802 



Fishery Bulletin 100(4) 



Estimates of current bycatch rates and size 

 frequency of species 



Current bycatch rates were obtained from research 

 and observer surveys. The research surveys and gear 

 are described in detail in Stobutzki et al. (2001b) and 

 Stobutzki et al.^ Briefly, the research surveys sampled 

 the nine major NPF fishing regions (1997 and 1998) to 

 describe the bycatch. A scientific observer conducted three 

 trips (of one-month duration) on commercial vessels in 

 the NPF during 1996-97. A crew member of the commer- 

 cial fleet was trained in elasmobranch identification and 

 recorded the elasmobranch catch on commercial vessels 

 during 1997. All elasmobranchs caught were identified, 

 most to species, and their total number and weight were 

 recorded. Where possible, each individual's sex, weight, 

 and length were recorded. Total length (TL) was recorded 

 for sharks, rhynchobatids, and pristids, and disc width 

 (DW) was recorded for the remaining rays. Trawls during 

 the research survey were of 0.5-h duration and a single 

 trawl net was used. The observer data were collected from 

 commercial trawls, 3-4 h in duration and where two nets 

 were towed. 



The overall catch rate for each species was calculated 

 from the three sources. Catch rates were corrected for 

 duration of the trawl and the length of the headrope. The 

 catch rates of species in each trawl were converted into 

 catch per swept area of the trawl as the numbers of in- 

 dividuals per square kilometer swept (no./km^). We used 

 the trawl speed recorded during the trawls and assumed 

 that the prawn trawls had a spread of 0.66 of the headrope 

 length (Bishop and Sterling, 1999). Individuals of the spe- 

 cies Carcharhinus tilstoni and C. limbatus are difficult to 

 distinguish. Genetic studies in this region have indicated 

 that C. limbatus is very rare (Lavery and Shaklee, 1991); 

 therefore all specimens were recorded as C. tilstoni. 



Size at first maturity and fecundity 



Because there is limited biological information on dasy- 

 atidids and gymnurids (Last and Stevens, 1994), we re- 

 tained specimens from the scientific surveys to obtain 

 preliminary estimates of size at maturity and fecundity. 

 For females, gonad weight, diameter of the largest ovum in 

 the ovary, and their fecundity status (whether they were 

 pregnant of not, and whether there were in iitero embryos 

 present) were recorded. For pregnant individuals, the 

 number of embryos was recorded. For males, we recorded 

 gonad weight, clasper length, and the calcification state of 

 the clasper (uncalcified, partially calcified, or totally calci- 

 fied). Size at sexual maturity for females was estimated as 



Stobutzki, I.. S. Blaber, D. Brewer, G. Frv, D. Heales, P. Jones, 

 M. Miller, D. Milton, J. Salini, T Van der Velde, Y-G. Wang, 

 T. Wassenberg, M. Dredge, A. Courtney, K. Chilcott, and S. 

 Eayrs. 2000. Ecological sustainability of bycatch and biodi- 

 versity in prawn trawl fisheries. Final report to the Fisher- 

 ies Research and Development Corporation (FRDC), Project 

 96/257, .512 p. FRDC, PO Box 222. Deakin West ACT 2600, 

 Australia. 



the length of the smallest pregnant individual; for males 

 it was determined from clasper size and calcification (Bass 

 etal.. 197.3). 



Within-net survival 



Currently there is no information on the survival rate of 

 elasmobranchs caught as bycatch in prawn trawlers. The 

 October 1998 research survey and crew-member observer 

 (Table 1) recorded whether individuals were dead or alive 

 when landed on the deck. This record provided an estimate 

 of the within-net mortality, which was no doubt lower than 

 the total mortality because some individuals recorded as 

 alive would subsequently die as a result of capture. Logis- 

 tic regressions (PROC LOGISTIC, SAS 1997) were used 

 to determine whether there was a relationship between 

 the likelihood of survival and the length or sex of an indi- 

 vidual. The species were analyzed in two groups: sharks 

 (species where TL was recorded) and rays (species where 

 DW was recorded). 



Assessment of the sustainability of 

 elasmobranch species 



The assessment was based on the method developed by 

 Stobutzki et al. (2001a) which was designed to accommo- 

 date a high diversity of and a limited amount of informa- 

 tion. The sustainability of the species was assumed to be 

 dependent on two overriding features; 1 ) the susceptibility 

 of the species to capture and mortality caused by trawl- 

 ing and 2) the capacity of the population to recover after 

 depletion. Biological and ecological information was col- 

 lated from the literature (Compagno, 1984a; 1984b; Last 

 and Stevens, 1994; Froese and Pauly^). This information 

 was used to rank the species along two axes describing the 

 overriding features: 



Axis 1: The susceptibility of a species to capture and mor- 

 tality due to a prawn trawl (susceptibility), 



Axis 2: The capacity of a species to recover once the popu- 

 lation is depleted (recovery). 



Each feature (or axis) was derived from several criteria 

 (listed below) that summarized aspects of the biology of 

 the species (six criteria for axis 1 and five criteria for axis 

 2). Each species was given a ranking from 1 to 3 for each 

 criterion (the definitions of the ranks for the criteria are 

 provided in Table 2). A rank of 1 suggested that the spe- 

 cies was highly susceptible to capture or had little capac- 

 ity to recover; a rank of 3 suggested that the species had a 

 low susceptibility to capture or a high capacity to recover. 

 Depending on the criterion, these ranks were based on cat- 

 egorical or continuous data (Table 2). Where continuous 

 data were used, because no information was available to 

 assign divisions between the ranks, the range of the data 

 was divided into thirds to create the categories. 



•5 Froese, R., and D. Pauly, eds. 1999. Fishbase 99. URL 

 htpp://www.fishbase.org. (Date accessed: November 1999.] 



