01 Jones FB108(4).Indd

365 Abstract—Commercial catches taken Species compositions of elasmobranchs in southwestern Australian waters by trawl fisheries targeting prawns caught by three different commercial fishing and scallops and from gillnet and longline fisheries targeting sharks methods off southwestern Australia, and were sampled at different times of biological data for four abundant bycatch species the year between 2002 and 2008. This sampling yielded 33 elasmobranch species representing 17 families. Ashlee A. Jones (contact author)1 Multivariate statistics elucidated Norman G. Hall1 the ways in which the species com- 1 positions of elasmobranchs differed Ian C. Potter among fishing methods and provided Email address for contact author: [email protected] benchmark data for detecting changes 1 Centre for Fish and Fisheries Research in the elasmobranch fauna in the Murdoch University future. Virtually all elasmobranchs Murdoch, Western Australia, 6150, Australia 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, The impact of commercial fisheries amount of bycatch (Cook, 2003) and Aptychotrema vincentiana, Squatina on the populations of sharks and rays that the mortality of individuals in australis, and Myliobatis australis, has, in recent years, become an issue that bycatch is substantial (Bonfil, were greater for females than males. of international concern (Stevens 1994). Indeed, it has been estimated The L determined for the males of 50 et al., 2000; Walker et al., 2005). It that approximately two thirds of the these species at maturity by using full clasper calcification as the criterion of is important to recognize, however, elasmobranchs caught as bycatch in maturity did not differ significantly that elasmobranchs are not only tar- Australia’s northern prawn trawl fish- geted by certain fisheries, but also ery die while in the net (Stobutzki et from the corresponding L50 derived by using gonadal data as the crite- comprise a substantial component of al., 2002). Furthermore, that study rion for maturity. The proportions of the bycatch of commercial fisheries, demonstrated that most of these indi- the individuals of these species with such as those employing trawl nets, viduals are small, and more than half lengths less than those at which 50% gillnets, and longlines (Stevens et al., are immature and some are caught reach maturity were far greater in 2000; Stobutzki et al., 2001; Walker, immediately after birth. trawl samples than in gillnet and 2005a). An assessment of the impacts Many elasmobranchs are at or near longline samples. This result was of commercial fishing on the elasmo- the apex of marine food webs and due to differences in gear selectiv- ity and to trawling being undertaken branchs taken as bycatch is hindered thus their removal can have a sig- in shallow inshore waters that act by the fact that most of that catch nificant impact on the trophic struc- as nursery areas for these species. is typically reported as “unidentified ture of an ecosystem (Camhi et al., Sound quantitative data on the spe- shark” or “mixed fish,” or not reported 1998; Stevens et al., 2000; Shepard cies compositions of elasmobranchs at all (Walker, 2005a). The numbers and Myers, 2005). Furthermore, caught by commercial fisheries and of sharks and rays taken as bycatch certain biological characteristics of the biological characteristics of the by commercial fisheries may, in some elasmobranchs, such as their long life main elasmobranch bycatch species cases, exceed those of the targeted spans, low fecundities, and late ages are crucial for developing strategies species, and many of those individuals at maturity, limit their ability both for conserving these important spe- die either during capture or after they to withstand fishing pressure, either cies and thus the marine ecosystems of which they are part. are discarded (Bonfil, 1994; Stobutzki when targeted or caught incidentally, et al., 2002; Walker, 2005a). Although, and to recover from overexploitation in most studies of commercial fisher- (Stevens et al., 2000; Walker, 2005a; ies, nontargeted species are referred Gallucci et al., 2006). In general, to as bycatch, Walker et al. (2005) the populations of endemic species emphasized that some of those species or those that have localized distribu- are usually retained and thus consti- tions tend to be most prone to over- Manuscript submitted 15 December 2009. Manuscript accepted 28 January 2010. tute byproduct, whereas the others fishing (Stevens et al., 2000). More- Fish. Bull. 108:365–381 (2010). are usually discarded and therefore over, there are little or no data on the constitute bycatch in