Feeding ecology of the southern lanternshark (Etmopterus baxteri) and the brown lanternshark (E. unicolor) off southeastern Australia

ICES Journal of Marine Science, Jan 2011

Hallett, C. S., and Daley, R. K. 2011. Feeding ecology of the southern lanternshark (Etmopterus baxteri) and the brown lanternshark (E. unicolor) off southeastern Australia. – ICES Journal of Marine Science, 68: 157–165. 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 deep-water fisheries off southeastern Australia.

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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)


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Chris S. Hallett, Ross K. Daley. Feeding ecology of the southern lanternshark (Etmopterus baxteri) and the brown lanternshark (E. unicolor) off southeastern Australia, ICES Journal of Marine Science, 2011, pp. 157-165, 68/1, DOI: 10.1093/icesjms/fsq143