Species compositions of elasmobranchs caught by three different commercial fishing methods off southwestern Australia, and biological data for four abundant bycatch species.
Article Type: Report
Subject: Trawling (Research)
Longlining (Fisheries) (Research)
Gillnetting (Research)
Chondrichthyes (Research)
Authors: Jones, Ashlee A.
Hall, Norman G.
Potter, Ian C.
Pub Date: 10/01/2010
Publication: Name: Fishery Bulletin Publisher: National Marine Fisheries Service Audience: Academic Format: Magazine/Journal Subject: Zoology and wildlife conservation Copyright: COPYRIGHT 2010 National Marine Fisheries Service ISSN: 0090-0656
Issue: Date: Oct, 2010 Source Volume: 108 Source Issue: 4
Topic: Event Code: 310 Science & research
Geographic: Geographic Scope: Australia Geographic Code: 8AUST Australia
Accession Number: 243451473
Full Text: Abstract--Commercial catches taken in southwestern Australian waters by trawl fisheries targeting prawns and scallops and from gillnet and longline fisheries targeting sharks were sampled at different times of the year between 2002 and 2008. This sampling yielded 33 elasmobranch species representing 17 families. Multivariate statistics elucidated the ways in which the species compositions of elasmobranchs differed among fishing methods and provided benchmark data for detecting changes in the elasmobranch fauna in the future. Virtually all elasmobranchs caught by trawling, which consisted predominantly of rays, were discarded as bycatch, as were approximately a quarter of the elasmobranchs caught by both gillnetting and longlining. The maximum lengths and the lengths at maturity of four abundant bycatch species, Heterodontus portusjacksoni, Aptychotrema vincentiana, Squatina australis, and Myliobatis australis, were greater for females than males. The [L.sub.50] determined for the males of these species at maturity by using full clasper calcification as the criterion of maturity did not differ significantly from the corresponding [L.sub.50] derived by using gonadal data as the criterion for maturity. The proportions of the individuals of these species with lengths less than those at which 50% reach maturity were far greater in trawl samples than in gillnet and longline samples. This result was due to differences in gear selectivity and to trawling being undertaken in shallow inshore waters that act as nursery areas for these species. Sound quantitative data on the species compositions of elasmobranchs caught by commercial fisheries and the biological characteristics of the main elasmobranch bycatch species are crucial for developing strategies for conserving these important species and thus the marine ecosystems of which they are part.

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The impact of commercial fisheries on the populations of sharks and rays has, in recent years, become an issue of international concern (Stevens et al., 2000; Walker et al., 2005). It is important to recognize, however, that elasmobranchs are not only targeted by certain fisheries, but also comprise a substantial component of the bycatch of commercial fisheries, such as those employing trawl nets, gillnets, and longlines (Stevens et al., 2000; Stobutzki et al., 2001; Walker, 2005a). An assessment of the impacts of commercial fishing on the elasmobranchs taken as bycatch is hindered by the fact that most of that catch is typically reported as "unidentified shark" or "mixed fish," or not reported at all (Walker, 2005a). The numbers of sharks and rays taken as bycatch by commercial fisheries may, in some cases, exceed those of the targeted species, and many of those individuals die either during capture or after they are discarded (Bonfil, 1994; Stobutzki et al., 2002; Walker, 2005a). Although, in most studies of commercial fisheries, nontargeted species are referred to as bycatch, Walker et al. (2005) emphasized that some of those species are usually retained and thus constitute byproduct, whereas the others are usually discarded and therefore constitute bycatch in the strict sense.

In an assessment of various commercial fisheries throughout the world, it was shown that trawl fisheries targeting prawns generate the largest amount of bycatch (Cook, 2003) and that the mortality of individuals in that bycatch is substantial (Bonfil, 1994). Indeed, it has been estimated that approximately two thirds of the elasmobranchs caught as bycatch in Australia's northern prawn trawl fishery die while in the net (Stobutzki et al., 2002). Furthermore, that study demonstrated that most of these individuals are small, and more than half are immature and some are caught immediately after birth.

Many elasmobranchs are at or near the apex of marine food webs and thus their removal can have a significant impact on the trophic structure of an ecosystem (Camhi et al., 1998; Stevens et al., 2000; Shepard and Myers, 2005). Furthermore, certain biological characteristics of elasmobranchs, such as their long life spans, low fecundities, and late ages at maturity, limit their ability both to withstand fishing pressure, either when targeted or caught incidentally, and to recover from overexploitation (Stevens et al., 2000; Walker, 2005a; Gallucci et al., 2006). In general, the populations of endemic species or those that have localized distributions tend to be most prone to overfishing (Stevens et al., 2000). Moreover, there are little or no data on the reproductive biology of most bycatch species. Such data are required for determining the resilience of these species to fishing pressure, thereby enabling the development of management plans for conserving their populations (Frisk et al., 2001; Stobutzki et al., 2002; Walker, 2005b).

Most of the species in Australia's rich diversity of elasmobranchs are endemic and occupy demersal habitats on the continental shelf or slope (Last and Stevens, 2009) and are thus potentially prone to depletion by demersal fishing methods such as trawling, gillnetting, and longlining (Stobutzki et al., 2001, 2002; Coelho et al., 2003; Perez and Wahrlich, 2005, Walker et al., 2005). Analyses of fisheries data for 1994 to 1999 showed that the bycatch species Heterodontus portusjacksoni (Heterodontidae) and Myliobatis australis (Myliobatidae) are abundant in catches of the temperate demersal gillnet and longline fisheries of southwestern Australia (McAuley and Simpfendorfer, 2003). These two species and the rhinobatid Aptychotrema vincentiana and the squatinid Squatina australis, which are also caught as bycatch by commercial fisheries, collectively contributed as much as 17% to the total biomass of the 172 species of fish caught during extensive trawling along the lower west coast of Australia (Hyndes et al., 1999). It has been estimated that approximately half of the elasmobranchs taken as bycatch by commercial trawlers in this region are likely to die during or after capture (Laurenson et al. (1)). Despite the potentially detrimental effects of commercial fishing on the above four elasmobranch species and their ecological importance in the temperate waters of southern Australia, sound biological data have been collected only for H. portusjacksoni (McLaughlin and O'Gower, 1971; Tovar-Avila et al., 2007; Jones et al., 2008; Powter and Gladstone, 2008).

The families to which the above four species belong are represented elsewhere in the world by species that are taken in substantial numbers as bycatch. For example, in the eastern Pacific, up to a thousand individuals of the heterodontid shark Heterodontus mexicanus may be caught in a single gillnet set, many of which die (Garayzar, 2006). In some parts of the world, the populations of several species of rhinobatid and squatinid have been so drastically depleted from overfishing that they have been listed as critically endangered (Morey et al., 2006; Lessa and Vooren, 2007). In eastern Australia, the commercial landings of Myliobatis australis have been steadily increasing to the point where their stocks now need to be monitored (White et al., 2006).

The first aim of the present study was to determine the numbers, and thereby the percent contributions, of the females and males of each shark and ray species in samples of commercial catches taken in southwestern Australian waters by demersal trawling for prawns and scallops and by demersal gillnetting and longlining for sharks. These data enabled the percent contributions made by the bycatch and byproduct to the total elasmobranch catch to be estimated for each fishing method. The percent contributions of each species to the catches obtained by trawling, gillnetting, and longlining were then used to compare statistically the species compositions of the elasmobranchs in the catches taken by each of these fishing methods.

Emphasis was next placed on determining the length compositions of the bycatch species H. portusjacksoni, A. vincentiana, S. australis, and M. australis in samples collected by trawling, gillnetting, and longlining and on estimating the lengths at maturity of the last three of these ecologically important species. The lengths of both the females and males at which 50% are mature ([L.sub.50]) were first determined by using gonadal data as the criterion for determining maturity and then, in the case of males, by employing full clasper calcification as that criterion. The [L.sub.50] calculated for the males of each species by using the two maturity criteria were then compared to ascertain whether the [L.sub.50] derived by using clasper calcification as the index of maturity is a reasonable proxy for that derived using gonadal stage as that criterion. The [L.sub.50] at maturity of A. vincentiana, S. australis, and M. australis and of H. portusjacksoni (Jones et al., 2008) were then employed to determine the proportions of both sexes of each of these four species that were caught by each fishing method before they had the opportunity to reproduce. Finally, the management and conservation implications of our data are considered.

