Investigating hawksbill turtle migratory paths and foraging grounds as strongholds or targets driving critical population declines

Chris Madden Hof

doi:10.25907/00794

Once abundant across most tropical regions of the world, only approximately 4,800 adult female hawksbill turtles (Eretmochelys imbricata) are thought to remain in the Pacific Ocean, a reduction of more than 75% when compared to historic levels (Mortimer and Donnelly, 2008). With an estimated 9 million hawksbills harvested over a 150 year period for the tortoiseshell trade (Miller et al., 2019) and a continued overexploitation of turtles to the present day (Ingram et al., 2022; Wallace et al., 2010), there is concern these critically endangered turtles will face imminent localised population collapse or extinctions, if not already extirpated (Pilcher et al., 2021; Senko et al., 2022). Yet, hawksbill turtles are the least studied marine turtle in the world (Limpus, 2009), and information regarding their genetic structure and composition, migratory pathways, and foraging grounds is lacking for most populations, leading to a lack of effective policy and conservation management efforts. In the western Pacific Ocean region, the north-east Queensland (neQld) genetic stock was once considered one of the world’s largest hawksbill turtle populations. However, this population is continuing to decline and likely to be extirpated within the decade, despite the highest level of national and marine park protection afforded to the neQld index nesting site (Bell et al., 2020). Conversely, Papua New Guinea (PNG) hawksbill turtles remain unprotected and are also reported to be in decline and assessed as overexploited (Kinch 2020a; Wangunu et al., 2004). While there is some evidence of breeding population connectivity between these two neighbouring countries, whether hawksbill migratory paths and foraging grounds are strongholds, or targeted sources driving critical population declines remains poorly understood. Prior to this thesis, no studies had identified the migration routes or foraging home ranges for these populations. The overarching aim of this thesis is to use data to inform policy and to identify mitigation management that is required to recover the neQld and PNG hawksbill populations. The conservation status, threats and legislative provisions of hawksbill turtles in the western Pacific Ocean region were assessed (Chapter 2, 3), leading to the population trend and trajectory of the once world’s largest neQld stock to be evaluated. The population continues to decline (58% over the past 28 years) and is likely to be extirpated as early as 2032 (Bell et al., 2020; Chapter 4). Using satellite tracking and genetic analysis, the migratory paths and foraging grounds of the neQld stock and the likely threats continuing to prevent population recovery were also quantified (Madden Hof et al., 2023a; Chapter 5). No satellite tracked turtle left the Australian continental shelf, and their migratory pathways and home range occurrence dominated western Queensland. As a result of this study, several policy and management recommendations were provided including reducing overharvesting , mitigating fisheries interactions (through both bycatch reduction and ghost net management), and increasing or expanding protected area management. A first in PNG, the migratory paths and foraging grounds of the Conflict Group of Islands nesting population in PNG were also identified, whereby approximately 90% of the satellite tracked turtles travelled to the NE coast of Queensland to forage. Using genetics, we also defined two new management units for PNG in Milne Bay and Kavieng, thereby raising the importance of managing these populations as separate regional management units from the neQld stock and others in the western Pacific Ocean region (Madden Hof et al., 2023b; Chapter 6). Given the importance of northern Queensland as a multi-stock hawksbill turtle foraging ‘sink’, further policy protection and management mitigation is urgently needed to reverse the declining trajectory of the neQld stock and PNG population, and to ensure Queensland waters remain a stronghold for these and other western Pacific Ocean hawksbill populations (e.g. Solomon Islands and Vanuatu). This new research highlighted the need for urgent and prioritised action to assess and manage many western Pacific Ocean hawksbill populations, resulting in the development (and endorsement) of a Convention of Migratory Species Single Species Action Plan (Chapter 7). This thesis provides new insight into hawksbill ecology and movement, and has identified critical policy and management interventions required to effectively secure hawksbill turtle populations in NE Australia and PNG before their populations are likely to become extinct.

Investigating hawksbill turtle migratory paths and foraging grounds as strongholds or targets driving critical population declines

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