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Dynamic predictive modelling and bycatch mitigation for threatened, endangered, and protected species, in Australia’s Eastern Tuna and Billfish Fishery
Dissertation

Dynamic predictive modelling and bycatch mitigation for threatened, endangered, and protected species, in Australia’s Eastern Tuna and Billfish Fishery

Lisa Walton
University of the Sunshine Coast, Queensland
Doctor of Philosophy, University of the Sunshine Coast, Queensland
2026
DOI:
https://doi.org/10.25907/01099
pdf
Thesis 14.01 MB
Embargoed Access, Embargo ends: 25-Aug-2027 CC BY-NC V4.0

Abstract

Marine and estuarine ecology (incl. marine ichthyology) Fisheries management bycatch longline fisheries marine turtles seabirds East Australian Current
The marine environment is experiencing a global biodiversity crisis. Predatory marine megafauna play a critical role keeping ocean ecosystems balanced, yet impacts of commercial fisheries remain a major threat to many of these species. Pelagic longline fisheries target high-value teleost fishes such as tuna and billfish worldwide, but also interact with highly threatened, endangered, or protected marine megafauna, including seabirds and marine turtles. Although substantial effort has been directed towards improving bycatch mitigation in longline fisheries, the diversity of available options remain limited. Effective mitigation is not a single solution and requires a combination of approaches, including improved understanding of the environmental drivers of bycatch risk alongside practical mitigation technologies that provide additional protection for the most vulnerable species. This thesis integrates dynamic predictive modelling of bycatch risk as a function of ocean conditions with experimental evaluation of mitigation strategies to reduce interactions between commercial fisheries and vulnerable marine megafauna. Chapter Two focuses on one of the most endangered leatherback turtle populations globally in the south-west Pacific Ocean, which nest in Indonesia and migrate to sub-tropical and temperate foraging grounds along the east coast of Australia and the Tasman Sea. Interactions between this population and Australia’s largest tuna longline fishery were modelled using oceanographic variables to identify areas of elevated bycatch risk, with moon phase and sub-surface seascape features identified as significant predictors. Chapter Three explores a suite of environmental drivers of bycatch risk for albatrosses and shearwaters. As surface-foraging species, ocean conditions at depth were less influential for seabirds than for leatherback turtles, while higher bycatch risk was associated with seasonal and latitudinal patterns, and the margins of eddies and currents. Finally, Chapter Four investigates a range of seabird bycatch mitigation approaches not previously examined in an operational fishery setting, assessing hook sink rates in a pelagic longline fishery across multiple experimental trials. Novel gear varieties, different setting speeds and current conditions, port and starboard setting under varying current directions, and alternative hook positions on live bait were all explored. Several of these effects have not been examined previously, producing findings that are new to the field of seabird bycatch mitigation. Overall, this thesis addresses critical knowledge gaps in sustainable fisheries management for highly vulnerable marine species. It provides new insight into the patterns, trends, and oceanographic conditions under which marine predators are more likely to interact with pelagic longline fisheries. It also delivers practical, evidence-based recommendations. Together, these findings offer globally relevant guidance for reducing interactions with threatened, endangered, and protected species such as leatherback turtles and Procellariiformes, while supporting the economic viability and sustainability of pelagic longline fisheries.

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