Materials and methods

Sampling regime

The elasmobranchs were examined in commercial catches from 69 demersal trawls, 24 demersal gillnets, and 19 longline sets of fishing vessels operating in southwestern Australian waters southwards of 32[degrees] Slat. on the west coast and then eastwards to 118[degrees] E long. on the south coast (Fig. 1). Trawling targets the western king prawn (Melicertus latisulcatus) and the Ballot's saucer scallop (Amusium balloti), whereas demersal gillnetting and longlining target mainly the gummy shark (Mustelus antarcticus), the dusky shark (Carcharhinus obscurus), and the whiskery shark (Furgaleus macki) (McAuley and Simpfendorfer, 2003). Sampling, which was undertaken between November 2002 and November 2008, was designed to ensure that the catches of each fishing method were sampled at least once, and generally on at least three occasions, in each season of the calendar year. The trawl, gillnet, and longline fishermen, whose catches were selected for sampling, were those that fished regularly and readily allowed us onboard, and whose methods were representative of those used in the area that is fished. The numbers of each elasmobranch species caught by each fishing method on each sampling occasion were recorded, as also were the sexes of all individuals except for those of a few species in a small number of trawl samples when the catches of those species were particularly large, in which case the sexes of each individual in a large, randomly selected subsample were recorded. Each elasmobranch species caught by gillnetting and longlining was categorized as either targeted, byproduct, or bycatch, whereas those taken by trawling, where prawns and scallops are the target species, were categorized as either byproduct or bycatch.

Trawling was undertaken mainly at depths of 8 to 13 m in a marine embayment on the lower west coast of Western Australia and, to a lesser extent, at depths [less than or equal to]32 m and at distances within 20 km from the mainland along that coast. The trawl net, in which the codend consisted of 45-mm mesh, was towed for 60-180 min at a speed of ~6.5 km/h. Commercial gillnet fishermen deployed up to 7000 m of either 165- or 178-mm stretched mesh net that was set for up to 24 hours at depths of 24-73 m, whereas longlines, consisting of 360 hooks attached to approximately 6400 m of mainline, were set for an average of 3 hours at depths of 65-73 m.

Multivariate analysis

The square root of the percent contribution of the number of each elasmobranch species to the total catch of all elasmobranchs recorded in each sample during regular onboard observations of the catches taken by each fishing method was used to construct a Bray-Curtis similarity matrix, which was then subjected to nonmetric multidimensional scaling (nMDS). One-way analysis of similarities (ANOSIM) was used to test whether the species compositions of the elasmobranch catches taken by the three fishing methods were significantly different and, if so, pair-wise ANOSIM tests were used to test for differences between the compositions of the elasmobranchs obtained by each pair of methods. The R-statistic value was then employed to ascertain the extent of any differences between the compositions of those catches (Clarke, 1993). R-statistic values approaching 1 demonstrate that the species composition of the a priori groups differed markedly, and a value of approximately 0 indicates that the species compositions of those groups are very similar. Similarity percentages (SIMPER; Clarke, 1993) were used to identify the species that typify the samples obtained by each fishing method and which species are responsible for discriminating between the samples caught by each pair of methods. The ordination and associated tests were undertaken with the PRIMER vers. 6 statistical package (Clarke and Gorley, 2006).

Length and weight measurements

A randomly selected subset of individuals collected during regular onboard observations, together with additional randomly selected individuals provided by commercial fishermen, yielded a total of 516 H. portusjacksoni, 340 A. vincentiana, 362 S. australis, and 218 M. australis, which were brought to the laboratory and processed. The data derived from the samples were used to construct length-frequency histograms and to derive sex ratios and reproductive data for the species. The sex of each individual was recorded and the total length (TL--tip of snout to tip of tail) and total weight (W) of each H. portusjacksoni, A. vincentiana and S. australis were measured and weighed to the nearest 1 mm and 1 g, respectively. The disc length (DL-- tip of snout to the junction of the tail and pelvic fins) of each M. australis was measured to the nearest 1 mm and the disc width (DW) and weight of each fully intact individual were recorded to the nearest 1 mm and 1 g, respectively. The relationships between DW and DL, DL and DW, and between the natural logarithms of W and DL and of W and DW for intact M. australis were calculated by using least trimmed squares (LTS) regression. A resampling test was used to demonstrate that the above data for the two sexes could be pooled. The above relationships were used to estimate the DW and W of the small number of M. australis (~15%) whose pectoral fins had been removed by fishermen for commercial sale. Note that, in the results, the sample size (n) and coefficient of determination ([r.sup.2]) refer to the trimmed data for the individuals used for the analyses.

[FIGURE 1 OMITTED]

Length at maturity

The reproductive tracts of the females and males of A. vincentiana, S. australis, and M. australis were assigned to one of the following maturity stages by using the criteria outlined in White et al. (2001). For females, stage 1 = uteri small and thin and oocytes not macroscopically visible; stage 2 = uteri enlarging but still thin and oocytes becoming visible but not yet containing yolk; stage 3 = uteri enlarged and oocytes yolked; stage 4 = pregnant, and stage 5 = uteri or cloaca distended, indicating that parturition had recently occurred. For males, stage 1 = seminal vesicles small and thin and testes not well defined; stage 2 = seminal vesicles enlarging and starting to become coiled, but testes not yet lobed; stage 3 = seminal vesicles tightly coiled and testes lobed; and stage 4 = similar to stage 3, but with semen present in the distal portion of the seminal vesicle. Individuals with reproductive tracts at stages 1 and 2 are regarded as sexually immature, whereas those at stages 3 and 4 and, in females, also at stage 5, can reproduce or have reproduced and are therefore considered mature. When we alternatively employed full clasper calcification as the indicator of maturity, we considered that males with noncalcified and partially calcified claspers were sexually immature, and those with fully calcified claspers were mature because they have the ability to copulate.

The probability (P) that an individual is mature was assumed to be a logistic function of its length (L):

P = [{1+ exp[-([alpha] + [beta]L)]}.sup.-1], (1)

where [alpha] and [beta] are parameters that determine the location and shape of the logistic curve.

The parameters [alpha] and [beta] were transformed to the lengths [L.sub.50] ([TL.sub.50] or [DL.sub.50]) and [L.sub.95] ([TL.sub.95] or [DL.sub.95]) by which 50% and 95% of fish have attained maturity, respectively, using the equations

[L.sub.50] = - [alpha]/[beta], (2)

[L.sub.95] = [[log.sub.e](19)-[alpha]]/[beta]. (3)

The equation for the probability that a fish is mature thus becomes

P=[{1 + exp[-[log.sub.e](19)(L-[L.sub.50])/([L.sub.95]-[L.sub.50])]}.sup.-1]. (4)

Logistic relationships of the above form were derived for females and males of A. vincentiana, S. australis, and M. australis by using gonadal maturity status, i.e., [greater than or equal to]stage 3, as the criterion for maturity and, in addition, for males, employing full clasper calcification as the criterion for maturity. Logistic regression analysis was used to fit these logistic curves by using Solver in Excel (Microsoft, Redmond, WA) to maximize the log-likelihood. We used likelihood-ratio tests (Cerrato, 1990) to compare the [L.sub.50] of the females and males of each species at maturity using gonadal status as the criterion for maturity and to compare the [L.sub.50] derived for males using both gonadal and clasper calcification status as the criteria for maturity. For the likelihood-ratio tests, the hypothesis that the data for both the females and males of each species could be described by a common logistic curve was rejected at the [alpha]=0.05 level of significance if the test statistic, calculated as twice the difference between the log-likelihoods obtained by fitting maturity curves with a common value of [L.sub.50] for both sexes and by fitting separate maturity curves for each sex, exceeded [[chi square].sub.[alpha]](q), where q is the difference between the numbers of parameters in the two approaches. Note that the [L.sub.50] of females and males of H. portusjacksoni at maturity had been determined previously with this approach (Jones et al., 2008). WinBUGS (Lunn et al., 2000) was also used to fit the logistic curves to the maturity-at-length data for the females and males of each species and to calculate the proportion that were mature at length, thereby enabling derivation of the upper and lower confidence intervals for the [L.sub.50] and [L.sub.95] values and for the proportion of mature males and females in each 50-mm length class (see Jones et al. [2008] for further details of this WinBUGS analysis).

