Sustainability of elasmobranchs caught as bycatch in a tropical prawn (shrimp) trawl fishery.
The relative sustainability of elasmobranchs caught as bycatch was examined by ranking species with respect to their susceptibility to capture and mortality due to prawn trawling and with respect to their capacity to recover once the population was depleted. The species that were least likely to be sustainable were four species of pristids, Dasyatis brevicaudata, and Himantura jenkinsii. These are bottom-associated batoids that feed on benthic organisms and are highly susceptible to capture in prawn trawls. The recovery capacity of these species was also low according to our criteria. Our results provide a valuable first step towards ensuring the sustainability of elasmobranchs that are caught as bycatch by highlighting species for management and research. The effectiveness of turtle excluder devices (TEDs) in reducing elasmobranch bycatch varied greatly among species but was generally not very effective because most of the captured species were small.
Worldwide, there is increasing concern over the capture of elasmobranchs (sharks and rays) as bycatch. The global landings of elasmobranchs are currently 760,000 metric tons (t) but a similar amount is part of unreported bycatch (Stevens et al., 2000). This bycatch is unmanaged in most fisheries and elasmobranchs are less able to sustain their populations under fishing regimes designed to sustain the target teleosts or invertebrates (Heuter, 1998). Some species have declined significantly because they are captured as bycatch, e.g. the common (Dipturus batis) and barndoor (D. laevis) skates (Brander, 1981; Casey and Myers, 1998). Despite these prevailing fishery practices, there have been few evaluations of the ability of elasmobranch species to sustain population levels (Walker and Hislop, 1998).
In general, an evaluation of the sustainability of any bycatch species is hampered by a lack of information. This is particularly so for elasmobranchs. Elasmobranch bycatch is often not recorded (Bonfil, 1994), or when it is recorded, the species composition is unknown. There is also limited biological information on most bycatch species, such as age at maturity, growth rate, and fecundity. This lack of information hampers the use of conventional stock assessment methods to determine the population status of these species.
Australia has a highly diverse elasmobranch fauna; almost half of the species are endemic (Last and Stevens, 1994). In northern Australian waters, elasmobranchs are impacted by a range of fisheries. Gillnet, longline, and dropline fisheries target shark species. Sharks, rays, and sawfish are also caught as bycatch in dropline and gillnet fisheries that target teleosts and in trawl fisheries that target teleosts or prawns. The current levels of elasmobranch bycatch are unknown for most of these fisheries. However, we know that the retained elasmobranch bycatch has increased because of the rising value of elasmobranch products, such as fins.
The largest fishery in northern Australia is the northern prawn fishery (NPF), which covers an area over 1,000,000 [km.sup.2] of ocean (Fig. 1) (McLoughlin et al., 1997). In the NPF, elasmobranchs contribute about 4% of the total bycatch weight (Stobutzki et al., 2001b). Prior to 2001, NPF trawlers were allowed to retain shark products but were restricted with respect to the amount on board at any one time. Management required fishermen to record retained bycatch in trawler logbooks, but the records were not validated. In 1999, 4177 kg of fillet, trunk, and whole shark and 1531 fins were recorded (Sharp et al. (1)). The compulsory use of turtle excluder devices (TEDs) in NPF trawls, beginning with the year 2000, have excluded some elasmobranchs from the bycatch (Brewer et al., 1998). However species-specific exclusion has not been examined.
This study is one of several (Milton, 2001; Stobutzki et al., 2001a) that broadly examine the sustainability of bycatch species groups in the NPF. The aim of this study was to assess the relative sustainability of elasmobranch species taken as bycatch in the NPF. We use a broadbrush method developed by Stobutzki et al (2001a) to encompass the high diversity of and limited amount of information. This semiquantitative technique assesses the sustainability of species according to two overriding characteristics: 1) their susceptibility to capture and mortality due to trawling; and 2) the ability of a population to recover after depletion. Traditional population assessment methods have attempted to measure or model these factors. The broadbrush method uses biological and ecological criteria to rank species with respect to these two characteristics, maximizing the use of the limited information available. The method identifies species that are least likely to be sustainable in the bycatch, so that these can be the focus of further research and management.
Species present in the NPF and those captured as bycatch
A list of the elasmobranchs species recorded in the area of the NPF was compiled from Last and Stevens (1994). A list of species taken as NPF bycatch has been collated from two sources: 1) fishery research surveys undertaken within the NPF fishing grounds (Crocos and Coman, 1997; Stobutzki et al., 2001b; Blaber et al. (2); Crocos et al. (3)); and 2) records of elasmobranch bycatch by observers on commercial vessels (these observers were either scientific staff or trained crew-members) (Stobutzki, 2001b; Pender et al. (4); Stobutzki et al. (5)).
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. (5) 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 commercial 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 individuals per square kilometer swept (no./[km.sup.2]). 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 species 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 dasyatidids and gymnurids (Last and Stevens, 1994), we retained 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 utero 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 calcified). Size at sexual maturity for females was estimated as the length of the smallest pregnant individual; for males it was determined from clasper size and calcification (Bass et al., 1973).
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. Logistic 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 individual. 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 accommodate a high diversity of and a limited amount of information. 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 trawling and 2) the capacity of the population to recover after depletion. Biological and ecological information was collated from the literature (Compagno, 1984a; 1984b; Last and Stevens, 1994; Froese and Pauly (6)). 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 mortality due to a prawn trawl (susceptibility),
Axis 2: The capacity of a species to recover once the population 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 I suggested that the species was highly susceptible to capture or had little capacity 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 categorical 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.
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 I 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.
Water column position Because prawn trawls fish close to the sea floor, demersal species are more likely to be captured than pelagic species.
Survival This estimate was based on the survival-in-the-net data outlined previously. The possible survival range of 0-100% was divided into thirds for the divisions between the ranks.
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 undertaken by Stobutzki et al. (5) and Stobutzki et al. (2001b). Commercial fishing is highly aggregated within the managed 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 predominantly a nighttime fishery (McLoughlin et al., 1997). Species with a higher catchability at night are more susceptible 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).
Diet This criterion reflects whether the diet of the species 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 demersal organisms are assumed to be more susceptible to prawn trawls than species that feed higher in the water column.
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 depth range of species will influence their susceptibility to capture: a higher proportion of a species' population is likely to be taken if there is an overlap. Species with a broader depth range may have a spatial refuge from trawling, making them less susceptible. The depth range of species was determined from previous research surveys in the NPF and from the literature.
Axis 2: The capacity of a species to recover once the population is depleted
There were five criteria (probability of breeding, maximum size, removal rate, annual fecundity, mortality index) on this axis.
Probability of breeding We assumed that a species is likely to have a greater capacity to recover from a decrease in population due to trawling if most individuals are captured after they have bred. The probability that an individual of a species had bred before capture was determined from the mean length at capture in relation to the species' recorded size at maturity. The mean length at capture of a species was recorded in the research and observer surveys 1996-98 (Table 1). Size at maturity was determined from the available literature and from our estimates outlined previously.
A t-test (Sokal and Rohlf, 1996) was used to determine whether the mean length at capture was significantly different from the size at maturity for each species.
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 populations 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 bycatch 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 bioregion (Table 3). Commercial fishing effort is recorded in log books in boat days (held by the Australian Fisheries Management Authority). One boat day was assumed to be the equivalent of 14 hours of trawling with two nets and with 14-fathom (25.48 m) headropes at a speed of 3.2 knots (5.9 km/h) (Bishop and Sterling, 1999).
The estimate of the total amount removed for a species within the whole fishery was calculated by summing the removal estimates for the bioregions. This estimate was then converted to a proportion of the estimated total biomass of the species.
An estimate of the total biomass of each species in the bioregions where tiger prawn trawling occurs was generated from all research and scientific observer surveys conducted in the NPF during the 1990s (Fig. 1, Table 1). The gears used were prawn trawls (Florida flyer nets) and two types offish trawls (Frank and Bryce trawls and Engel trawls). Both night and daytime trawling were undertaken. Both prawn-trawl and fish-trawl surveys were analyzed in order to cover the management area of the fishery.
The catch rates of species in each trawl were converted to the catch per swept area of the trawl as described previously. The fish trawls were assumed to have a spread of 0.6 of the headrope length (Blaber et al., 1994). A mean catch rate for each gear at each time (day or night) was calculated in each bioregion, resulting in up to six catch rate estimates for a species in a bioregion. The highest of these means was used for each species in that bioregion. This catch rate was then multiplied by the area of the bioregion to give an estimate of total numbers of individuals in the bioregion. Currently there are no robust estimates of the catchability coefficients for the various trawl gears and therefore a catchability coefficient of one was assumed for all species. Such a high catchability coefficient is unlikely to be valid for most species and results in an underestimate of the total biomass. For 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.
