Jones et al.: Species compositions of elasmobranchs caught by three commercial fishing methods 
379 
On the basis of our calculations of the L 50 at matu- 
rity for females and males of each of the four selected 
bycatch species, the overall percentages, in trawl sam- 
ples, 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 nurs- 
ery areas has been emphasized by Walker (2005a). The 
far lower percentage of A. vincentiana with lengths less 
than the L 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. portus- 
jacksoni, A. vincentiana, S. australis, and M. austt'alis 
with lengths less than their L go at maturity in gillnet 
than in trawl catches reflects a combination of the fol- 
lowing: 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; McAu- 
ley 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 mor- 
tality 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 appar- 
ent 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 elasmo- 
branchs 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 mortal- 
ity, 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. ant- 
arcticus, 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 (Mona- 
canthid 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 ma- 
jority 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 at- 
tributable, 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 protec- 
tion for bycatch species, and thus commercial fisheries 
can have an equal or even greater impact on the popu- 
lations of bycatch species. In contrast, ecosystem-based 
fisheries management, i.e., an ecosystem approach to 
fisheries, requires that the populations of bycatch spe- 
cies 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 biologi- 
cal productivity and are therefore susceptible to over- 
exploitation (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). 