the strict sense. reproductive biology of most bycatch The views and opinions expressed In an assessment of various com- species. Such data are required for or implied in this article are those of the author (or authors) and do not necessarily mercial fisheries throughout the world, determining the resilience of these reflect the position of the National Marine it was shown that trawl fisheries tar- species to fishing pressure, thereby Fisheries Service, NOAA. geting prawns generate the largest enabling the development of manage- 366 Fishery Bulletin 108(4) ment plans for conserving their populations (Frisk et branch catch to be estimated for each fishing method. al., 2001; Stobutzki et al., 2002; Walker, 2005b). The percent contributions of each species to the catches Most of the species in Australia’s rich diversity of obtained by trawling, gillnetting, and longlining were elasmobranchs are endemic and occupy demersal habi- then used to compare statistically the species composi- tats on the continental shelf or slope (Last and Ste- tions of the elasmobranchs in the catches taken by each vens, 2009) and are thus potentially prone to depletion of these fishing methods. by demersal fishing methods such as trawling, gill- Emphasis was next placed on determining the length netting, and longlining (Stobutzki et al., 2001, 2002; compositions of the bycatch species H. portusjacksoni, Coelho et al., 2003; Perez and Wahrlich, 2005, Walker A. vincentiana, S. australis, and M. australis in samples et al., 2005). Analyses of fisheries data for 1994 to collected by trawling, gillnetting, and longlining and 1999 showed that the bycatch species Heterodontus on estimating the lengths at maturity of the last three portusjacksoni (Heterodontidae) and Myliobatis australis of these ecologically important species. The lengths of (Myliobatidae) are abundant in catches of the temperate both the females and males at which 50% are mature demersal gillnet and longline fisheries of southwestern (L50) were first determined by using gonadal data as the Australia (McAuley and Simpfendorfer, 2003). These criterion for determining maturity and then, in the case two species and the rhinobatid Aptychotrema vincentia- of males, by employing full clasper calcification as that na and the squatinid Squatina australis, which are also criterion. The L50 calculated for the males of each species caught as bycatch by commercial fisheries, collectively by using the two maturity criteria were then compared contributed as much as 17% to the total biomass of the to ascertain whether the L50 derived by using clasper cal- 172 species of fish caught during extensive trawling cification as the index of maturity is a reasonable proxy along the lower west coast of Australia (Hyndes et al., for that derived using gonadal stage as that criterion. 1999). It has been estimated that approximately half The L50 at maturity of A. vincentiana, S. australis, and of the elasmobranchs taken as bycatch by commercial M. australis and of H. portusjacksoni (Jones et al., 2008) trawlers in this region are likely to die during or after were then employed to determine the proportions of both capture (Laurenson et al.1). Despite the potentially det- sexes of each of these four species that were caught by rimental effects of commercial fishing on the above four each fishing method before they had the opportunity to elasmobranch species and their ecological importance in reproduce. Finally, the management and conservation the temperate waters of southern Australia, sound bio- implications of our data are considered. logical data have been collected only for H. portusjacksoni (McLaughlin and O’Gower, 1971; Tovar-Ávila et al., 2007; Jones et al., 2008; Powter and Gladstone, 2008). Materials and methods The families to which the above four species belong are represented elsewhere in the world by species that Sampling regime are taken in substantial numbers as bycatch. For ex- ample, in the eastern Pacific, up to a thousand individu- The elasmobranchs were examined in commercial catches als of the heterodontid shark Heterodontus mexicanus from 69 demersal trawls, 24 demersal gillnets, and 19 may be caught in a single gillnet set, many of which die longline sets of fishing vessels operating in southwestern (��Garayzar, 2006). In some parts of the world, t��he popu- Australian waters southwards of 32° S lat. on the west lations of several species of rhinobatid and squatinid coast and then eastwards to 118° E long. on the south have been so drastically depleted from overfishing that coast (Fig. 1). Trawling targets the western

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