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 Bertlanffy 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:
(1) Mortality index = ([L.sub.max] - [L.sub.ave])/([L.sub.ave] - [L.sub.min]),
where [L.sub.max] = the maximum length;
[L.sub.ave] = the mean length at capture in the fishery; and
[L.sub.min = the smallest length caught.
The closer the mean length of captured individuals ([L.sub.ave]) to the maximum length ([L.sub.max]) 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.sub.min]). For our analysis, we assumed constant catchability and mortality across the whole length range caught. The Lave and [L.sub.min] 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 criteria. 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:
(2) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
where [S.sub.i] = the total susceptibility or recovery ranks for species i;
[w.sub.j] = the weighting for criterion j;
[R.sub.i] = the rank of species i for criterion j; and
n = the number of criteria on each axes.
The criteria were weighted to reflect the relative importance of each criterion in determining the overall characteristic and the robustness and quality of the data (Table 2), the latter in terms of the amount of species-specific information and the scale of the information available. The criteria that were seen as major determinants of susceptibility 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 species 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 index 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
(3) 16(y - 0.75) (x - 0.75) = 4, 9, 16, 25, 36, 49.
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 available 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 elasmobranchs 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 elasmobranchs 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 transformed (log (length + 1)) prior to analysis to normalize 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.
Species captured as bycatch in prawn trawls of the NPF
At least 79 species of elasmobranchs from 18 families, inhabit the NPF region (Table 3). 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 overall catch rates were for Carcharhinus tilstoni, C. dussumieri, R. djiddensis, and H. toshi (Table 4). These four species contributed almost 65% of the observed elasmobranch bycatch. Carcharhinus dussumieri and C. tilstoni were recorded in 20% of all trawls, R. 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 fecundity (Table 5). None of the species showed a change in gonadosomatic index (GSI) or diameter of the largest egg, both of which would clearly indicate maturity. The average number of embryos was low (Table 5); most species had one or two, with the exception of Gymnurus australis, 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 maturity 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 Carcharhinus sorrah to 1643 mm for Rhina ancylostoma (Table 6). For 30 species, a size at birth was available from the literature and, of these, eight species were caught in bycatch at this size (Table 6).
Where an estimate of the size at first maturity ([L.sub.m]) 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.sub.m], indicating that, on average, half the individuals caught had reached maturity before capture. Species such as R. acutus, with an average size less than [L.sub.m], 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).
Whether an individual was alive or dead when landed on the deck was recorded for 847 animals. Overall 56% 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 [[chi].sup.2]=19.7, P<0.001, rays [[chi].sup.2]=10.5, P=0.0012) and that survival increased with length of the individual (sharks [[chi].sup.2]=4.8, P=0.029, rays [[chi].sup.2]=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 ([+ or -] 10 SE) mm, rays 424 ([+ or -] 41 SE) mm) was smaller than the mean size of those that survived (sharks 797 ([+ or -] 17 SE) mm, rays 546 ([+ or -] 33 SE) mm). The overall percentage of individuals 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 criteria (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 sufficient difference between them; the correlation coefficient (r) 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 and mortality. The next 19 species had a rank of 1.15, also low. Carcharhinus tilstoni, C. macloti, Sphyrna lewini, Prionace glauca, C. brevipinna, and Aetomyleus nichofii had the highest ranks on this axis (>1.92), indicating that they were the least susceptible to capture and mortality.
On the recovery axis (Appendix 2) Aetomyleus vespertillo, Dasyatis brevicaudatus, Pristis clavate, and P. pectinata had the lowest ranks, indicating that they had the lowest capacity to recover. Gymnura australis, H. toshi, Hemigaleus microstoma, and R. taylori had the highest ranks on this axis and therefore the highest capacity to recover.
When the ranks of the species on the two axes were plotted (Fig. 2), Dasyatis brevicaudatus, P. pectina, P. clavata, P. microdon, P. zijsron, and Himantura jenkinsii ranked the lowest on the combination of the two axes, indicating that they were the least likely to be able to survive capture as bycatch. The species Eusphyrna blochii, H. toshi, C. macloti, and C. tilstoni ranked the highest on the two axes, indicating that they were the most likely to be able to survive capture as bycatch.
[FIGURE 2 OMITTED]
The impact of turtle exclusion devices on elasmobranch bycatch
Both sharks and rays taken as bycatch were significantly smaller in nets with a codend fitted with a TED (Table 9). The length frequency of the sharks and rays caught in the nets with TEDs showed a lower proportion of the larger individuals (Fig. 3). Where individual species were examined, there was a decrease in the size of C. dussumieri and R. djiddensis caught in the net with a TED (Table 9). There was no significant difference in size for H. microstoma, A. annotata, and H. toshi (Table 9). However, significantly larger individuals of Rhizoprionodon acutus were caught in the net with a TED (Table 9).
[FIGURE 3 OMITTED]
Of the elasmobranch species known to inhabit this region, 71% were taken as bycatch in the NPF. The highly diverse bycatch is characteristic of tropical prawn trawl fisheries (Hall, 1999). Two critical pieces of information for assessing the impact of trawling in this region on elasmobranchs are the catch rates and survival of species.
Current catch rates
Although the bycatch was highly diverse, four species dominated the catch of the present study (C. tilstoni, C. dussumieri, R. djiddensis, and H. toshi, Table 4), occurring in 14-20% of trawls, so that one individual was seen at least every seven trawls. However, most species (75%) contributed <1% of the catch and had low catch rates (Table 4). However, even low catch rates can result in a large overall take of individuals. The fishery recorded 18,314 days of fishing in 1999 (Sharp et al. (1)) and if each day consisted of four trawls (Bishop and Sterling, 1999), 73,256 trawls (with two nets) would have been undertaken in the year. Hence for a species occurring in 1% of trawls, 733 individuals would have been caught in the year.
There are no long-term catch data available that can be examined for changes in catch rates of elasmobranch species. Although shark byproducts are recorded in NPF logbooks, the data are of limited value because they are not validated and not species-specific. Pender et al. (4) surveyed the bycatch in Northern Territory waters of the NPF during the 1980s. Rhynchobatids (71% of the elasmobranch catch), carcharhinids (12%) and dasyatids (11%) dominated the catch (Pender et al.4). All species recorded by Pender et al. (4) were recorded in our study. Direct comparisons of the catch rates of Pender et al.4 with those of our study were not possible because of differences in gear, season, and region.
Most elasmobranchs caught in bycatch are small (<1000 mm). For some species, this means that most individuals have not bred before capture (Table 6) and therefore the fishery will have a greater adverse impact on the species. At least eight species were caught at sizes close to their known birth size (Table 6). This finding suggests that pupping may occur in the area of the fishery. Whether these species have restricted pupping grounds is unknown.
Our estimates of within-net survival are the first for elasmobranchs in prawn trawls. The results suggest that most sharks and rays, particularly the smaller individuals, die within the trawl net (56%). The lower survival rates of male individuals is possibly because the males of most elasmobranch species are smaller than the females. The rhynchobatid R. djiddensis had a higher survival rate (90%) than most other species, whereas the lowest survival rate was seen in C. tilstoni and R. acutus (18%). Although the larger elasmobranchs appeared to have a higher within-net survival, in the commercial fishery these were the very individuals killed for their fins and therefore their mortality was ultimately higher than that predicted by their size alone. In 2001 the NPF introduced an industry-initiated ban on all shark products, so that the only mortality these species are subject to is that caused by the capture process. Differences between species in survival rates may influence changes in the relative abundance of species.
