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Marine turtle populations worldwide are threatened by a number of anthropogenic activities, of which fishing is unquestionably one of the most harmful. Though the effects of fisheries mortality have been documented across species, gear types and regions, management intervention remains constrained by data limitations even in well-monitored fisheries in developed nations. The issue of illegal, unreported and unregulated (IUU) fishing introduces further uncertainty into fisheries management regimes, with a lack of understanding of how IUU fishing activities affect marine turtle populations. For both legal and IUU fisheries, such data deficiencies hinder the development of targeted mitigation strategies for reducing fisheries-related injury and mortality of marine turtles. I chose to examine specific issues of data deficiency for marine turtles and fisheries in the Indian Ocean and Southeast Asia region (IOSEA), an area identified as having critical data needs for multiple fleet types and turtle populations.  I began by demonstrating the need for cross-jurisdictional assessment of turtle bycatch in legal, regulated fisheries. I evaluated the cumulative patterns of turtle bycatch in Australia using longitudinal datasets from commercial fisheries in three separate management jurisdictions (Chapter 2). The results of this chapter point to potential cumulative impacts to vulnerable turtle populations (i.e. leatherback and olive ridley) arising from interactions with multiple fisheries. Spatial analysis revealed a bycatch 'hotspot' in the Gulf of Carpentaria, where commercial fisheries impact multiple turtle species and genetic stocks. These results illustrate the need to set cumulative, cross-jurisdictional bycatch quotas for marine turtles, and to evaluate turtle bycatch at the population level instead of separately within individual fisheries. I also stress the need for timely collaboration between management agencies in order to implement effective, biologically relevant management strategies for marine turtles and other vulnerable taxa.  Next, I evaluated IUU fishing as a threat to marine turtles in the IOSEA (Chapter 3). The unlawful nature of IUU fishing makes it difficult to study directly and thus required gathering data from outside traditional bureaucratic reporting frameworks. Using the expert elicitation technique, I determined that IUU fishing is likely to have potentially significant impacts on marine turtle populations in the IOSEA through targeted exploitation, use of prohibited gears and international wildlife trafficking. IUU fishing activities were found to vary in nature and magnitude on sub-regional scales, with Southeast Asia emerging as an area of concern due to the targeted exploitation and trafficking of marine turtles by IUU vessels. This chapter constitutes the first expert consensus characterising IUU fishing as a serious threat to marine turtles. These results indicate a demonstrable need to strengthen monitoring, control and surveillance (MCS) efforts and to employ regional coordination to help build capacity in lessdeveloped nations.  To complement the broad-scale assessment of IUU fishing performed in Chapter 3, I then examined IUU fishing dynamics relating to marine turtles within the Southeast Asia subregion (Chapter 4). During interviews with commercial fishers in two Malaysian states (i.e. Terengganu and Sabah), I found that the root causes of IUU fishing differ considerably between states, as do the dynamics of marine turtle capture and trade. This chapter also provides evidence linking IUU fishing vessels to the direct capture, trade and transshipment of marine turtles in Malaysia; such activities are likely to occur in other nations throughout the IOSEA, particularly where underlying situational factors are similar. Given these statespecific differences in IUU fishing practices and motivations, I conclude that the enforcement response must be similarly nuanced in order to address the varying drivers of IUU fishing in each state context. Further, an international, collaborative and pluralistic regulatory approach is needed to reduce IUU fishing and wildlife trafficking, as these are interconnected facets of a broader issue of unlawful marine resource extraction.  As IUU fishing persists despite the large number of political instruments and initiatives aimed at eliminating it, an examination of the barriers to effective policy implementation and enforcement is needed. To address this need, I designed a structured survey for officials working in marine conservation, fisheries management and enforcement throughout the IOSEA (Chapter 5). Survey results indicate that while IUU fishing is considered a management priority throughout the IOSEA, on-ground action is hindered by scale mismatches and capacity shortfalls. Among management agencies in the IOSEA, there is a mismatch between the acknowledgement that inter-agency collaboration is important and the reported degree of its implementation. These results also identified a number of knowledge gaps that managers believed would be useful in reducing the incidence of IUU fishing and marine turtle exploitation. I emphasise that decentralised fisheries management strategies have the potential to develop targeted, locally-based solutions, and also present an opportunity for much-needed data-gathering. Finally, drawing partly on the results of Chapter 4, I conclude that improvements in MCS measures must develop alongside advances in understanding of the drivers and barriers present in each local context.  Based on the combined results of my interdisciplinary thesis, legal and IUU fisheries interact with marine turtles in a number of ways that have consequences for the survival of turtle populations. Existing national and international instruments are not sufficiently equipped to tackle the variety of fishing-related threats to marine turtles, and instead will require adoption of cross-jurisdictional, pluralistic and potentially decentralised management approaches in order to enact change at the level of individual fishers. In Chapter 6, I emphasise the value of my approach for evaluating widespread, complex and data-limited threats, and that my contribution informs management efforts for both fisheries and marine turtles. I then conclude the thesis by identifying avenues for useful future research.

Addressing sources of data deficiency for sea turtles and fisheries in the Indian Ocean and Southeast Asia