had 

 reached maturity before capture (Table 6). 



The female-to-male ratio of individuals caught was 

 close to 1:1 for the two common species, D. leylandi and C. 

 dussumieri (Table 6). However, other species had a range 

 from predominantly male (e.g. R. acutus) to predominantly 

 female (e.g. H. toshi) (Table 6). 



Within-net survival 



Whether an individual was alive or dead when landed on 

 the deck was recorded for 847 animals. Overall 56%f were 

 dead after capture in the trawl and 44% were alive. Both 

 sharks and rays showed that the probability of survival was 

 lower for males than for females (sharks ;^-=19.7,P<0.001, 

 rays ;f-=10.5, P=0.0012) and that survival increased with 

 length of the individual (sharks ;f^=4.8, P=0.029, rays 

 ;f-'= 11.08, P=0.0009). Two-thirds of male sharks and rays 

 were recorded as dead after capture in the trawl (Table 7). 

 The mean size of rays and sharks that died (sharks 684 

 (±10 SE) mm, rays 424 (±41 SE) mm) was smaller than the 

 mean size of those that survived (sharks 797 (±17 SE) mm, 

 rays 546 (±33 SE) mm). The overall percentage of individu- 

 als of a species that died varied from 10% (R. djiddensis) to 

 82% (C. dussumieri and R. acutus) (Table 7). 



Assessment of the sustainability of 

 elasmobranch species 



The 56 species of elasmobranchs recorded as bycatch in 

 the NPF were ranked on each of the criteria on the two 

 axes (Appendices 1 and 2). The extent to which species- 

 specific information was available varied among the cri- 

 teria (Table 2). Water column position, depth range, and 

 maximum size had species-specific information for all 

 species. Survival and day and night catchability had little 

 species-specific information. 



Most of the criteria were not correlated (Table 8). On the 

 susceptibility axis the strongest correlation was between 

 diet and water column position (Table 8). However, both 

 criteria were retained because we believed there was suf- 

 ficient difference between them; the correlation coefficient 

 ir) was only 0.67. On the recovery axis no correlations 

 were significant (Table 8). 



On the susceptibility axis (Appendix 1 ) the four species of 

 Pristidae, Atelomycterus fasciatus, Himantura jenkinsi, and 

 Stegostoma fasciatum had a rank of 1, the lowest possible 

 rank, suggesting they were the most susceptible to capture 