Assessment of the sustainability of elasmobranch species
Elasmobranchs, in general, are more susceptible to over-fishing than are bony fishes, but there is likely to be a range of sensitivities among the species (Walker, 1998; Stevens et al., 2000). The process we applied in our study allowed us to examine these different sensitivities and to highlight those species whose populations were most likely to be affected by the NPF. The process was designed to deal with the high diversity of the bycatch and the paucity of information available for most species. Our process was similar to that used by the International Union for the Conservation of Nature and Natural Resources (IUCN) red lists (IUCN, 1995) that categorize species with respect to the threat of extinction worldwide. The IUCN uses criteria on the extent of population decrease, area of occurrence, percentage of population that is mature, and the probability of extinction (IUCN, 1995). The IUCN criteria have been modified for application to marine fishes and to smaller geographic scales (Musick, 1998). With respect to elasmobranchs, several authors have examined the variable resilience of species to fishing pressure. These approaches have focused on life history characteristics that influence the recovery of populations, including reproductive and growth parameters (reviewed by Stevens et al., 2000). Our process is similar to these but focuses at the level of an individual fishery, incorporating fishery-specific information on the susceptibility of species to the fishery. Of significant importance with all methods is the ability to calculate the range of parameters required for a large number of species (Stevens et al., 2000). The semi quantitative method used in our study maximizes what can be determined from the data available and enables consistency across the species. The criteria include characteristics that influence the probability of extinction of a species and its sensitivity to overfishing (McKinney, 1997: Carlton et al., 1999; Roberts and Hawkins, 1999; Stevens et al., 2000). Our analysis provides a process for highlighting gaps in information and for prioritizing species for future management and research. This process does not replace traditional methods of population assessment but provides a rapid assessment of the species, so that traditional methods can be focused on the high-risk species.
The species that were least likely to be sustainable in the bycatch of the NPF were D. brevicaudatus, P. pectinata, P. clavata, P. microdon, P. zijsron, and Himantura jenkinsii (Fig. 3). The pristids and H. jenkinsii had ranks of 1 on the susceptibility axis, the lowest possible rank, and D. brevicaudatus ranked 1.15 (Appendix 1). These species are demersal, are rare in the bycatch, and at least for the pristids (which have restricted depth distributions) are likely to be rare. Nothing is known about their survival. Their diets include benthic organisms and are likely to include commercial prawns; their range and day and night catchability is unknown. The combination of these factors means that these species are likely to occur in trawl grounds and that they are highly susceptible to capture and mortality due to trawlers. The recovery capacity of populations of these species is also low (Appendix 2). The rarity of the species in the bycatch means that no data are available to estimate the probability of breeding before capture, removal rate, total biomass, or the mortality index for most of these species, and they therefore received ranks of 1 for these criteria. In general these are large animals and are therefore likely to have slower recovery rates for their population than those of smaller species. The annual fecundity was low for all species.
[FIGURE 3 OMITTED]
The pristids are the focus of increasing international concern because their populations are declining worldwide (Stevens et al. 2000). They are rarely seen today in areas where they were previously abundant (Simpfendorfer, 2000). This decrease in pristid populations has resulted in four species being listed on the IUCN 1996 red list (Bailie and Groombridge, 1996). Of the species studied in our study, P. pectinata and P. microdon are listed as endangered. Recent demographic analysis of pristid populations has indicated that their recovery will take several decades even if they are given effective conservation (Simpfendorfer, 2000).
In comparison, the species that were most likely to be able to sustain capture in the bycatch of the NPF were H. toshi, E. blockii, C. macloti, and C. tilstoni. These species had a lower susceptibility to capture and mortality due to trawling (Appendix 1). With the exception of H. toshi, these are pelagic species and there is little likelihood of their capture in prawn trawls. For the species for which data were available, their survival was higher in trawls. The depth range of the species was wide and their catch rates during the day were the same as or higher than at night. This range provides partial refuge from the nighttime commercial trawling. The data available suggest that their recovery capacity is higher than that of most elasmobranch species (Appendix 2). Individuals of most of these species are likely to have bred before capture and they are smaller. These species were common in the bycatch, and estimates of their removal rate (which was low) and their mortality index (average) were therefore easy to determine. However, all species had low annual fecundities.
This assessment of the elasmobranch bycatch is an important first step in ensuring their sustainability because it provides a focus for future research and management. The current ranking is constrained, however, by the available data and by the assumptions outlined in the "Methods" section. The effect of the lack of species-specific information on the ranks should be taken into account because it may reduce the rank of some species. The application of our assessment has highlighted important information gaps, which should be the focus of research, particularly for the species that are least likely to be sustainable.
It is also important that the assessment of the sustainability of elasmobranch species is extended to include the impact of other fisheries in the region. There are, for instance, fisheries targeting sharks, as well as other fisheries that capture elasmobranchs as bycatch. Because elasmobranch species may have a wide distribution range, their populations could be impacted by several fisheries, which might create an unsustainable status for the population overall. For example, the pristids are likely to be impacted by the inshore and estuarine gillnet fisheries in this region.
