Feeding ecology of the southern lanternshark (Etmopterus baxteri) and the brown lanternshark (E. unicolor) off southeastern Australia
Chris S. Hallett
1
Ross K. Daley
0
0
CSIRO Marine and Atmospheric Research
,
Castray Esplanade, Hobart, Tasmania 7001
,
Australia
1
Centre for Fish and Fisheries Research, School of Biological Sciences and Biotechnology, Murdoch University
,
South Street, Murdoch, Western Australia 6150
,
Australia
Little is known about the ecological interactions between bycatch species and orange roughy (Hoplostethus atlanticus) within exploited slope and seamount communities off Tasmania. The diet of Etmopterus baxteri and E. unicolor, two important bycatch species of these fisheries, is described using four indices [percentage frequency of occurrence, percentage by number, percentage by weight, and percentage by the index of relative importance (%IRI)] calculated for broad prey categories, individual prey taxa, and functional prey groups. The identifiable diet of E. unicolor was dominated by benthic cephalopods (96%IRI), whereas benthic teleost prey, notably orange roughy (43%IRI), dominated the diet of E. baxteri. Similar trophic mechanisms appear to support aggregations of orange roughy and Etmopterus spp. off Tasmania; they feed on demersal species and mesopelagic or vertically migrating nekton advected laterally onto the mid-slope. The importance of teleost prey in the diet of E. baxteri apparently increases with shark length, whereas crustaceans become less important, which is a similar diet shift to that of orange roughy. Etmopterus baxteri is both a potential competitor and predator of orange roughy in these ecosystems. Fishery managers need to understand and consider the complex trophic interactions between orange roughy, sharks, and other exploited species in managing recently reopened deepwater fisheries off southeastern Australia.
Introduction
Deep-sea ecosystems, most notably seamounts, often support
diverse fish communities dominated by aggregations of long-lived,
slow-growing, pelagic and demersal species which are exploited by
deep-water fisheries (Koslow et al., 2000). Since the late 1980s, the
orange roughy (Hoplostethus atlanticus) has been heavily targeted
in mid-slope and seamount communities (700 1200 m) off
southern and eastern Tasmania (Larcombe and Begg, 2007).
Oreo dories (Oreosomatidae) and dogfish (Squalidae) have also
been taken from these depths, initially as bycatch of orange
roughy fishing, but later targeted to a lesser extent. Together,
these teleosts and sharks dominate mid-slope demersal fish
communities, with orange roughy, oreos, and dogfish each constituting
significant proportions of the total biomass of the fish caught in
demersal trawls (40 110 mm codend mesh) over these areas
(23, 22, and 20%, respectively; Koslow et al., 1994).
To prevent overfishing, orange roughy catches have been
regulated by a range of measures in Australia since the early 1990s,
including a total allowable catch set annually. Despite these
measures, orange roughy became overfished in 2006 and the
species is now listed as conservation-dependent under Australias
Environment Protection and Biodiversity Conservation (EPBC)
Act. Consequently, most of the historical fishing grounds deeper
than 700 m have been closed to fishing, except the Cascade
Plateau (Larcombe and Begg, 2007). Observer data show that
catch rates of some species of mid-slope dogfish have also declined
off southeastern Australia, and several of these species have been
assessed as high risk following ecological risk assessment, requiring
careful monitoring and management (Larcombe and Begg, 2007;
Walker and Gason, 2007).
To facilitate recovery from overfishing, a conservation
programme for orange roughy is now under development. The
management plan allows sustainable fishing for blue grenadier
(Macruronus novaezelandiae) and king dory (Cyttus traversi) in
some recently reopened areas (Deepwater Resource Assessment
Group, 2009), but it is required to consider the effect of the
previous and any future fishery on bycatch species (including
orange roughy and sharks) in addition to the target species, as
well as on habitats and the vulnerable deep-sea ecosystem.
Ecologically sustainable development (ESD) of these fisheries,
including those that continue to target aggregations of orange
roughy on the Cascade Plateau, therefore requires an
understanding of the ecological interactions between target and bycatch
species (Fletcher et al., 2005). However, little is known of the
trophic mechanisms sustaining bycatch species such as dogfish, or
of the ecological interactions between those species and orange
roughy within the exploited communities off southeastern
Australia.
The southern lanternshark (Etmopterus baxteri) is the most
abundant deep-water dogfish on the seamounts around eastern
and southern Tasmania and an important component of the
bycatch in orange roughy fisheries (Daley et al., 2002). However,
the diets of this and other lanternsharks have not been studied
in detail. In New Zealand, E. baxteri is reported to feed mainly
on fish, in (...truncated)