Results

Species compositions by fishing method

A total of 4820 individual elasmobranchs, representing 10 families and 22 species of sharks and 7 families and 11 species of rays, were recorded during regular onboard examinations of the catches of commercial trawl, gillnet, and longline vessels operating off the southwestern coast of Australia (Tables 1-3).

The 2986 elasmobranchs caught by prawn and scallop trawling were dominated by rays, which comprised 10 of the 14 species and contributed 87% to the total elasmobranch catch (Table 1). The species of a single family of rays, the Urolophidae, comprising four species and two genera, contributed as much as 67% to the total trawl catch of elasmobranchs. The two species of shark (H. portusjacksoni and S. australis) and the two species of ray (A. vincentiana and M. australis), whose biological characteristics were determined (see later), each contributed between 4.5% and 8% to the total number of elasmobranchs caught by trawling and collectively as much as 25% (Table 1). Two species of shark, M. antarcticus and C. brevipinna, which were caught in very small numbers, were retained and thus constituted byproduct.

Gillnet catches yielded 1260 elasmobranchs, representing 19 species of shark and 6 species of ray, with sharks contributing 96% to the total catch of elasmobranchs (Table 2). The most abundant species in the gillnet catches was a targeted shark, Carcharhinus obscurus, which contributed more than a third to the total elasmobranch catch. The other two targeted species, Mustelus antarcticus and Furgaleus macki, which were dominated by females, ranked third and fifth in terms of abundance, respectively, and contributed an additional 14.2% and 6.5%, respectively (Table 2). The second most numerous species, however, was the shark H. portusjacksoni, a bycatch species, which constituted one-fifth of the total catch. None of the six species of ray caught by gillnetting was abundant in the catches obtained by this method. The byproduct and bycatch species contributed 18.7% and 25.7%, respectively, to the total gillnet catch. Thirteen of these species, which contributed more than one third to the total number of individuals of elasmobranchs caught, were always discarded as bycatch.

The 22 species of elasmobranch caught by longlining were dominated by one of the three targeted species, M. antarcticus, which made up 63% of the total catch of 574 individual elasmobranchs (Table 3). The next four most abundant species, which were all bycatch, consisted of three species of rays and the shark H. portusjacksoni and collectively contributed nearly a quarter of the individual elasmobranchs obtained by longlining. The other two targeted species, C. obscurus and F. macki, contributed only 3.8% and 0.9%, respectively, to the total catch taken by longlining. The 19 nontargeted species caught by longlining comprised eight species that were always discarded as bycatch and represented one quarter of the total catch.

On the ordination plot, derived from the similarity matrix constructed by using percent contributions of the various species to the elasmobranch catches obtained by the three fishing methods, the samples for longlining lie above those for gillnetting, and both of these lie to the right of the discrete group comprising the trawl samples (Fig. 2). One-way ANOSIM confirmed that the compositions of the samples obtained by the three fishing methods were significantly different (P=0.001, global R=0.753). Pair-wise ANOSIM tests revealed that the compositions in the samples collected by each method differed significantly from those in the samples obtained by each other method (all P<0.001), and the R-statistic was similarly high for trawling vs. gillnetting (0.797) and trawling vs longlining (0.774) and greater than that for gillnetting vs longlining (0.515).

The most important of the typifying species for the trawl samples, i.e., those that were most abundant and were found most frequently, comprised two ray species, A. vincentiana and Urolophus paucimaculatus, and two shark species, H. portusjacksoni and S. australis (Table 4). Heterodontus portusjacksoni and M. antarcticus were also important typifying species for the elasmobranch catches taken by both gillnetting and longlining. Carcharhinus obscurus is a particularly important typifying species for the gillnet samples and the same is true for the bycatch ray species Dasyatis brevicaudata for the longline samples. Relatively greater and more consistent numbers of A. vincentiana were particularly important for discriminating between the compositions of the samples caught by trawling and those obtained by both gillnetting and longlining, and greater and more consistent numbers of C. obscurus were especially important for discriminating between the samples taken by gillnetting from those obtained by both trawling and longlining (Table 4). The longline samples were discriminated from those obtained by both trawling and gillnetting by consistently greater numbers of D. brevicaudata.

[FIGURE 2 OMITTED]

Length-frequency compositions of the four selected bycatch species by fishing method

Wide size ranges of H. portusjacksoni, A. vincentiana, and S. australis and, to a certain extent, M. australis, were caught by trawling. However, the lengths of most H. portusjacksoni and M. australis were small and thus lay toward the lower end of their length ranges (Fig. 3). Although gillnetting also caught a broad size range of both H. portusjacksoni and A. vincentiana, it yielded predominantly larger S. australis and medium-size M. australis (Fig. 3). Although longline catches contained a wide size range of H. portusjacksoni and M. australis, they did not include the smallest individuals of these two species and only one of the A. vincentiana caught by this method was small (Fig. 3). No S. australis was caught by longlining.

The H. portusjacksoni obtained by all three fishing methods ranged from 180 to 1300 mm TL (Table 5), the latter length rarely being exceeded by this species throughout its range (Last and Stevens, 2009). The smallest individuals possessed conspicuous umbilical scars and were therefore neonates. The length-frequency distribution of female H. portusjacksoni is trimodal, whereas that of males is bimodal, and these modes correspond to the first two modes of females (Fig. 4). These differences account for the lengths of many females greatly exceeding the maximum length of 815 mm for males (Table 5). The weights of H. portusjacksoni ranged from 39 to 12,250 g (Table 5). The ratio of females to males of H. portusjacksoni differed significantly from parity among all individuals collectively (1 female:0.76 males; [chi square] = 9.46, P < 0.01), but not for juveniles (1 female:l.20 males; [chi square] = 2.65, P > 0.05). Note that, when calculating the sex ratios for juveniles, the term juvenile refers to females and males with lengths less than the smallest mature individual of their respective sex.

The A. vincentiana caught by all three fishing methods ranged from 201 to 1001 mm TL (Fig. 4; Table 5), and the smallest individuals lay within the length range recorded for the embryos of this species (A. Jones, unpubl. data) and the largest individuals exceeded the length of "at least 840 mm" reported for this species by Last and Stevens (2009). The length-frequency distributions of female and male A. vincentiana were both broadly bimodal and the numbers of both sexes were relatively low, between 500 and 699 mm (Fig. 4). However, the modal length class of 850-899 mm for the group of large females far exceeded that of 700-749 mm for the group of large males. Furthermore, the largest female A. vincentiana was both far longer (1001 mm) and heavier (3634 g) than the largest male, i.e., 872 mm and 1886 g, respectively (Table 5). The ratio of females to males differed significantly from parity among all individuals (1 female:0.67 males; [chi square] = 12.81, P < 0.001), but not among juveniles (1 female:0.76 males; [chi square] = 3.52, P > 0.05).

The smallest S. australis caught by all three fishing methods was 228 mm in TL (Fig. 4; Table 5) and thus only slightly longer than the length of 220 mm recorded for the largest embryo of this species in a concomitant study (A. Jones, unpubl, data). Although the maximum length of 1004 mm for S. australis in our samples is considerably less than the maximum length reported for this species by Last and Stevens (2009), it is still far greater than the [TL.sub.50] for either females or males at maturity in southwestern Australian waters. Although individuals were represented in all 50-mm length classes between 200 and 1049 ram, the length-frequency distributions of females and males were both dominated by their 250-299 mm length classes (Fig. 4). The largest female S. australis was far longer (1004 mm) and heavier (10,970 g) than the largest male (859 mm and 5500 g) (Table 5). The ratio of females to males of S. australis did not differ significantly from parity among either all individuals collectively (1 female:l.05 males; [chi square] = 0.18, P > 0.05) or among juveniles (1 female:1.11 males; [chi square] = 0.75, P > 0.05).