The results of our analysis, it is to be hoped, will help in the management of elasmobranch species and in earmarking the least sustainable of these species. Future management may include the use of exclusion devices (TEDs and BRDs), closures, or further limits on retaining shark products. The compulsory introduction of TEDs and BRDs into the NPF in 2000 is likely to affect catch rates of elasmobranchs. The TEDs have the potential to exclude large individuals. However, the majority of elasmobranchs caught are <1000 mm (Fig. 3) and may escape through TEDs. The effectiveness of TEDs will depend on their configuration (particularly the width between the bars) and the size and shape of the bycatch species. Rhynchobatus djiddensis, a large, broad species, appeared to be excluded well by TEDs (Table 9). In comparison, the smaller rays and small, slim sharks were not excluded well (Fig. 3, Table 9). With the introduction of TEDs to the fishery, species-specific exclusion rates should be monitored so that these can be taken into account in assessing the sustainability of a species. Juveniles of many elasmobranch species are still likely to be captured and their capture could potentially have a large impact on their respective populations. The TEDs may also be ineffective for species, such as the pristids, that may tangle their saw in the net or the TED. Species and the life stages of species, for which exclusion devices are not effective, may require different management strategies, such as marine protected areas.
This research is the first large-scale assessment of its kind on elasmobranch bycatch. The results highlight the diversity of elasmobranch bycatch in the NPF and the species that are least likely to be sustainable. We have also highlighted the limited information available for making this assessment. However, our method was designed to maximize the use of the limited information. The process we have used is applicable to other fisheries and also across fisheries, particularly where bycatch diversity is high.
Appendix 1 The ranking of elasmobranch species that occurred in the bycatch of the northern prawn fishery with respect to criteria that influence their susceptibility to capture and mortality due to prawn trawls. The weights of the criteria are shown in parentheses; * indicates where species-specific information was not available. The information was obtained from Compagno (19984a; 1984b), Last and Stevens (1994), and Froese and Pauly (6). Criteria Water column position Survival Range Family Species (3) (3) (2) Dasyatidae Himantura jenkinsii 1 1 * 1 Pristidae Pristis clavata 1 1 * 1 * Pristidae Pristis microdon 1 1 * 1 * Pristidae Pristis pectinata 1 1 * 1 * Pristidae Pristis zijsron 1 1 * 1 Stegostomatidae Stegostoma fasciatum 1 1 * 1 * Scyliorhinidae Atelomycterus fasciatus 1 1 * 1 Carcharhinidae Carcharhinus amboinensis 1 1 * 1 Carcharhinidae Carcharhinus leucas 1 1 * 1 * Dasyatidae Dasyatis brevicaudatus 1 1 * 1 * Dasyatidae Dasyatis sp. A 1 1 * 1 Dasyatidae Dasyatis thetidis 1 1 * 1 Scyliorhinidae Galeus sp. A 1 1 * 1 * Dasyatidae Himantura fai 1 1 * 1 Dasyatidae Himantura granulata 1 1 * 1 Dasyatidae Himantura uarnak 1 1 * 1 * Ginglymostomatidae Nebrius ferrugineus 1 1 * 1 Carcharhinidae Negaprion acutidens 1 1 * 1 Orectolobidae Orectolobus ornatus 1 1 * 1 Rhinobatidae Rhinobatos typus 1 1 * 1 Squatinidae Squatina sp. A 1 1 * 1 * Dasyatidae Taeniura meyeni 1 1 * 1 Dasyatidae Urogymnus asperrimus 1 1 * 1 Pristidae Anoxypristis cuspidata 1 1 * 1 * Dasyatidae Pastinachus sephen 1 1 * 2 Hemiscylliidae Chiloscyllium punctatum 1 1 * 2 Dasyatidae Dasyatis kuhlii 1 1 * 2 Dasyatidae Himantura sp. A 1 1 * 2 Narcinidae Narcine westraliensis 1 1 * 1 * Rhynchobatidae Rhina ancylostoma 1 1 * 2 Carcharhinidae Carcharhinus fitztroyensis 3 1 * 1 Dasyatidae Amphotistius annotata 1 1 * 2 Dasyatidae Himantura undulata 1 1 * 2 Carcharhinidae Rhizoprionodon taylori 1 1 * 1 Carcharhinidae Galeocerdo cuvier 3 1 * 1 Hemiscylliidae Hemigaleus microstoma 1 1 3 Dasyatidae Dasyatis leylandi 1 2 3 Gymnuridae Gymnura australis 1 2 3 Carcharhinidae Carcharhinus sorrah 3 1 2 Carcharhinidae Rhizoprionodon acutus 1 1 3 Rhynchobatidae Rhynchobatus djiddensis 1 3 2 Myliobatidae Aetobatus narinari 3 1 * 1 Myliobatidae Aetomylaeus vespertilio 3 1 * 1 * Carcharhinidae Carcharhinus albimarginatus 3 1 * 1 Hemiscylliidae Hemipristis elongatus 3 1 * 1 Sphyrnidae Sphyrna mokarran 3 1 * 1 Carcharhinidae Carcharhinus limbatus 3 1 1 * Sphyrnidae Eusphyra blochii 3 1 * 1 Carcharhinidae Carcharhinus dussumieri 1 2 2 Dasyatidae Himantura toshi 1 2 3 Myliobatidae Aetomyleus nichofii 3 1 * 2 Carcharhinidae Carcharhinus brevipinna 3 1 * 1 * Carcharhinidae Prionace glauca 3 1 * 1 * Sphyrnidae Sphyrna lewini 3 1 * 1 * Carcharhinidae Carcharhinus macloti 3 1 * 1 Carcharhinidae Carcharhinus tilstoni 3 2 2 Criteria Day and night catchability Diet Family Species (2) (2) Dasyatidae Himantura jenkinsii 1 * 1 * Pristidae Pristis clavata 1 * 1 * Pristidae Pristis microdon 1 * 1 * Pristidae Pristis pectinata 1 * 1 * Pristidae Pristis zijsron 1 * 1 * Stegostomatidae Stegostoma fasciatum 1 1 Scyliorhinidae Atelomycterus fasciatus 1 * 1 * Carcharhinidae Carcharhinus amboinensis 1 * 1 Carcharhinidae Carcharhinus leucas 1 * 1 Dasyatidae Dasyatis brevicaudatus 1 * 1 * Dasyatidae Dasyatis sp. A 1 * 1 * Dasyatidae Dasyatis thetidis 1 * 1 * Scyliorhinidae Galeus sp. A 1 * 1 * Dasyatidae Himantura fai 1 * 1 * Dasyatidae Himantura granulata 1 * 1 Dasyatidae Himantura uarnak 1 * 1 * Ginglymostomatidae Nebrius ferrugineus 1 * 1 Carcharhinidae Negaprion acutidens 1 * 1 Orectolobidae Orectolobus ornatus 1 1 Rhinobatidae Rhinobatos typus 1 * 1 * Squatinidae Squatina sp. A 1 * 1 * Dasyatidae Taeniura meyeni 1 * 1 * Dasyatidae Urogymnus asperrimus 1 * 1 * Pristidae Anoxypristis cuspidata 2 1 * Dasyatidae Pastinachus sephen 1 * 1 Hemiscylliidae Chiloscyllium punctatum 1 1 Dasyatidae Dasyatis kuhlii 1 1 * Dasyatidae Himantura sp. A 1 * 1 * Narcinidae Narcine westraliensis 1 * 2 Rhynchobatidae Rhina ancylostoma 1 1 Carcharhinidae Carcharhinus fitztroyensis 1 * 1 Dasyatidae Amphotistius annotata 2 1 * Dasyatidae Himantura undulata 2 1 * Carcharhinidae Rhizoprionodon taylori 3 1 Carcharhinidae Galeocerdo cuvier 1 * 1 Hemiscylliidae Hemigaleus microstoma 1 * 2 Dasyatidae Dasyatis leylandi 1 1 Gymnuridae Gymnura australis 2 1 * Carcharhinidae Carcharhinus sorrah 1 * 1 Carcharhinidae Rhizoprionodon acutus 3 1 Rhynchobatidae Rhynchobatus djiddensis 1 * 1 * Myliobatidae Aetobatus narinari 1 * 2 Myliobatidae Aetomylaeus vespertilio 1 * 2 * Carcharhinidae Carcharhinus albimarginatus 1 * 2 Hemiscylliidae Hemipristis elongatus 1 * 2 Sphyrnidae Sphyrna mokarran 1 * 2 Carcharhinidae Carcharhinus limbatus 1 * 3 Sphyrnidae Eusphyra blochii 2 1 Carcharhinidae Carcharhinus dussumieri 3 1 Dasyatidae Himantura toshi 2 1 Myliobatidae Aetomyleus nichofii 1 2 * Carcharhinidae Carcharhinus brevipinna 1 * 3 Carcharhinidae Prionace glauca 1 * 3 Sphyrnidae Sphyrna lewini 1 * 3 Carcharhinidae Carcharhinus macloti 3 2 Carcharhinidae Carcharhinus tilstoni 2 1 Criteria Depth range Susceptibility Family Species (1) ranking Dasyatidae Himantura jenkinsii 1 1.00 Pristidae Pristis clavata 1 1.00 Pristidae Pristis microdon 1 1.00 Pristidae Pristis pectinata 1 1.00 Pristidae Pristis zijsron i 