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



803 



Where species-specific information was not available, a 

 species was given the same rank as other species within 

 its family for the criteria water column position, diet, and 

 day and night catchability. For the other criteria, where it 

 was not necessarily logical that family members would be 

 similar, or where family information was not available, a 

 rank of 1 was assigned as a precautionary approach. 



Axis 1 : Susceptibility of a species to capture and 

 mortality induced by the prawn trawl 



There were six criteria (water column position, survival, 

 range, day and night catchability, diet, and depth range) 

 on axis 1. 



undertaken by Stobutzki et al. ^ and Stobutzki et al. (2001b). 

 Commercial fishing is highly aggregated within the man- 

 aged area of the fishery. The nine regions of highest effort 

 were surveyed in 1997 (Table 1 ) and the presence or absence 

 of each species was recorded in each region. We assumed 

 that species with a restricted range could be impacted more 

 heavily by trawling than those with a broader range. 



Day and night catchability The tiger prawn fishery is pre- 

 dominantly a nighttime fishery (McLoughlin et al., 1997). 

 Species with a higher catchability at night are more sus- 

 ceptible to capture as bycatch. The relative catch rate of 

 species during night and daytime trawling was compared 

 during research surveys in October 1997 (Table 1). 



Water column position Because prawn trawls fish close 

 to the sea floor, demersal species are more likely to be cap- 

 tured than pelagic species. 



Survival This estimate was based on the survival-in- 

 the-net data outlined previously. The possible sui^vival 

 range of 0-100% was divided into thirds for the divisions 

 between the ranks. 



Diet This criterion reflects whether the diet of the spe- 

 cies may attract them to trawl grounds and whether they 

 feed within the area of the water column swept by a prawn 

 trawl. Species that feed on commercial prawns may be 

 attracted to the commercial fishing grounds, increasing 

 their susceptibility to capture. Species that feed on demer- 

 sal organisms are assumed to be more susceptible to prawn 

 trawls than species that feed higher in the water column. 



Range This criterion reflects the geographic spread of 

 a species within the NPF and was determined from the 

 research, scientific, and crew-member observer surveys 



Depth range Commercial trawls in the NPF are made 

 mainly between 15 m and 40 m (Somers, 1994). An overlap 

 between the depth range of trawling and the preferred 



