365 
Species compositions of elasmobranchs 
caught by three different commercial fishing 
methods off southwestern Australia, and 
biological data for four abundant bycatch species 
Ashlee A. Jones (contact author ) 1 
Norman G. HalS 1 
Ian C. Potter 1 
Email address for contact author: ashlee.|ones@murdoch. edu.au 
1 Centre for Fish and Fisheries Research 
Murdoch University 
Murdoch, Western Australia, 6150, Australia 
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 com- 
positions 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 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 50 derived 
by using gonadal data as the crite- 
rion 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 selectiv- 
ity and to trawling being undertaken 
in shallow inshore waters that act 
as nursery areas for these species. 
Sound quantitative data on the spe- 
cies 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 spe- 
cies and thus the marine ecosystems 
of which they are part. 
Manuscript submitted 15 December 2009. 
Manuscript accepted 28 January 2010. 
Fish. Bull. 108:365-381 (2010). 
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. 
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 tar- 
geted 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 elasmo- 
branchs 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 fisher- 
ies, nontargeted species are referred 
to as bycatch, Walker et al. (2005) 
emphasized that some of those species 
are usually retained and thus consti- 
tute byproduct, whereas the others 
are usually discarded and therefore 
constitute bycatch in the strict sense. 
In an assessment of various com- 
mercial fisheries throughout the world, 
it was shown that trawl fisheries tar- 
geting 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 fish- 
ery die while in the net (Stobutzki et 
al., 2002). Furthermore, that study 
demonstrated that most of these indi- 
viduals 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 sig- 
nificant impact on the trophic struc- 
ture 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 distribu- 
tions tend to be most prone to over- 
fishing (Stevens et al., 2000). More- 
over, 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 manage- 