1.00 Stegostomatidae Stegostoma fasciatum 1 1.00 Scyliorhinidae Atelomycterus fasciatus 3 1.15 Carcharhinidae Carcharhinus amboinensis 3 1.15 Carcharhinidae Carcharhinus leucas 3 1.15 Dasyatidae Dasyatis brevicaudatus 3 1.15 Dasyatidae Dasyatis sp. A 3 1.15 Dasyatidae Dasyatis thetidis 3 1.15 Scyliorhinidae Galeus sp. A 3 1.15 Dasyatidae Himantura fai 3 1.15 Dasyatidae Himantura granulata 3 1.15 Dasyatidae Himantura uarnak 3 1.15 Ginglymostomatidae Nebrius ferrugineus 3 1.15 Carcharhinidae Negaprion acutidens 3 1.15 Orectolobidae Orectolobus ornatus 3 1.15 Rhinobatidae Rhinobatos typus 3 1.15 Squatinidae Squatina sp. A 3 1.15 Dasyatidae Taeniura meyeni 3 1.15 Dasyatidae Urogymnus asperrimus 3 1.15 Pristidae Anoxypristis cuspidata 1 1.15 Dasyatidae Pastinachus sephen 1 1.15 Hemiscylliidae Chiloscyllium punctatum 3 1.31 Dasyatidae Dasyatis kuhlii 3 1.31 Dasyatidae Himantura sp. A 3 1.31 Narcinidae Narcine westraliensis 3 1.31 Rhynchobatidae Rhina ancylostoma 3 1.31 Carcharhinidae Carcharhinus fitztroyensis 1 1.46 Dasyatidae Amphotistius annotata 3 1.46 Dasyatidae Himantura undulata 3 1.46 Carcharhinidae Rhizoprionodon taylori 3 1.46 Carcharhinidae Galeocerdo cuvier 3 1.62 Hemiscylliidae Hemigaleus microstoma 3 1.62 Dasyatidae Dasyatis leylandi 3 1.69 Gymnuridae Gymnura australis 1 1.69 Carcharhinidae Carcharhinus sorrah 3 1.77 Carcharhinidae Rhizoprionodon acutus 3 1.77 Rhynchobatidae Rhynchobatus djiddensis 3 1.77 Myliobatidae Aetobatus narinari 3 1.77 Myliobatidae Aetomylaeus vespertilio 3 1.77 Carcharhinidae Carcharhinus albimarginatus 3 1.77 Hemiscylliidae Hemipristis elongatus 3 1.77 Sphyrnidae Sphyrna mokarran 3 1.77 Carcharhinidae Carcharhinus limbatus 1 1.77 Sphyrnidae Eusphyra blochii 3 1.77 Carcharhinidae Carcharhinus dussumieri 3 1.85 Dasyatidae Himantura toshi 3 1.85 Myliobatidae Aetomyleus nichofii 3 1.92 Carcharhinidae Carcharhinus brevipinna 3 1.92 Carcharhinidae Prionace glauca 3 1.92 Sphyrnidae Sphyrna lewini 3 1.92 Carcharhinidae Carcharhinus macloti 3 2.08 Carcharhinidae Carcharhinus tilstoni 3 2.15 Appendix 2 The ranking of elasmobranch species that occurrred in the bycatch of the northern prawn fishery with respect to criteria that reflect their capacity to recover after depletion by trawling. The weights of the criteria are shown in parentheses; * indicates where species-specific information was not available. The information was obtained from Compagno (1984a; 1984b), Last and Stevens (1994), and Froese and Pauly (6). Criteria Probability Maximum Removal of breeding size rate Family Species (3) (3) (3) Dasyatidae Dasyatis brevicaudatus 1 1 1 * Pristidae Pristis pectinata 1 * 1 1 * Pristidae Pristis clavata 1 * 2 1 * Myliobatidae Aetomylaeus vespertilio 1 * 2 1 * Dasyatidae Taeniura meyeni 1 1 3 Dasyatidae Himantura jenkinsii 1 * 2 2 Carcharhinidae Carcharhinus amboinensis 1 2 1 Carcharhinidae Carcharhinus leucas 1 3 1 * Scyliorhinidae Galeus sp. A 1 * 3 1 * Narcinidae Narcine westraliensis 1 * 3 1 * Pristidae Pristis microdon 1 * 3 1 * Squatinidae Squatina sp. A 1 * 3 1 * Carcharhinidae Prionace glauca 1 * 2 1 * Myliobatidae Aetobatus narinari 1 * 1 3 Carcharhinidae Carcharhinus limbatus 1 * 3 1 * Carcharhinidae Carcharhinus albimargi- natus 1 * 2 2 Carcharhinidae Carcharhinus brevipinna 1 * 3 1 * Dasyatidae Dasyatis thetidis 1 * 2 3 Pristidae Pristis zijsron 1 * 2 3 Dasyatidae Himantura fai 1 * 2 3 Dasyatidae Himantura granulata 1 * 2 3 Dasyatidae Himantura uarnak 1 * 2 3 Dasyatidae Himantura undulata 1 * 2 3 Orectolobidae Orectolobus ornatus 1 * 2 3 Dasyatidae Urogymnus asperrimus 1 2 3 Scyliorhinidae Atelomycterus fasciatus 2 2 2 Dasyatidae Dasyatis sp. A 2 3 1 Dasyatidae Amphotistius annotata 2 * 1 3 Rhynchobatidae Rhynchobatus djiddensis 1 2 3 Carcharhinidae Carcharhinus fitztroyensis 2 2 2 Sphyrnidae Sphyrna mokarran 2 1 3 Carcharhinidae Negaprion acutidens 3 2 1 Rhynchobatidae Rhina ancylostoma 1 * 3 3 Rhinobatidae Rhinobatos typus 1 * 3 3 Pristidae Anoxypristis cuspidata 1 3 3 Hemiscylliidae Chiloscyllium punctatum 1 * 3 3 Hemiscylliidae Hemipristis elongatus 2 2 3 Carcharhinidae Rhizoprionodon acutus 1 3 3 Stegostomatidae Stegostoma fasciatum 2 * 2 3 Sphyrnidae Sphyrna lewini 1 * 2 3 Myliobatidae Aetomyleus nichofii 1 * 3 3 Carcharhinidae Carcharhinus macloti 2 2 3 Carcharhinidae Carcharhinus sorrah 1 3 3 Dasyatidae Pastinachus sephen 3 1 3 Carcharhinidae Galeocerdo cuvier 2 1 3 Dasyatidae Himantura sp. A 1 * 3 3 Carcharhinidae Carcharhinus tilstoni 1 3 3 Carcharhinidae Carcharhinus dussumieri 3 2 3 Dasyatidae Dasyatis kuhlii 2 3 3 Dasyatidae Dasyatis leylandi 2 3 3 Ginglymostomatidae Nebrius ferrugineus 3 2 3 Sphyrnidae Eusphyra blochii 2 3 3 Gymnuridae Gymnura australis 3 3 3 Dasyatidae Himantura toshi 3 3 3 Carcharhinidae Rhizoprionodon taylori 3 3 3 Hemiscylliidae Hemigaleus microstoma 2 3 3 Criteria Annual Mortality fecundity index Recovery Family Species (2) (1) ranking Dasyatidae Dasyatis brevicaudatus 1 2 1.08 Pristidae Pristis pectinata 2 1 * 1.17 Pristidae Pristis clavata 1 * 1 * 1.25 Myliobatidae Aetomylaeus vespertilio 1 * 2 1.33 Dasyatidae Taeniura meyeni 1 1 1.50 Dasyatidae Himantura jenkinsii 1 * 1 1.50 Carcharhinidae Carcharhinus amboinensis 2 2 1.50 Carcharhinidae Carcharhinus leucas 1 1 1.50 Scyliorhinidae Galeus sp. A 1 * 1 * 1.50 Narcinidae Narcine westraliensis 1 * 1 * 1.50 Pristidae Pristis microdon 1 * 1 * 1.50 Squatinidae Squatina sp. A 1 * 1 * 1.50 Carcharhinidae Prionace glauca 3 1 * 1.58 Myliobatidae Aetobatus narinari 1 2 1.58 Carcharhinidae Carcharhinus limbatus 1 2 1.58 Carcharhinidae Carcharhinus albimargi- natus 2 1 * 1.67 Carcharhinidae Carcharhinus brevipinna 2 1 * 1.67 Dasyatidae Dasyatis thetidis 1 * 1 1.75 Pristidae Pristis zijsron 1 * 1 * 1.75 Dasyatidae Himantura fai 1 * 1 * 1.75 Dasyatidae Himantura granulata 1 * 1 * 1.75 Dasyatidae Himantura uarnak 1 * 1 1.75 Dasyatidae Himantura undulata 1 * 1 1.75 Orectolobidae Orectolobus ornatus 1 * 1 * 1.75 Dasyatidae Urogymnus asperrimus 1 1 1.75 Scyliorhinidae Atelomycterus fasciatus 1 1 1.75 Dasyatidae Dasyatis sp. A 1 * 1 * 1.75 Dasyatidae Amphotistius annotata 1 * 2 1.83 Rhynchobatidae Rhynchobatus djiddensis 1 * 2 1.83 Carcharhinidae Carcharhinus fitztroyensis 1 2 1.83 Sphyrnidae Sphyrna mokarran 2 * 1 1.92 Carcharhinidae Negaprion acutidens 2 1 * 1.92 Rhynchobatidae Rhina ancylostoma 1 * 1 2.00 Rhinobatidae Rhinobatos typus 1 * 1 2.00 Pristidae Anoxypristis cuspidata 1 1 * 2.00 Hemiscylliidae Chiloscyllium punctatum 1 * 1 2.00 Hemiscylliidae Hemipristis elongatus 1 1 2.00 Carcharhinidae Rhizoprionodon acutus 1 1 2.00 Stegostomatidae Stegostoma fasciatum 1 * 1 2.00 Sphyrnidae Sphyrna lewini 2 * 2 * 2.00 Myliobatidae Aetomyleus nichofii 1 2 2.08 Carcharhinidae Carcharhinus macloti 1 2 2.08 Carcharhinidae Carcharhinus sorrah 1 * 2 2.08 Dasyatidae Pastinachus sephen 1 * 2 2.08 Carcharhinidae Galeocerdo cuvier 3 2 2.17 Dasyatidae Himantura sp. A 1 * 3 2.17 Carcharhinidae Carcharhinus tilstoni 1 3 2.17 Carcharhinidae Carcharhinus dussumieri 1 1 2.25 Dasyatidae Dasyatis kuhlii 1 1 * 2.25 