The relationships between DW and DL for females and males of M. australis collectively are described by the following equations:

DW = 1.70 DL + 8.16 ([r.sup.2] = 0.997, n = 96), (5)

DL = 0.58 DW - 3.01 ([r.sup.2] = 0.998, n = 96). (6)

From the values obtained from the above equations, the following relationships between the natural logarithms of W and DL, and of W and DW for both sexes are described as

[log.sub.e] W = 2.91 [log.sub.e] DL - 8.92 ([r.sup.2] = 0.999, n = 91), (7)

[log.sub.e] W = 3.19 [log.sub.e] DW - 12.25 ([r.sup.2] = 0.999, n = 91). (8)

After correction for bias (Beauchamp and Olson, 1973), the respective back-transformed relationships became

W = 0.0001344 [DL.sup.2-91], (9)

W = 0.000004787 [DW.sup.3.19] (10)

The M. australis caught by using the three sampling methods ranged from 118 to 800 mm DL (Fig. 4, Table 5), which corresponds to 198-1192 mm DW. The minimum DW is at the extreme lower end of the range reported for this species at birth, and the maximum DW is appreciably less than the maximum DW of 1600 mm recorded for M. australis (Last and Stevens, 2009). The largest female caught was 800 mm DL and 37,811 g W, which greatly exceeded the 545 mm DL and 12,373 g W of the largest male (Table 5). Both sexes were represented in each DL class between 100 and 549 mm, and females were also present in each subsequent size class up to 800-849 mm (Fig. 4). The length-frequency distributions for both sexes contained a single prominent modal length class at 150-199 mm. The ratio of 1 female:l.27 males of M. australis among the individuals caught by all methods collectively did not differ significantly from parity ([chi square] = 3.10, P > 0.05), and the same was true for juveniles, i.e., 1 female:l.04 males ([chi square] = 0.06, P>0.05).

Lengths of females and males at maturity

The smallest female and male of H. portusjacksoni with mature gonads measured 715 and 595 mm TL, respectively (Table 5). Using gonadal stage as the index, we found that the [TL.sub.50] for female H. portusjacksoni at maturity was 805 mm, and the [TL.sub.50] for males was 593 mm, which represent 62% and 73% of their respective maximum TL. The [TL.sub.50] for males was only 12 mm greater and not significantly different from the 581 mm derived by using full clasper calcification as the index of maturity (Table 6, see also Jones et al., 2008).

The smallest female and male of A. vincentiana with mature gonads were 754 and 642 mm, respectively, and all females [greater than or equal to]896 mm and males [greater than or equal to]793 mm were mature (Fig. 5, Table 5). The [TL.sub.50] for females of 798 mm at maturity differed significantly (P<0.001) from the corresponding [TL.sub.50] for males of 671 mm when using gonadal stage as the criterion for maturity (Table 6). The latter [TL.sub.50] for males did not differ significantly from the [TL.sub.50] of 654 mm derived by using full clasper calcification as the criterion for maturity (P>0.05) (Fig. 5, Table 6). The [TL.sub.50] for female and male A. vincentiana at maturity, using gonadal status as the criterion for maturity, were 80% and 77% of their respective maximum TL.

On the basis of gonadal data, the TL of the smallest mature female and male S. australis were 825 and 754 mm, respectively, and all females [greater than or equal to]840 mm and all males [greater than or equal to]754 mm were mature (Fig. 5, Table 5). The [TL.sub.50] for females (823 mm) and males of S. australis (734 mm), derived by employing gonadal stage as the criterion for maturity, were significantly different (P<0.001; Table 6). The latter [TL.sub.50] was not significantly different (P>0.05) from the [TL.sub.50] of 721 mm derived for male S. australis when using clasper calcification as the criterion for maturity (Fig. 5, Table 6). The [TL.sub.50] calculated for females and males of S. australis, with gonadal stage as the criterion for maturity, were 82% and 85% of their respective maximum TL.

[FIGURE 3 OMITTED]

The DL of the smallest females and males of M. australis with mature gonads were 444 and 365 mm, respectively, and all females and males with DL [greater than or equal to]513 and 433 mm, respectively, were mature (Fig. 5, Table 5). The [DL.sub.50] of 511mm ([DW.sub.50]=879 mm) for females at maturity differed significantly (P<0.001) from the 399 mm ([DW.sub.50]=689 mm) of males when gonadal status was used as the criterion for maturity (Table 6). The latter [DL.sub.50] did not differ significantly (P>0.05) from the 388 mm ([DW.sub.50]=670 mm) derived for males at maturity with clasper calcification as the criterion for maturity (Table 6). On the basis of gonadal criteria, the [DL.sub.50] for females and males of M. australis at maturity were 64% and 73% of their [DL.sub.max], respectively.

[FIGURE 4 OMITTED]

Percentage of females and males caught by each fishing method

The percentage of females of H. portusjacksoni, A. vincentiana, S. australis and M. australis caught in trawls with lengths below the [L.sub.50] ([TL.sub.50] or [DL.sub.50]) at maturity were very high and similar to those of males (Table 7). The percentage in trawl samples of both sexes with lengths less than their [L.sub.50] at maturity ranged from 63% for A. vincentiana, to 90% for both M. australis and S. australis, respectively, to 97% for H. portusjacksoni (Table 7). In the case of gillnet samples for three of the four species, the percentage of females with lengths less than their [L.sub.50] at maturity exceeded those of males and particularly so for M. australis, for which the values were 86% and 40%, respectively (Table 7). In longline samples, the percentage of males of H. portusjacksoni with lengths below their [L.sub.50] at maturity slightly exceeded the corresponding value for females (37% and 32%, respectively).

Discussion

This study is the first to quantify the contribution of each elasmobranch species to the total elasmobranch catch obtained by co-occurring trawl, gillnet, and longline fisheries, and to calculate the contributions made by the bycatch and byproduct species to the catches taken by each fishing method. In addition, our results indicate that nMDS ordination and associated tests would be invaluable for detecting whether the species composition of elasmobranchs in the catch produced by each fishing method changes in the future in response to either variations in fishing activity or environmental factors and, if so, also for elucidating the magnitude of that effect. This study has also produced, for four abundant bycatch species, the sound quantitative biological data of the types required by managers for developing plans for conserving stocks and which are deficient for the vast majority of bycatch species (Stobutzki et al., 2002). The sizes at maturity that were determined for the four bycatch species in this study enabled the proportion of each species, which was caught by each fishing method before it had the potential to reproduce, to be estimated.

[FIGURE 5 OMITTED]

Compositions of the catches taken by the three fishing methods

Our results indicate that the elasmobranch component of the catches taken by demersal trawlers on the lower west coast of Australia is dominated by rays, which comprise 10 of the 14 elasmobranch species caught by this method. Furthermore, these batoids were either small species, e.g., stingarees, or represented by smaller individuals, e.g., Aptychotrema vincentiana and M. australis. Indeed, the four small species of stingaree (Urolophidae) caught by trawling contributed as much as two-thirds to the total trawl catch of elasmobranchs and were so abundant in samples collected during an extensive trawling study along the lower west coast of Australia that they collectively contributed 17.5% to the total biomass of the 172 fish species caught during that study (Hyndes et al., 1999). The large number of small batoids caught paralleled the situation recorded for other trawl fisheries, including the multispecies bottom trawl fishery off Argentina (Tamini et al., 2006). As with the large ray species, the only two shark species taken in appreciable numbers in our study, H. portusjacksoni and S. australis, were represented predominantly by small individuals.

The large number of elasmobranchs taken by prawn and scallop trawling emphasizes the lack of selectivity of this fishing method for its targeted species. Indeed, Bonfil (2) has stated that towed nets are the most indiscriminant of all fishing gears because they are designed to capture everything in their path and thus inevitably encounter a large number of nontarget species. The susceptibility of small demersal elasmobranchs to capture by trawling is due to their limited mobility and benthic lifestyle (see also Stobutzki et al., 2001, 2002; Walker, 2005a; Tamini et al., 2006) and to the trawlers often operating in nearshore waters, which typically act as nursery areas for several elasmobranch species, including H. portusjacksoni (White and Potter, 2004; Jones et al., 2008; Kinney and Simpfendorfer, 2009). The catches of elasmobranchs taken by trawls in our study comprised only six individuals that were retained as byproduct compared with the 2980 individuals discarded as bycatch.

