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The trophic ecology and food web dynamics of the mid-slope demersal fish community (700-1200 m) off southern Tasmania were investigated. The top predators in the community were orange roughy, the oreos and squalids. Orange roughy Hoplostethus atlanticus is dominant on the flat ground but even more so on the seamounts, and consequently fishing has been intense on these seamounts. Oreos are also fished on the seamounts. During 10 research cruises around south-eastern Australia, more than 9000 stomachs from orange roughy and 23 other species were collected and examined. Orange roughy ate predominantly mesopelagic and benthopelagic fish, crustaceans and squid as adults, and mainly crustaceans as juveniles suggesting resource partitioning between juveniles and adults. Orange roughy, warty dory Allocyttus verrucosus, squalids and macrourids are benthopelagic omnivores. In contrast, the smooth oreo Pseudocyttus maculatus, and the alepocephalids ate pyrosomes. Daily rates of food consumption for adult orange roughy were 1.15% body weight and for juveniles were 0.91%. The metabolism of orange roughy estimated from a mass balance model, was similar to that of active, migratory mesopelagic fishes and greater than non-migratory bathypelagic fishes. The use of enzyme activity rates of white muscle as proxies for oxygen consumption was investigated for orange roughy and seven other dominant species in the community. Lactate dehydrogenase (LDH), malate dehydrogenase (MDH) & citrate synthase (CS) activities were assayed. The fishes were all anaerobically poised, with LDH activities greater than CS activities. The LDH activities of smooth oreo, dogfish Centroscymnus crepidater and orange roughy were higher than the majority of deepliving species studied previously. The oxygen consumption rate for orange roughy, estimated from the bioenergetic model, was best estimated from LDH activities. Oxygen consumption for the seamount-associated fish similar to orange roughy, might also be best predicted from LDH activities. Food web models using Ecopath with Ecosim models were developed to examine scenarios of fishing impacts and pelagic inputs to the community through advection. Diet composition, species composition, biological and physiological parameters for each food web group, collected from the research cruises, as well as fishery catch statistics, were used in construction of the models. After simulating a decade of fishing, orange roughy declined to about 60% of the starting biomass but recovered to 99% of starting biomass over the next 40 years once fishing ceased. The oreos declined more and only recovered to 57% once fishing ceased. Advection of prey was the most important mechanism by which the community biomass on the seamounts could be sustained. High rates allowed the orange roughy stocks to recover in less than 20 years and oreos in 55 years While low advection rates would allow recovery to only two-thirds pre-fishing biomass for both fisheries.

We used the Ecopath with Ecosim software to develop a trophic mass-balance model of the eastern Great Australian Bight ecosystem, off southern Australia. Results provide an ecosystem perspective of Australia's largest fishery, the South Australian sardine fishery, by placing its establishment and growth in the context of other dynamic changes in the ecosystem, including: the development of other fisheries; changing abundances of apex predator populations and oceanographic change. We investigated the potential impacts of the sardine fishery on high tropic level predators, particularly land-breeding seals and seabirds which may be suitable ecological performance indicators of ecosystem health. Results indicate that despite the rapid growth of the sardine fishery since 1991, there has likely been a negligible fishery impact on other modelled groups, suggesting that current levels of fishing effort are not impacting negatively on the broader ecosystem structure and function in the eastern Great Australian Bight. Results highlight the importance of small pelagic fish to higher trophic levels, the trophic changes that have resulted from loss and recovery of apex predator populations, and the potential pivotal role of cephalopod biomass in regulating 'bottom-up' trophic processes. The ability to resolve and attribute potential impacts from multiple fisheries, other human impacts and ecological change in this poorly understood region is highlighted by the study, and will be critical to ensure future ecologically sustainable development within the region. Refereed/Peer-reviewed