Dasyatidae Dasyatis leylandi 1 * 1 * 2.25 Ginglymostomatidae Nebrius ferrugineus 2 1 * 2.42 Sphyrnidae Eusphyra blochii 2 2 2.50 Gymnuridae Gymnura australis 1 2 2.58 Dasyatidae Himantura toshi 1 2 2.58 Carcharhinidae Rhizoprionodon taylori 1 2 2.58 Hemiscylliidae Hemigaleus microstoma 2 3 2.58 Table 1 The surveys that contributed to the estimate of the removal rate (*), total biomass, and within-net survival for elasmobranch species in the Northern Prawn Fishery, Australia. # indicates the surveys whose data contributed to the list of bycatch species. Year Month Type Gear 1998 * (#) Sep-Oct research survey Florida flyer 1997 * (#) Oct research survey Florida flyer 1997 * (#) Sep-Oct scientific observer Florida flyer 1997 * (#) Aug-Oct crew member observer Florida flyer 1997 * (#) May-Jun scientific observer Florida flyer 1997 * (#) Feb-Mar research survey Florida flyer, Engels 1996 * (#) Sep scientific observer Florida flyer 1995 (#) Jun research survey Florida flyer 1995 (#) Oct-Nov research survey Florida flyer 1995 (#) Feb-Mar research survey Florida Flyer 1994 (#) Nov research survey Florida flyer 1994 (#) Jul research survey Florida flyer 1994 (#) Ma research survey Florida flyer 1994 (#) Mar research survey Florida flyer 1993 (#) Nov research survey Florida flyer 1993 (#) Oct research survey Florida Flyer 1993 (#) Aug research survey Florida Flyer 1993 Jan-Feb research survey Engels, Frank and Bryce 1991 Nov research survey Frank and Bryce 1990 Nov-Dec research survey Frank and Bryce No. No of of nets Year trawls used Reference 1998 * (#) 366 1 Stobutzki et al. (5) 1997 * (#) 424 1 Stobutzki et al. (2001b) 1997 * (#) 60 2 Stobutzki et al. (2001b) 1997 * (#) 141 2 Stobutzki et al. (5) 1997 * (#) 76 2 Stobutzki et al. (2001b) 1997 * (#) 248 1 Stobutzki et al. (2001b) 1996 * (#) 83 2 Stobutzki et al. (2001b) 1995 (#) 38 1 Blaber et al. (2) 1995 (#) 39 1 Blaber et al. (2) 1995 (#) 39 1 Blaber et al. (2) 1994 (#) 7 2 Crocos and Coman (1997); Crocos et al. (3) 1994 (#) 7 2 Crocos and Coman (1997); Crocos et al. (3) 1994 (#) 4 2 Crocos and Coman (1997); Crocos et al. (3) 1994 (#) 5 2 Crocos and Coman (1997); Crocos et al. (3) 1993 (#) 81 1 Crocos and Coman 1997; Crocos et al. (3) 1993 (#) 5 2 Blaber et al. (2) 1993 (#) 9 2 Crocos and Coman (1997; Crocos et al. (3) 1993 71 1 Milton et al. (1995) 1991 62 1 Milton et al. (1995) 1990 128 1 Blaber et al. (1994); Milton et al. (1995) Table 2 The criteria used to assess 1) the relative susceptibility of bycatch species to capture and mortality due to prawn trawls and 2) their recovery capacity after depletion due to trawling. These combine to provide the ranks for the axes in Figure 2. For each criterion the definition of the three ranks is given, as well as the weighting score and the percentage of species for which species-specific information was used to rank them. Species- Rank specific Criteria Weight information (%) 1 Susceptibility Water column 3 100 Demersal or benthic position Survival 3 18 Probability of survival <33% Range 2 71 Species range [less than or equal to] 3 fishery regions Day and night 2 32 Higher catch rate catchability at night Diet 2 55 Known to, or capable of, feeding on commer- cial prawns or benthic organisms Depth range 1 100 Less than 60 m Recovery Probability of 3 42 Probability of breeding breeding before cap- ture <50% Maximum size 3 100 Maximum disc width >1755 mm Maximum total length >4781 mm Removal rate 3 79 Removal rate >66% Annual 1 52 Annual fecundity fecundity [less than or equal to] 5 young per year Mortality 1 64 mortality index >3.47 index Rank Criteria 2 3 Susceptibility Water column Not applicable Benthopelagic or position pelagic Survival Probability of survi- Probability of survival val between 33% and >66% 66%, inclusive Range 3 fishery regions Species range < species range >6 fishery regions [less than or equal to] 6 fishery regions Day and night No difference between Higher catch rate catchability night and day at day Diet Not applicable Feed on pelagic organisms Depth range Not applicable Deeper than 60 m Recovery Probability of Probability of Probability of breeding breeding breeding before cap- before capture >50% ture not significantly different from 50% Maximum size 853 mm < maximum Maximum disc width disc width [less than [less than or equal to] or equal to] 1755 mm 853 mm 1861 mm < maximum Maximum total length total length [less than [less than or equal to] or equal to] 4781 mm 1861 mm Removal rate 33% < removal rate 33% [less than or equal [less than or equal to] to] removal rate 66% Annual 5 young per year Annual fecundity fecundity < annual fecundity >19 young per year [less than or equal to] 19 young per year Mortality 0.92 < mortality index mortality index [less index [less than or equal to] than or equal to] 0.92 3.47 Table 3 The elasmobranch species that are known to occur in the region of the northern prawn fishery (NPF), Australia, and of these species, those that have been recorded in NPF bycatch (Table 1). The labels in parentheses refer to the species abbreviations in Figure 2. Recorded in bycatch Family Yes Carcharhinidae Carcharhinus albimarginatus (Cal) Carcharhinus amboinensis (Cam) Carcharhinus brevipinna (Cb) Carcharhinus dussumieri (Cd) Carcharhinus fitztroyensis (Cf) Carcharhinus leucas (Cle) Carcharhinus limbatus (Cli) Carcharhinus macloti (Cm) Carcharhinus sorrah (Cs) Carcharhinus tilstoni (Ct) Galeocerdo cuvier (Gc) Negaprion acutidens (Na) Prionace glauca (Pg) Rhizoprionodon acutus (Rac) Rhizoprionodon taylori (Rta) Dasyatidae Amphotistius annotata (Aa) Dasyatis brevicaudatus (Db) Dasyatis leylandi (Dl) Dasyatis kuhlii (Dk) Dasyatis sp. A (Dsa) Dasyatis thetidis (Dt) Himantura fai (Hf) Himantura granulata (Hg) Himantura jenkinsii (Hj) Himantura sp. A (Hsa) Himantura toshi (Ht) Himantura uarnak (Hua) Himantura undulata (Hun) Pastinachus sephen (Ps) Taeniura meyeni (Tm) Urogymnus asperrimus (Ua) Ginglymostomatidae Nebrius ferrugineus (Nf) Gymnuridae Grymnura australis (Ga) Hemigaleidae Hemigaleus microstoma (Hm) Hemipristis elongatus (He) Hemiscylliidae Chiloscyllium punctatum (Cp) Mobulidae Myliobatidae Aetobatus narinari (Ana) Aetomylaeus vespertilio (Av) Aetomyleus nichofii (Ani) Narcinidae Narcine westraliensis (Nw) Orectolobidae Orectolobus ornatus (Oo) Pristidae Anoxypristis cuspidata (Ac) Pristis clavata (Pc) Pristis microdon (Pm) Pristis pectinata (Pp) Pristis zijsron (Pz) Scyliorhinidae Atelomycterus fasciatus (Af) Galeus sp. A (Gsa) Sphyrnidae Eusphyra blochii (Eb) Sphyrna lewini (Sl) Sphyrna mokarran (Sm) Squatinidae Squatina sp. A (Ssa) Stegostomatidae Stegostoma fasciatum (Sf) Rhincodontidae Rhinobatidae Rhinobatos typus (Rty) Rhynchobatidae Rhynchobatus djiddensis (Rd) Rhina ancylostoma (Ran) Recorded in bycatch Family No Carcharhinidae Carcharhinus amblyrhynchoides Carcharhinus amblyrhynchos Carcharhinus cautus Carcharhinus obscurus Carcharhinus plumbeus Carcharias taurus Carcharinus falciformis Carcharinus melanopterus Loxodon macrorhinus Rhizoprionodon