Although the three species targeted by gillnetting, C. obscurus, M. antarcticus, and Furgaleus macki, collectively accounted for just over half of the total number of elasmobranchs caught by this method, the contribution of all retained species (i.e. including byproduct), accounted for three quarters of the total. This is very similar to the situation recorded for a gillnet fishery in southeastern Australia (Walker et al., 2005). The remainder of the catch (i.e., the bycatch) was still substantial, however, emphasizing that a considerable number of the sharks and rays caught by gillnet were discarded, as has been reported in gillnet fisheries elsewhere in the world (e.g., Perez and Warhlich, 2005).

The mesh sizes of the gillnets (165 and 178 mm) were selected to catch the three targeted species, C. obscurus, M. antarcticus, and F. macki, at a marketable size which, depending on the species, typically corresponded to modal fork lengths of between 800 and 1200 mm (McAuley and Simpfendorfer, 2003). This mesh selectivity accounts for the fact that the TL of the majority of A. vincentiana and S. australis in our gillnet catches fell within a relatively narrow range of 700-1000 ram, with the latter length closely approximating the maximum length recorded for these two species during the present study (Table 5). However, the TL for H. portusjacksoni, the second most abundant species in the gillnet catches, spanned the full range of this species in southwestern Australian waters (Jones et al., 2008). The capture by gillnet of a substantial number of H. portusjacksoni with lengths less than 700 mm is attributable to the tendency for all sizes of H. portusjacksoni to become entangled in gillnets as a result of their possessing prominent dorsal fin spines (Walker, 2005a). In the case of the ray M. australis, the larger individuals were proportionately less in gillnet than longline catches because this wide disc-shaped species becomes increasingly deflected from the net as it grows larger (Walker, 2005a).

Longlining was so successful at targeting M. antarcticus that this shark contributed nearly two thirds to the total elasmobranch catch taken by this method and, together with the other two targeted species, C. obscurus and F. macki, represented nearly 70% of that total catch. However, those last two species were not abundant in these catches. In fact, the second to fifth most abundant species were bycatch species. Although 11 of the 19 nontargeted elasmobranch species were typically retained as byproduct, none of those species was numerous and collectively accounted for only ~7% of the elasmobranch catch. From the above, it follows that the contribution of bycatch to the longline catches (~26%) was similar to that in the gillnet catches. However, unlike the situation with gillnetting, the bycatch species S. australis was never caught on longline hooks, presumably because this squatinid is an ambush predator that targets mobile prey such as teleosts and cephalopods (E. Sommerville, personal commun. (3)).

Although the use of nMDS ordination and associated tests emphasized that the species compositions of the elasmobranchs caught by trawling, gillnetting, and longlining differed markedly, SIMPER showed that H. portusjacksoni was a major typifying species for the elasmobranchs taken by trawling in relatively shallow waters and by gillnetting and longlining in deeper and more offshore waters. This bycatch species is thus clearly abundant and widely distributed throughout the inshore and more offshore coastal waters of southwestern Australia. The finding that the ray D. brevicaudata was the most important typifying species in the longline samples indicates that this bycatch species was consistently present in reasonable numbers in water depths of approximately 70 m. Although the total number of M. antarcticus obtained by longlining was far greater than that of D. brevicaudata, M. antarcticus was occasionally taken in large numbers and, at other times, not caught at all, reflecting the tendency for this shark species to school (Lenanton et al., 1990; Last and Stevens, 2009).

Length compositions, sex ratios, and habitats of the four selected bycatch species

Our data indicate that the maximum lengths of the females of H. portusjacksoni, A. vincentiana, S. australis, and M. australis exceed those of males by 37%, 13%, 14%, and 31%, respectively. Moreover, for each species, the sex ratio did not differ significantly from parity among their juveniles, and the females dominated the length classes of the larger individuals. The resultant trend for the overall sex ratios for H. portusjacksoni and A. vincentiana to significantly favor females indicates that the females of at least these two species live longer than their males. Such a conclusion for H. portusjacksoni in southwestern Australian waters is consistent with the results of Tovar-Avila et al. (2009), who showed that, in southeastern Australia, the maximum age of the females (35 years) of this species is greater than that of males (28 years). Although the overall sex ratios of S. australis and M. australis did not differ from parity, those ratios were almost certainly attributable to the larger individuals of these two species being proportionately less well represented in the overall samples of those species. In the case of S. australis, the capture of fewer larger individuals was mainly due to this species not being taken by longlining fisheries in deeper, offshore waters, where they had been caught by gillnetting, and thus the overall samples were swamped by juveniles caught by trawls in their nursery areas. The conclusion that larger females were under-represented in the overall catch of M. australis is consistent with a tendency for the larger individuals, which would presumably have been predominantly females, to be deflected from the net.

The difference between the maximum lengths of the females and males of H. portusjacksoni in southwestern Australian waters (37%) was far greater than the 19% and 10% differences recorded for two populations of this species in southeastern Australian (Tovar-Avila et al., 2007) and the 15% difference found farther north in eastern Australia (Powter and Gladstone, 2008). The trend for females to attain a larger size than that of males parallels that reported by Cortes (2000) for populations representing 164 species and 19 families of sharks. However, the average difference between the sizes of females and males reported by Cortes (2000) was 10% and thus smaller than the differences observed for particularly H. portusjacksoni and M. australis.

Comparisons of the length-frequency distributions for each of H. portusjacksoni, A. vincentiana, S. australis, and M. australis in samples obtained from trawling in inshore waters, and by gillnetting and longlining farther offshore, indicated that all four species use shallow, inshore waters as nursery areas and that substantial numbers of the adults of S. australis and M. australis are also present in these inshore habitats. The tendency for M. australis to use inshore areas as nursery areas in southwestern Australia is emphasized by the considerable numbers of juveniles of this species that were caught during a study of a permanently open estuary on the south coast of Western Australia (Potter and Hyndes, 1994). Furthermore, the juveniles of H. portusjacksoni are also abundant in inshore waters off eastern Australia (McLaughlin and O'Gower, 1971). Because several of the mature females of A. vincentiana caught in inshore waters contained full-term embryos, they would have been poised to give birth in those waters. However, because adults of H. portusjacksoni, A. vincentiana, and M. australis were caught by gillnetting and longlining, and S. australis was caught by longlining, some individuals of each of these species move offshore as they increase in size and some may even live permanently in offshore waters.

Lengths at maturity and their implications

The greater maximum length attained by females than by males of each of the four elasmobranch species was accompanied, on the basis of gonadal criteria, by a greater [L.sub.50] at maturity, and the differences in [L.sub.50] ranged from 12% for S. australis and 19% for A. vincentiana to as high as 28% for M. australis and 36% for H. portusjacksoni. The trend for the females of these species to mature at a larger size than that of males follows the overall trend exhibited by shark species (Cortes, 2000).

The values derived for the [L.sub.50] at maturity for the males of A. vincentiana, S. australis, and M. australis by using full clasper calcification as the criterion for maturity did not differ significantly from the corresponding values derived by using gonadal maturity as that criterion. This result parallels that recorded for H. portusjacksoni by Jones et al. (2008), for Squalus magalops by Braccini et al. (2006), and for Trygonorrhina dumerilii by Marshall et al. (2007). Thus, to obtain data that can subsequently be used to derive a reasonable proxy for the [L.sub.50] for males at maturity, scientists can rapidly record the lengths of males, determine whether or not their claspers are calcified, and then immediately return them live to the sea. Furthermore, for each of the four species, all of the males with uncalcified claspers and most of those with partially calcified claspers possessed immature (stage 1 or 2) gonads, whereas those with fully calcified claspers almost invariably contained mature (stage 3 or 4) gonads and this finding thus accounts for the [L.sub.50] at maturity not being significantly different when gonadal and clasper calcification criteria are used.

On the basis of our calculations of the [L.sub.50] at maturity for females and males of each of the four selected bycatch species, the overall percentages, in trawl samples, of H. portusjacksoni, S. australis, and M. australis, whose individuals would not typically have had the potential to reproduce, were particularly high, with values ranging from 90% to 97%. These high values are attributable to trawling taking place mainly in shallow inshore waters that typically act as nursery areas. The importance of protecting the newborn and young juveniles of species that have well-defined nursery areas has been emphasized by Walker (2005a). The far lower percentage of A. vincentiana with lengths less than the [L.sub.50] at maturity in trawl samples than was the case with the other three species is attributable to the tendency for this species to occupy inshore waters throughout its life.

