Stobulzki et al Sustainability of elasmobranrhs caught as bycatch in a tropical prawn trawl fishery 



807 



all species. Such a hiixh catchahility coefficient is unlikely 

 to be valid for most s[)ecies and results in an underesti- 

 mate of the total biomass. P^jr the two bioregions where 

 commercial tiger prawn trawlers operate, there was no 

 survey data from which to estimate catch rates (bioregions 

 4 and 5, Fig. 1). Therefore, the mean catch rate of the other 

 bioregions was used to allow an estimate of catch rate. The 

 total biomass of each species was calculated by summing 

 the estimates for the bioregions. The removal rate would 

 range between 0^< and 100%; this range was divided into 

 thirds for the division between the ranks. 



s, = ^ 



i n 



1". 



(2) 



where S^ = the total susceptibility or recovery ranks for 

 species /; 

 if = the weighting for criterion 7; 

 R^ = the rank of species ( for criterion y; and 

 n = the number of criteria on each axes. 



Annual fecundity The annual fecundity of species was 

 estimated from data in the literature and the biological 

 samples collected during our study. The annual fecundity 

 of a species was estimated as the average number of pups 

 per female multiplied by the number of times the females 

 bred per year. Where the frequency of breeding was not 

 known, it was assumed to be annual, unless the known 

 gestation period was longer than 12 months. The range of 

 fecundities was calculated and divided into thirds for the 

 divisions between the ranks. 



Mortality index The recovery capacity of a population is 

 likely to be related to its fishing mortality rate (Sparre 

 and Venema, 1992). A measure of this rate can be derived 

 from the length-frequency of a species and the von Bert- 

 lanffy growth parameters (Sparre and Venema, 1992). 

 However, for most species von Bertalanffy parameters 

 were not available and therefore an index of mortality was 

 calculated as follows: 



Mortalifv index = (L„ 



L ,)/(L„ 



(1) 



where L„,„j = the maximum length; 



L^^^ = the mean length at capture in the fishery; 



and 

 ^nuii - ^-he smallest length caught. 



The closer the mean length of captured individuals (L^^,^,) 

 to the maximum length (L^^^) the lower the mortality 

 the population is subject to. As mortality due to fishing 

 increases, the mean length of species in a population 

 approaches the minimum length (L„,,„). For our analysis, 

 we assumed constant catchahility and mortality across 

 the whole length range caught. The L^^,^, and L„„„ were 

 calculated from length data collected during our study. 



The range of mortality estimates was calculated and 

 divided into thirds for the divisions between the ranks. 



Analysis of criteria 



Partial correlations (Sokal and Rohlf 1996) were used to 

 determine whether there was any redundancy in the cri- 

 teria. Strong correlations would suggest that two or more 

 criteria explained the same factors, which would lead to 

 overemphasis of their effect. One of the correlated criteria 

 was, therefore, removed. 



The total susceptibility, or removal ranking, of a species 

 was determined by the following equation: 



The criteria were weighted to reflect the relative impor- 

 tance of each criterion in determining the overall charac- 

 teristic and the robustness and quality of the data (Table 2), 

 the latter in terms of the amount of species-specific in- 

 formation and the scale of the information available. The 

 criteria that were seen as major determinants of suscep- 

 tibility or recovery and for which there were more robust 

 data were weighted highest. This weighting was done in 

 collaboration with the NPF Fishery Assessment Group. 



The total susceptibility and recovery ranks for the spe- 

 cies were graphed to determine the relative sustainability 

 of the species caught as bycatch by prawn trawlers. The 

 species least likely to be sustainable would be identified as 

 the species with the lowest ranks on both axes. 



Contour lines were drawn on the graph to group species 

 that would be similar with respect to their sustainablity. 

 Because neither susceptibility, nor recovery alone, provide 

 a complete index to the sustainability of species, the in- 

 dex is a combination of these two features. Recovery is 

 likely to be conditionally important on susceptibility, and 

 therefore, a multiplicative relationship between the two 

 axes is appropriate. We assumed that this relationship is 

 symmetrical and given this assumption, the contour lines 

 followed the equation 



16(.v - 0.75) (.V - 0.75) = 4, 9, 16, 25, 36, 49. (3) 



The impact of turtle excluder devices on 

 elasmobranch bycatch 



Data on the size of species captured in nets fitted with 

 TEDs and with nets with standard codends were avail- 

 able from two sources. The crew-member observer recorded 

 seven pairs of trawls in which one net was fitted with a TED 

 and one had a standard codend. The TED was a Seymour 

 TED with 110-mm bar spacing. Previous research surveys 

 from one area of the NPF also recorded information on elas- 

 mobranchs captured in nets with and without TEDs. The 

 TEDs were AusTEDs, NordMore Grids, and SuperShooters; 

 the design of these nets is detailed in Brewer et al. ( 1998). 

 The length frequency of elasmobranchs caught in nets 

 with TEDs was compared to the length frequency of elas- 

 mobranchs caught in nets without a TED. First, species 

 were grouped into sharks (TL measured) and rays (DW 

 measured) for analysis. The mean length of individuals 

 captured in nets fitted with a TED was compared with 

 that of elasmobranchs caught in nets with a standard 

 codend by using a one-way ANOVA. The lengths were 



