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



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Maximum size The maximum size of a species was used 

 as an indicator of the species' relative recovery rate. In 

 general, larger species tend to live longer and their popu- 

 lations recover more slowly (Roberts and Hawkins, 1999). 

 Size appears to be a good predictor of vulnerability for 

 marine fishes (Jennings et al., 1999), and in particular 

 skates (Walker and Hislop, 1998; Dulvy et al., 2000). 

 Estimates of maximum size came from the literature. 

 Species were grouped according to whether DW or TL 

 was measured. The range of the maximum sizes of species 

 was calculated and divided into thirds for the divisions 

 between the ranks. 



Removal rate We assumed that species with a higher 

 proportion of their biomass removed as bycatch would 

 have a lower capacity to recover. The estimate of removal 

 rate was based on the catch rates from research surveys 

 and scientific observer collections undertaken between 



1996 and 1998 (Table 1). We assumed that these catch 

 rates were representative of the overall catch rates in the 

 commercial fishery. 



The catch rates of bycatch species vary spatially within 

 the NPF (Stobutzki et al., 2001b). Therefore, the fishery 

 was stratified before we estimated the mean catch rate, 

 using the bioregions identified in the Interim Marine and 

 Coastal Regionalization for Australia (IMCRA) process 

 (Thackway and Cresswell, 1998) (Fig. 1). A mean catch 

 rate for each species was calculated for each bioregion 

 where commercial tiger prawn trawling occurs. 



The biomass (in numbers of individuals per year) of by- 

 catch removed by the commercial fishery was estimated by 

 multiplying the mean catch rate calculated above by the 



1997 commercial tiger prawn fishery effort in each biore- 

 gion (Table 3). Commercial fishing effort is recorded in log 

 books in boat days (held by the Australian Fisheries Man- 

 agement Authority). One boat day was assumed to be the 