The smaller proportion of individuals of H. portusjacksoni, A. vincentiana, S. australis, and M. australis with lengths less than their [L.sub.50] at maturity in gillnet than in trawl catches reflects a combination of the following: 1) differences in gear selectivity; the mesh sizes of gillnets are chosen with the purpose of catching targeted species at a sufficient size to be commercially marketed (Simpfendorfer and Unsworth, 1998; McAuley and Simpfendorfer, 2003) and thereby obtaining proportionately more of the larger, mature individuals of these four bycatch species than their juveniles; and 2) the location of gillnetting in deeper, offshore waters and thus beyond typical nursery areas.

Mortality

Onboard sampling of the catches obtained by the three fishing methods during commercial operations was made difficult by the small size of the fishing boats and the rapidity with which the fishermen worked to retrieve the retained species and discard the bycatch. It was thus not possible to determine precisely the fishing-induced mortality of the individuals of each bycatch species taken by each fishing method and, in particular, the mortality of H. portusjacksoni, A. vincentiana, S. australis, and M. australis. From our observations, however, it is apparent that, in the case of trawling, many individuals died while in the net and those that survived were often so badly injured during sorting that they would have been unlikely to survive after release. From our observations, we believe that the level of mortality associated with trawling is greater than that suggested by Laurenson et al. (1), who proposed that approximately half of the elasmobranchs taken by trawling during a research project on the lower west coast of Australia was likely to die during capture or after being discarded. Indeed, in a detailed study of the bycatch of Australia's northern prawn trawl fishery, Stobutzki et al. (2002) showed that as much as two-thirds of the sharks and rays caught by this method died while still in the net, and thus the overall mortality, i.e., including individuals that subsequently died from the trauma of capture, would have been higher. It should also be recognized that the pregnant females of the smaller species of rays (White and Potter, 2004) and S. australis and M. australis tended to abort after capture, thus adding a further detrimental effect to the populations of those species.

It was evident that the individuals of most species, including those of the three targeted species, M. antarcticus, C. obscurus, and F. macki, and several bycatch species, such as M. australis and S. australis, had died by the time the gillnet was retrieved. It is proposed that this high mortality was related to the very long soak times of the gillnets (up to 24 h), which is consistent with the observation that mortality was far lower in a gillnet study conducted in southeastern Australia in which soak times were only ~8 h (Walker et al., 2005). Furthermore, the long soak times led to the elasmobranchs in the nets becoming infested by sea lice (Cirolana sp.) and to attack by leatherjackets (Monacanthid spp.), thus hastening the death of species such as M. australis. Moreover, although H. portusjacksoni frequently survived capture by gillnets, the individuals of this species were often severely injured while being forcibly removed from the gillnets.

In contrast to the situation with gillnetting, the majority of elasmobranchs survived capture by longlining and thus, in general, the individuals of bycatch species caught by hooks were able to be returned to the sea alive. The high survival of elasmobranchs caught by longlining in southwestern Australia is presumably attributable, at least in part, to the short set times (~3 h) for this fishing method.

Implications for ecosystem-based fisheries management

Traditional fisheries management has focused on ensuring that the populations of targeted species are maintained at levels that are considered sustainable. However, fisheries regulations aimed at constraining the exploitation of targeted species may provide little protection for bycatch species, and thus commercial fisheries can have an equal or even greater impact on the populations of bycatch species. In contrast, ecosystem-based fisheries management, i.e., an ecosystem approach to fisheries, requires that the populations of bycatch species in an ecosystem, as well as those of the targeted and byproduct species, are sustained. It is thus relevant that our study revealed that many individuals of the elasmobranch bycatch species caught by commercial fisheries, and particularly by trawling, were immature and, together with our observations and the results of other studies, strongly indicated that mortality is high among these individuals. The problems posed by such fisheries-induced mortalities on these bycatch species are exacerbated because elasmobranchs have low biological productivity and are therefore susceptible to overexploitation (Stevens et al., 2000; Walker, 2005a). Thus, while it is recognized that the trawl, gillnet, and longline fisheries of southwestern Australia are not large, the removal of even moderate numbers from the population of any elasmobranch species has the potential to affect that population at a local level (Walker, 2005a).

In the context of the ecosystem-based fisheries management framework, every attempt should be made to reduce, where practical, the amount of bycatch. In view of the high mortality of the bycatch species taken by trawlers and gillnetters, commercial fishermen should be encouraged to explore ways of reducing the amount of bycatch and of increasing survival among discarded individuals. Thus, with trawling for example, fishermen could explore the effectiveness of bycatch reduction devices, reduce the duration of trawls and onboard handling time, and unload the catch into water tanks before sorting. In the case of gillnetting, fishermen could explore whether a reduction in the soak time of their nets leads to a decrease in the mortality of the bycatch and an increase in the market quality and thus the value of the flesh of the retained species.

The biological and catch data produced during this and other studies on locally abundant bycatch species of elasmobranchs (White et al., 2001; 2002; White and Potter, 2005; Marshall et al., 2007) indicate that trawl, gillnet, and longline fisheries may be having a direct impact on the populations of these species in southwestern Australia. The acquisition of these data now enables managers to determine whether the impacts of commercial fisheries on the populations of bycatch species justify modifying fishing regulations to ensure that the risks to the sustainability of these species are reduced and that the integrity of the ecosystem is thus maintained. Such risks could readily be achieved through the use of rapid assessment techniques, such as those described by Stobutzki (2001; 2002) and Walker et al. (2005).

The views and opinions expressed or implied in this article are those of the author (or authors) and do not necessarily reflect the position of the National Marine Fisheries Service, NOAA.

Acknowledgments

Special thanks are extended to the commercial fishermen, and particularly to H. Gilbert, P. Dyer and A. Butler, whose help was invaluable for obtaining samples, to P. Coulson, D. French, and B. Farmer, who assisted in sampling, and to A. Hesp for his statistical advice. We also thank three anonymous referees for their constructive comments, which have led to an improved manuscript. Funding for this project was provided by FISHCARE WA and Murdoch University.

Manuscript submitted 15 December 2009. Manuscript accepted 28 January 2010. Fish. Bull. 108:365-381 (2010).

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Ashlee A. Jones (contact author) [1]

Norman G. Hall [1]

Ian C. Potter [1]

Email address for contact author: ashlee.jones@murdoch.edu.au

[1] Centre for Fish and Fisheries Research Murdoch University Murdoch, Western Australia, 6150, Australia

(1) Laurenson, L. J. B., P. Unsworth, J. W. Penn, and R. C. J. Lenanton. 1993. The impact of trawling for saucer scallops and western king prawns on the benthic communities in coastal waters off south-western Australia, 93 p. Fisheries Res. Report, Department of Fisheries, no.100 (part 1), Western Australia.

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(3) Sommerville, Emma. 2007. Centre for Fish and Fisheries Research, Murdoch Univ., Murdoch, Western Australia, 6150.
Table 1

Number of females and males and percentage contribution of females of
each elasmobranch species (sex determined during regular onboard
observations) in the catches of trawl vessels fishing for prawns and
scallops on the lower west coast of Australia. The total number of
individuals of each species (including those whose sex could not be
determined owing to logistic constraints) and the percent contribution
of each species to the total elasmobranch catch are also given. Plain
font * = byproduct species, i.e., those that are not targeted but
usually retained; Bold font = bycatch species, i.e., those that are
usually discarded.