oligolinx Triaenodon obesus Dasyatidae Dasyatis fluviorum Taeniura lymma Ginglymostomatidae Gymnuridae Hemigaleidae Hemiscyllium ocellatum Hemiscyllium trispeculare Hemiscylliidae Mobulidae Manta birostris Mobula eregoodootenkee Myliobatidae Narcinidae Narcine sp. A 0rectolobidae Eucrossorhinus dasypogon Orectolobus wardi Pristidae Scyliorhinidae Atelomycterus macleayi Sphyrnidae Squatinidae Stegostomatidae Rhincodontidae Rhiniodon typus Rhinobatidae Aptychotrema sp. A Rhynchobatidae Table 4 The percentage of trawls in which species were caught, mean catch rate (SE=standard error), and the percentage of catch contributed by each species. No./[km.sup.2] % of % of Family Species trawls Mean SE catch Carcharhinidae Carcharhinus albimarginatus 0.10 0.58 0.41 0.26 Carcharhinus amboinensis 0.20 0.04 0.04 0.02 Carcharhinus dussumieri 20.57 38.80 4.89 17.54 Carcharhinus fitztroyensis 0.20 0.80 0.40 0.35 Carcharhinus macloti 0.20 0.98 0.50 0.43 Carcharhinus sorrah 1.67 1.47 0.57 0.65 Carcharhinus tilstoni 19.49 44.20 5.98 20.07 Galeocerdo cuvier 0.20 0.01 0.00 <0.01 Negaprion acutides 0.10 0.00 0.00 <0.01 Rhizoprionodon acutus 9.15 10.61 1.63 4.83 Rhizoprionodon taylori 0.10 0.00 0.00 <0.01 Dasyatidae Amphotistius annotata 1.97 1.56 0.41 0.74 Dasyatis kuhlii 2.56 1.48 0.56 0.69 Dasyatis leylandi 15.35 9.44 1.18 4.48 Dasyatis sp. A 0.10 0.00 0.00 <0.01 Dasyatis thetidis 0.49 0.03 0.01 0.01 Himantura fai 0.10 0.01 0.00 <0.01 Himantura granulata 0.20 0.01 0.01 <0.01 Himantura jenkinsii 0.59 2.11 0.77 0.95 Himantura sp. A 2.17 0.11 0.04 0.05 Himantura toshi 17.72 27.85 3.10 12.84 Himantura uarnak 0.98 1.44 0.58 0.70 Himantura undulata 0.89 0.96 0.40 0.43 Pastinachus sephen 3.44 0.69 0.31 0.31 Taeniura meyeni 0.10 0.40 0.28 0.18 Urogymnus asperrimus 0.39 0.40 0.28 0.18 Ginglymostomatidae Nebrius ferrugineus 0.10 0.58 0.41 0.26 Gymnuridae Gymnura australis 5.91 8.02 1.64 3.82 Hemiscylliidae Chiloscyllium punctatum 5.41 11.83 1.96 5.42 Hemigaleus microstoma 9.84 9.64 1.55 4.56 Hemipristis elongatus 0.20 0.02 0.02 0.01 Myliobatidae Aetobatus narinari 0.30 0.60 0.41 0.27 Aetomylaeus nichofii 1.08 1.57 0.61 0.74 Orectolobidae Orectolobus ornatus 0.10 0.52 0.52 0.27 Pristidae Anoxypristis cuspidata 0.98 0.71 0.42 0.32 Pristis zijsron 0.10 0.02 0.02 0.01 Rhinobatidae Rhinobatos typus 0.39 0.02 0.01 0.01 Rhynchobatidae Rhina ancylostoma 0.89 0.10 0.04 0.05 Rhynchobatus djiddensis 14.27 30.87 3.39 14.26 Scyliorhinidae Atelomycterus fasciatus 0.49 0.18 0.08 0.08 Sphyrnidae Eusphyra blochii 0.20 0.04 0.04 0.02 Sphyrna lewini 2.95 6.91 1.52 3.07 Sphyrna mokarran 0.39 0.02 0.02 0.01 Stegostomatidae Stegostoma fasciatum 2.17 2.17 1.02 1.10 Table 5 The estimated size (disc width) at first maturity and mean number of pups for ray species (sample size is shown in parentheses; SE=standard error). Size at maturity (mm) Species Male Female Amphotistius annotata 200 (9) 233 (8) Dasyatis kuhlii 300 (10) 378 (6) Dasyatis leylandi 185 (103) 180 (110) Gymnura australis 350 (29) 610 (16) Himantura toshi 400 (31) 660 (21) Number of pups Species Mean SE Amphotistius annotata 1.5 0.7 (2) Dasyatis kuhlii 2 -- (1) Dasyatis leylandi 1.1 0.3 (17) Gymnura australis 3.2 1.2 (6) Himantura toshi 1.5 0.7 (2) Table 6 The mean, minimum (min), and maximum (max) size (TL or DW) of elasmobranch species caught in nighttime prawn trawling. The size at maturity ([L.sub.m]) and at birth (pup size) are shown based on Last and Stevens (1994) or Table 5. The percentage of individuals caught that were mature (% mature) and the sex ratio are also shown. SE = standard error; n = sample size; P is the probability that the mean length at capture is different from [L.sub.m]. Size (mm) Family Species Mean SE Min. Max. Carcharhinidae Carcharhinus 850 -- -- -- albimarginatus Carcharhinus 1700 -- -- -- amboinensis Carcharhinus 636 6 270 850 dussumieri Carcharhinus 1045 225 820 1270 fitzroyensis Carcharhinus 745 75 670 820 macloti Carcharhinus 542 43 300 950 sorrah Carcharhinus 794.3 9 100 1950 tilstoni Galeocerdo cuvier 1175 285 890 1460 Negaprion 2600 -- -- -- acutidens Rhizoprionodon 689 14 280 960 acutus Rhizoprionodon 546 -- -- -- taylori Dasyatidae Amphotistius 211.4 12 140 452 annotata Dasyatis kuhlii 297.3 12 190 400 Dasyatis leylandi 182.2 3 110 400 Dasyatis sp. A 350 -- -- -- Dasyatis thetidis 1162 129 800 1420 Himantura fai 1900 -- -- -- Himantura 960 -- -- -- granulata Himantura 890 150 300 1140 jenkinsii Himantura sp. A 350 65 80 1800 Himantura toshi 456 11 150 1330 Himantura uarnak 1055 132 290 1600 Himantura undulata 1117 131 400 1500 Pastinachus sephen 1076 53 450 2000 Taeniura meyeni 1300 -- -- -- Urogymnus 850 106 530 1150 asperrimus Ginglymostomatidae Nebrius 2400 -- -- -- ferrugineus Gymnuridae Gymnura australis 462 19 120 860 Hemigaleidae Hemigaleus 609 19 250 950 microstoma Hemipristis 1340 190 1150 1530 elongata Hemiscylliidae Chiloscyllium 668 23 230 1000 punctatum Myliobatidae Aetobatus narinari 625 125 500 750 Aetomylaeus 437 42 240 720 nichofii Pristidae Anoxypristis 1930 193 1240 2550 cuspidata Rhinobatidae Rhinobatos typus 1953 188 1500 2340 Rhynchobatidae Rhina ancylostoma 1643 112 1010 2090 Rhynchobatus 869 36 230 2650 djiddensis Scyliorhinidae Atelomycterus 300 0 300 300 fasciatus Sphyrnidae Eusphyra blochii 990 320 670 1310 Sphyrna lewini 832 54 400 2400 Sphyrna mokarran 1780 457 400 2400 Stegostomatidae Stegostoma 1305 82 400 2000 fasciatum Sex ratio Family Species n F:M n [L.sub.m] Carcharhinidae Carcharhinus 1 -- -- 1700 albimarginatus Carcharhinus 1 -- -- 2100 amboinensis Carcharhinus 377 1.08 139 700 dussumieri Carcharhinus 2 -- -- 800 fitzroyensis Carcharhinus 2 -- -- 690 macloti Carcharhinus 25 3.03 16 900 sorrah Carcharhinus 344 0.95 84 1200 tilstoni Galeocerdo cuvier 2 all M 1 3000 Negaprion 1 -- -- 2200 acutidens Rhizoprionodon 140 0.56 81 750 acutus Rhizoprionodon 1 all F 1 400 taylori Dasyatidae Amphotistius 25 1.43 3 200 annotata Dasyatis kuhlii 24 4.00 10 300 Dasyatis leylandi 206 1.05 162 180 Dasyatis sp. A 1 -- -- 360 Dasyatis thetidis 5 all M 1 -- Himantura fai 1 -- -- -- Himantura 1 -- -- -- granulata Himantura 5 all M 1 -- jenkinsii Himantura sp. A 57 all F 12 -- Himantura toshi 235 4.17 52 400 Himantura uarnak 12 all F 2 -- Himantura undulata 7 all F 1 -- Pastinachus sephen 43 3.03 12 -- Taeniura meyeni 1 -- -- -- Urogymnus 5 all M 2 -- asperrimus Ginglymostomatidae Nebrius 1 -- -- 2250 ferrugineus Gymnuridae Gymnura australis 87 2.00 42 350 Hemigaleidae Hemigaleus 152 0.68 91 600 microstoma Hemipristis 2 all F 1 1100 elongata Hemiscylliidae Chiloscyllium 63 2.50 7 700 punctatum Myliobatidae Aetobatus narinari 2 all F 1 -- Aetomylaeus 11 all F 3 -- nichofii Pristidae Anoxypristis 8 all F 1 -- cuspidata Rhinobatidae Rhinobatos typus 4 -- -- -- Rhynchobatidae Rhina ancylostoma 9 0.33 3 -- Rhynchobatus 187 4.76 35 1100 djiddensis Scyliorhinidae Atelomycterus 2 -- -- 320 fasciatus Sphyrnidae Eusphyra blochii 2 -- -- 1080 Sphyrna lewini 37 all F 3 1400 Sphyrna mokarran 3 1.00 2 2100 Stegostomatidae Stegostoma 26 0.80 3 1700 