Common name                      Species name

Lobed stingaree#                 Urolophus lobatus#
Sparsely-spotted stingaree#      Urolophus paucimaculatus#
Masked stingaree#                Trygonoptera personata#
Western shovelnose ray#          Aptychotrema vincentiana#
Port Jackson shark#              Heterodontus portusjacksoni#
Southern fiddler ray#            Trygonorrhina dumerilii#
Western shovelnose stingaree#    Trygonoptera mucosa#
Southern eagle ray#              Myliobatis australis#
Australian angelshark#           Squatina australis#
Smooth stingray#                 Dasyatis breuicaudata#
Gummy shark *                    Mustelus antarcticus *
White-spotted guitarfish#        Rhynchobatus australiae#
Spinner shark *                  Carcharhinus breuipinna *
Coffin ray#                      Hypnos monopterygius#

Total

                                 Female   Male   Female   Total   Total
Common name                        n       n       %        n       %

Lobed stingaree#                   422     288    59.4     851     28.5
Sparsely-spotted stingaree#        235     349    40.2     726     24.3
Masked stingaree#                   84      66    56.0     277      9.3
Western shovelnose ray#            124      99    55.6     237      7.9
Port Jackson shark#                112     110    50.5     223      7.5
Southern fiddler ray#               94     118    44.3     220      7.4
Western shovelnose stingaree#       23      27    46.0     153      5.1
Southern eagle ray#                 67      81    45.3     148      5.0
Australian angelshark#              67      66    50.4     135      4.5
Smooth stingray#                     2       6    25.0       8      0.3
Gummy shark *                        4       1    80.0       5      0.2
White-spotted guitarfish#            1       0   100.0       1    < 0.1
Spinner shark *                      0       1       0       1    < 0.1
Coffin ray#                          0       1       0       1    < 0.1

Total                             1235    1213            2986

Note: Bold font = bycatch species, i.e., those that are usually
discarded are indicated with #.

Table 2
Number of females and males, percent contribution of females, and the
total number and percent contributions of each elasmobranch species
that were recorded during regular onboard observations of catches from
gillnet vessels on the southwest coast of Australia. Plain font =
targeted species; Plain font * = byproduct species i.e., those species
not targeted but usually retained; Bold font = bycatch species, i.e.,
those species that are usually discarded.

Common name                      Species name

Dusky shark                      Carcharhinus obscurus
Port Jackson shark#              Heterodontus portusjacksoni#
Gummy shark                      Mustelus antarcticus
Sandbar shark *                  Carcharhinus plumbeus *
Whiskery shark                   Furgaleus macki
Southern eagle ray#              Myliobatis australis#
Spinner shark *                  Carcharhinus brevipinna *
Western wobbegong *              Orectolobus hutchinsi *
Gulf wobbegong *                 Orectolobus halei *
Smooth hammerhead *              Sphyrna zygaena *
Cobbler wobbegong#               Sutorectus tentaculatus#
Bronze whaler *                  Carcharhinus brachyurus *
Spotted wobbegong *              Orectolobus maculatus *
Western shovelnose ray#          Aptychotrema vincentiana#
Australian angelshark#           Squatina australis#
Common sawshark#                 Pristiophorus cirratus#
Southern fiddler ray#            Trygonorrhina dumerilii#
Grey nurse shark#                Carcharias taurus#
Lobed stingaree#                 Urolophus lobatus#
Floral banded wobbegong#         Orectolobus floridus#
Smooth stingray#                 Dasyatis brevicaudata#
Pencil shark *                   Hypogaleus hyugaensis *
Ornate angelshark#               Squatina tergocellata#
Scalloped hammerhead *           Sphyrna lewini *
Western shovelnose stingaree#    Trygonoptera mucosa#

Total

                                 Female   Male   Female   Total   Total
Common name                        n       n       %        n       %

Dusky shark                       229     208     52.4     437     34.7
Port Jackson shark#               131     120     52.2     251     19.9
Gummy shark                       151      28     84.4     179     14.2
Sandbar shark *                    86      59     59.3     145     11.5
Whiskery shark                     69      13     82.9      82      6.5
Southern eagle ray#                18      14     56.3      32      2.5
Spinner shark *                    17       8     68.0      25      2.0
Western wobbegong *                 7      11     38.9      18      1.4
Gulf wobbegong *                    3      12     20.0      15      1.2
Smooth hammerhead *                 8       4     66.7      12      1.0
Cobbler wobbegong#                  7       4     63.6      11      0.9
Bronze whaler *                    10       1     90.9      11      0.9
Spotted wobbegong *                 2       7     22.2       9      0.7
Western shovelnose ray#             2       4     33.3       6      0.5
Australian angelshark#              2       4     33.3       6      0.5
Common sawshark#                    4       1     80.0       5      0.4
Southern fiddler ray#               3       2     60.0       5      0.4
Grey nurse shark#                   2       0    100.0       2      0.2
Lobed stingaree#                    1       1     50.0       2      0.2
Floral banded wobbegong#            2       0    100.0       2      0.2
Smooth stingray#                    1       0    100.0       1    < 0.1
Pencil shark *                      0       1      0.0       1    < 0.1
Ornate angelshark#                  1       0    100.0       1    < 0.1
Scalloped hammerhead *              0       1      0.0       1    < 0.1
Western shovelnose stingaree#       0       1      0.0       1    < 0.1

Total                             756     504             1260

Note: Bold font = bycatch species, i.e., those species that are
usually discarded are indicated with #.

Table 3

Number of females and males, percent contribution of females and the
total number and percent contributions of all individuals of each
elasmobranch species that were recorded during regular onboard
observations of the catches from longline vessels on the southwest
coast of Australia. Plain font = targeted species; Plain font * =
byproduct species, i.e., those that are not targeted but usually
retained; Bold font = bycatch species, i.e., those species that are
usually discarded.

Common name                     Species name

Gummy shark                     Mustelus antarcticus
Smooth stingray#                Dasyatis brevicaudata#
Southern eagle ray#             Myliobatis australis#
Southern fiddler ray#           Trygonorrhina dumerilii#
Port Jackson shark#             Heterodontus portusjacksoni#
Dusky shark                     Carcharhinus obscurus
Smooth hammerhead *             Sphyrna zygaena *
Bronze whaler *                 Carcharhinus brachyurus *
Whiskery shark                  Furgaleus macki
Western wobbegong *             Orectolobus hutchinsi *
Common sawshark *               Pristiophorus cirratus *
School shark *                  Galeorhinus galeus *
Western shovelnose ray#         Aptychotrema vincentiana#
Gulf wobbegong *                Orectolobus halei *
Sandbar shark *                 Carcharhinus plumbeus *
Spotted wobbegong *             Orectolobus maculatus *
Rusty carpetshark#              Parascyllium ferrugineum#
Melbourne skate#                Spinirija whitleyi#
Spinner shark *                 Carcharhinus brevipinna *
Australian sawtail catshark#    Figaro boardmani#
Pencil shark *                  Hypogaleus hyugaensis *
Scalloped hammerhead *          Sphyrna lewini *

Total

                                Female   Male   Female   Total   Total
Common name                       n       n       %        n       %

Gummy shark                      234     129     64.5     363    63.2
Smooth stingray#                  21      20     51.2      41     7.1
Southern eagle ray#               15      19     44.1      34     5.9
Southern fiddler ray#             24       8     75.0      32     5.6
Port Jackson shark#               20      11     64.5      31     5.4
Dusky shark                       16       6     72.7      22     3.8
Smooth hammerhead *                8       1     88.9       9     1.6
Bronze whaler *                    4       1     80.0       5     0.9
Whiskery shark                     5       0    100.0       5     0.9
Western wobbegong *                1       4     20.0       5     0.9
Common sawshark *                  4       1     80.0       5     0.9
School shark *                     2       2     50.0       4     0.7
Western shovelnose ray#            3       0    100.0       3     0.5
Gulf wobbegong *                   1       2     33.3       3     0.5
Sandbar shark *                    2       0    100.0       2     0.4
Spotted wobbegong *                0       2      0.0       2     0.4
Rusty carpetshark#                 0       2      0.0       2     0.4
Melbourne skate#                   2       0    100.0       2     0.4
Spinner shark *                    0       1      0.0       1     0.2
Australian sawtail catshark#       1       0    100.0       1     0.2
Pencil shark *                     0       1      0.0       1     0.2
Scalloped hammerhead *             1       0    100.0       1     0.2

Total                            364     210              574

Note: Bold font = bycatch species, i.e., those species that are
usually discarded are indicated with #.

Table 4

Main species that typified the catches of elasmobranchs recorded
onboard trawl, gillnet, and longline vessels (shaded back ground), and
those that discriminate between the catches of elasmobranchs obtained
by each pair of fishing methods (unshaded background). Plain font =
targeted species; Bold font = bycatch species; * Denotes that the
species is relatively more abundant and consistently caught by the
sampling method on the top (horizontal) row than on the side
(vertical) column.

                         Trawl

Trawl      Aptychotrema vincentiana#^
           Heterodontus portusjacksoni#^
           Urolophus paucimaculatus#^
           Squatina australis#^

Gillnet    Carcharhinus obscurus##
           Aptychotrema vincentiana *##^
           Heterodontus portusjacksoni##^
           Mustelus antarcticus##
           Myliobatis australis *##^
           Furgaleus macki##

Longline   Dasyatis brevicaudata##^
           Aptychotrema vincentiana *##^
           Trygonorrhina dumerilii *##^
           Heterodontus portusjacksoni *##^

                        Gillnet

Trawl

Gillnet
           Carcharhinus obscurus#
           Heterodontus portusjacksoni#^
           Mustelus antarcticus#
           Furgaleus macki#

Longline   Carcharhinus obscurus *##
           Dasyatis brevicaudata##^
           Mustelus antarcticus##
           Heterodontus portusjacksoni *##^
           Furgaleus macki *##

                      Longline

Trawl

Gillnet

Longline
           Dasyatis brevicaudata#^
           Mustelus antarcticus#
           Trygonorrhina dumerili#^
           Heterodontus portusjacksoni#^

Note: Main species that typified the catches of elasmobranchs recorded
onboard trawl, gillnet, and longline vessels (shaded back ground) are
indicated with #, and those that discriminate between the catches of
elasmobranchs obtained by each pair of fishing methods (unshaded
background) are indicated with ##.

Note: Bold font = bycatch species are indicated with ^.

Table 5
Biological characteristics of four elasmobranch species caught as
bycatch by commercial trawl, gillnet, and longline fisheries operating
off southwestern Australia. Length measurements are given as total
lengths (TL) for Heterodontus portusjacksoni, Aptychotrema
vincentiana, and Squatina australis, and as disc lengths (DL) for
Myliobatis australis. * denotes a value extrapolated from the
regression equation of the relation between DL and W. The true weight
could not be recorded because the pectoral fins had been removed by
fishermen. Sample size for each sex of each species is shown on
Figure 4.

                                   Heterodontus    Aptychotrema
                                  portusjacksoni   vincentiana

Females   Length range (mm)          198-1300        201-1001
          Weight range (g)          39-12,250        32-3634
          Smallest mature (mm)         715             754
          Largest immature (mm)        869             895

Males     Length range (mm)          180-815         214-872
          Weight range (g)           39-3920         33-1886
          Smallest mature (mm)         595             642
          Largest immature (mm)        654             792

                                  Squatina     Myliobatis
                                  australis    australis

Females   Length range (mm)       228-1004      118-800
          Weight range (g)        94-10,970   117-37,811 *
          Smallest mature (mm)       825          444
          Largest immature (mm)      834          472

Males     Length range (mm)        246-859      129-545
          Weight range (g)        115-5500    152-12,373 *
          Smallest mature (mm)       754          365
          Largest immature (mm)      707          433

Table 6
Estimates of the [L.sub.50] and [L.sub.95] at maturity, and the upper
and lower 95% confidence limits (CL), for females and males of
Heterodontus portusjacksoni, Aptychotrema uincentiana, and Squatina
australis recorded as total length (TL). For Myliobatis austra lis,
these values were recorded as disc length (DL); extrapolated values
for disc widths (DW) are also provided. Estimates were derived by
using gonadal status as an index of maturity for females and males and
also by using full clasper calcification as that index for males.
Sample sizes for females and males of each species are provided in
Figure 4.

                                                  Female (gonads)

                                              [L.sub.50]    [L.sub.95]

Heterodontus portusjacksoni    Estimate           805           896
                               Upper 95% CL       826           931
                               Lower 95% CL       781           866

Aptychotrema uincentiana       Estimate           798           877
                               Upper 95% CL       815           920
                               Lower 95% CL       774           855

Squatina australis             Estimate           823           852
                               Upper 95% CL       842           927
                               Lower 95% CL       771           826

Myliobatis australis           Estimate           511           585
                               Upper 95% CL       558           696
                               Lower 95% CL       480           538

                                                   Female (gonads)

                                              [DW.sub.50]   [DW.sub.95]

Myliobatis australis           Estimate           879          1006
                               Upper 95% CL       960          1195
                               Lower 95% CL       827           926

                                                   Male (gonads)

                                              [L.sub.50]    [L.sub.95]

Heterodontus portusjacksoni    Estimate           593           647
                               Upper 95% CL       605           674
                               Lower 95% CL       579           628

Aptychotrema uincentiana       Estimate           671           766
                               Upper 95% CL       695           833
                               Lower 95% CL       631           736

Squatina australis             Estimate           734           735
                               Upper 95% CL       753           806
                               Lower 95% CL       673           714

Myliobatis australis           Estimate           399           472
                               Upper 95% CL       416           518
                               Lower 95% CL       376           440

                                                   Male (gonads)

                                              [DW.sub.50]   [DW.sub.95]

Myliobatis australis           Estimate           689           813
                               Upper 95% CL       717           891
                               Lower 95% CL       649           758

                                                   Male (claspers)

                                              [L.sub.50]    [L.sub.95]

Heterodontus portusjacksoni    Estimate           581           652
                               Upper 95% CL       594           689
                               Lower 95% CL       563           629

Aptychotrema uincentiana       Estimate           654           707
                               Upper 95% CL       676           756
                               Lower 95% CL       618           679

Squatina australis             Estimate           721           723
                               Upper 95% CL       753           806
                               Lower 95% CL       674           714

Myliobatis australis           Estimate           388           453
                               Upper 95% CL       404           491
                               Lower 95% CL       366           420

                                                   Male (claspers)

                                              [DW.sub.50]   [DW.sub.95]

Myliobatis australis           Estimate           670           781
                               Upper 95% CL       697           845
                               Lower 95% CL       632           724

Table 7
Numbers of females and males and total number of Heterodontus
portusjacksoni, Aptychotrema vincentiana, Squatina australis, and
Myliobatis australis that were caught by each fishing method and
examined in the laboratory, and the percentages of individuals with
lengths less than their [L.sub.50] at maturity when using gonadal
status as the index of maturity. [L.sub.50] refers to total length
([TL.sub.50]), except in the case of M. australis where it refers to
disc length ([DL.sub.50]).

Fishing method   Species name                        Females

                                                n    % < [L.sub.50]

Trawl            Heterodontus portusjacksoni   121         98
                 Aptychotrema vincentiana      182         62
                 Squatina australis            155         89
                 Myliobatis australis           71         92

Gillnet          Heterodontus portusjacksoni   115         44
                 Aptychotrema vincentiana       16         38
                 Squatina australis             22         18
                 Myliobatis australis           14         86

Longline         Heterodontus portusjacksoni    57         32
                 Aptychotrema vincentiana        5          0
                 Squatina australis             --         --
                 Myliobatis australis           11         36

Fishing method   Species name                         Males

                                                n    % < [L.sub.50]

Trawl            Heterodontus portusjacksoni   128         96
                 Aptychotrema vincentiana      116         66
                 Squatina australis            169         91
                 Myliobatis australis           83         87

Gillnet          Heterodontus portusjacksoni    76         30
                 Aptychotrema vincentiana       20         30
                 Squatina australis             16         19
                 Myliobatis australis           25         40

Longline         Heterodontus portusjacksoni    19         37
                 Aptychotrema vincentiana        1        100
                 Squatina australis             --         --
                 Myliobatis australis           14          7

Fishing method   Species name                    Sexes combined

                                                n    % < [L.sub.50]

Trawl            Heterodontus portusjacksoni   249         97
                 Aptychotrema vincentiana      298         63
                 Squatina australis            324         90
                 Myliobatis australis          154         90

Gillnet          Heterodontus portusjacksoni   191         38
                 Aptychotrema vincentiana       36         33
                 Squatina australis             38         18
                 Myliobatis australis           39         56

Longline         Heterodontus portusjacksoni    76         33
                 Aptychotrema vincentiana        6         17
                 Squatina australis             --         --
                 Myliobatis australis           25         20
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