fasciatum Pup % size Family Species P mature (mm) Carcharhinidae Carcharhinus -- 100 550 albimarginatus Carcharhinus -- 0 600 amboinensis Carcharhinus <0.001 41 350 dussumieri Carcharhinus >0.5 50 500 fitzroyensis Carcharhinus >0.5 20 450 macloti Carcharhinus <0.001 8 500 sorrah Carcharhinus <0.001 0.6 600 tilstoni Galeocerdo cuvier >0.1 0 500 Negaprion -- -- 500 acutidens Rhizoprionodon <0.001 54 -- acutus Rhizoprionodon -- 100 250 taylori Dasyatidae Amphotistius >0.5 24 -- annotata Dasyatis kuhlii >0.5 8 160 Dasyatis leylandi >0.2 46 110 Dasyatis sp. A -- 0 -- Dasyatis thetidis -- -- 350 Himantura fai -- -- 550 Himantura -- -- 280 granulata Himantura -- -- -- jenkinsii Himantura sp. A -- -- -- Himantura toshi <0.001 12 200 Himantura uarnak -- -- 280 Himantura undulata -- -- 200 Pastinachus sephen -- -- 180 Taeniura meyeni -- -- 350 Urogymnus -- -- -- asperrimus Ginglymostomatidae Nebrius -- 100 400 ferrugineus Gymnuridae Gymnura australis <0.001 24 -- Hemigaleidae Hemigaleus >0.5 47 300 microstoma Hemipristis >0.5 50 520 elongata Hemiscylliidae Chiloscyllium <0.001 52 170 punctatum Myliobatidae Aetobatus narinari -- -- 260 Aetomylaeus -- -- 170 nichofii Pristidae Anoxypristis -- -- -- cuspidata Rhinobatidae Rhinobatos typus -- -- -- Rhynchobatidae Rhina ancylostoma -- -- -- Rhynchobatus <0.001 8 -- djiddensis Scyliorhinidae Atelomycterus -- 0 -- fasciatus Sphyrnidae Eusphyra blochii >0.5 50 450 Sphyrna lewini <0.001 3 450 Sphyrna mokarran >0.5 33 650 Stegostomatidae Stegostoma <0.001 23 200 fasciatum Table 7 The percentage of elasmobranchs that died within the trawl net, recorded on research and crew-member observer surveys. "Combined sharks" refers to all species where total length was recorded; "combined rays" refers to all species where disc width was recorded; n = number of specimens measured. % dead Family Taxa Female n Male combined sharks 23 149 66 combined rays 56 360 67 Carcharhinidae Carcharhinus dussumieri 48 207 58 Carcharhinus sorrah 73 15 50 Carcharhinus tilstoni 78 40 85 Rhizoprionodon acutus 75 44 86 Dasyatidae Dasyatis leylandi 27 22 95 Himantura toshi 43 40 78 Gymnuridae Gymnura australis 31 26 75 Hemigaleidae Hemigaleus microstoma 44 29 64 Rhynchobatidae Rhynchobatus djiddensis 21 24 20 % dead Family Taxa n Overall n combined sharks 59 61 639 combined rays 279 40 208 Carcharhinidae Carcharhinus dussumieri 114 52 321 Carcharhinus sorrah 8 65 23 Carcharhinus tilstoni 33 82 73 Rhizoprionodon acutus 72 82 116 Dasyatidae Dasyatis leylandi 19 59 41 Himantura toshi 18 53 58 Gymnuridae Gymnura australis 8 41 34 Hemigaleidae Hemigaleus microstoma 39 62 68 Rhynchobatidae Rhynchobatus djiddensis 5 10 59 Table 8 The correlations between the criteria on 1) the susceptibility axis and 2) the recovery axis. * indicates significance at P < 0.05. Day and night Depth Susceptibility Survival Range catchability Diet range Water column position 0.07 -0.18 0.13 0.67 * 0.07 Survival 0.48 * 0.07 -0.11 -0.00 Range 0.25 0.06 0.09 Day and night catchability -0.15 0.04 Diet 0.05 Maximum Removal Annual Mortality Recovery size rate fecundity index Probability of breeding 0.07 0.25 0.16 0.08 Maximum size -0.22 -0.25 0.17 Removal rate -0.27 0.21 Annual fecundity 0.12 Table 9 The mean size of elasmobranch species caught in nets with codends fitted with TEDs and with standard codends, and the ANOVA results from the comparison of these nets. "Combined sharks" refers to all species where total length was recorded; "combined rays" refers to all species where disc width was recorded. SE = standard error; n = number of trawls. Size (mm) Species Codend mean SE n Combined sharks standard 887 59 168 TED 596 36 269 Combined rays standard 330 19 157 TED 286 7 414 Carcharhinus dussumieri standard 844.4 96.4 60 TED 489.1 13.8 81 Rhizoprionodon acutus standard 636.9 34.2 45 TED 724.2 17.4 91 Hemigaleus microstoma standard 708.9 148.5 23 TED 508.0 19.9 58 Amphotistius annotata standard 206.4 31.1 50 TED 169.8 3.4 156 Himantura toshi standard 371.0 19.4 51 TED 351.6 9.7 151 Rhynchobatus djiddensis standard 1076.9 125.6 19 TED 611.6 32.3 24 ANOVA results Species Codend F df P Combined sharks standard 4.25 1,569 0.0398 TED Combined rays standard 26.77 1,435 <0.0001 TED Carcharhinus dussumieri standard 26.88 1,139 <0.0001 TED Rhizoprionodon acutus standard 7.15 1,134 0.0084 TED Hemigaleus microstoma standard 2.77 1,79 0.0988 TED Amphotistius annotata standard 2.97 1,200 0.4395 TED Himantura toshi standard 0.60 1,200 0.4395 TED Rhynchobatus djiddensis standard 20.81 1,41 <0.0001 TED Figure 1 The management area of the northern prawn fishery (NPF) and the bioregions defined through the interim marine and coastal rationalization (IMCR) process (Thackway and Cresswell, 1998). The shaded area represents the regions fished by commercial prawn trawlers. The dots mark the positions of the trawls that were sampled to estimate the removal rates and total biomass of bycatch species (Table 1). The numbers refer to the bioregions (1=Oceanic Shoals, 2=Tiwi, 3=Cobourg, 4=Arnhem Wessel, 5=Arafura, 6=Groote, 7=Pellew, 8=Wellesley, 9=Karumba-Nassau, 10=West Cape York, 11=Carpentaria). Bioregion Area Effort ([km.sup.2]) (Boat days) 1 253,343 198 2 5,134 45 3 8,380 97 4 22,752 21 5 155,114 92 6 16,717 2,909 7 21,494 558 8 26,771 2,195 9 56,700 1,524 10 22,269 2,846 11 229,974 2,349 [ILLUSTRATION OMITTED]
We thank the commercial fishermen who took the time to record their catch in logbooks and support this project. The skippers and crews of the boats on which our scientific observer sampled are also thanked for their cooperation and patience. We also thank the Northern Prawn Fishery Management Advisory Committee for its support; the Northern Prawn Fishery, Fishery Assessment Group for their participation in this process and feedback; C. Rose for collection of the crew-member observer data; and T. Walker for valuable comments on the manuscript. CSIRO Marine Research colleagues helped in many ways, M. Haywood, F. Manson and J. Bishop for assistance with the bioregion data and commercial effort data, C. Burridge and W. Venables for assistance with the statistical analysis; S. Blaber, G. Fry, D. Milton, J. Salini, J. Stevens and T. Wassenberg for constructive comments on the manuscript. We also thank the fishing, scientific, and electronics crew of the RV Southern Surveyor who made the research surveys possible. This project was funded by the Australian Fisheries Research and Development Corporation (Project no. 96/257) and CSIRO Marine Research.
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Manuscript accepted 29 May 2002. Fish. Bull. 100:800-821 (2002).
Ilona C. Stobutzki Margaret J. Miller Don S. Heales David T. Brewer CSIRO Marine Research PO Box 120 Cleveland Queensland 4163, Australia E-mail address: ilona.stobutzki@.csiro.au
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|Author:||Stobutzki, Ilona C.; Miller, Margaret J.; Heales, Don S.; Brewer, David T.|
|Date:||Oct 1, 2002|
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