Threatened sawfishes of Australia: Spatial ecology and species composition through time

Nikki Biskis

doi:10.25907/01001

Northern Australia represents one of the last global strongholds for sawfishes, iconic marine predators facing severe population declines worldwide as a result of bycatch and trophy-keeping. Despite both international and domestic protection measures, significant knowledge gaps persist regarding historical baselines and current distributions within Queensland. This data deficiency, common among elasmobranch bycatch species, limits the capacity to evaluate population trajectories, assess management effectiveness, or quantify biodiversity loss. In this thesis, I use sawfishes as a case study to investigate distribution patterns using a range of monitoring techniques, including rostral morphometrics, citizen science, microchemical analysis, and historical archives. For historically exploited trophy species, the scale of population declines is often mirrored by an overall reduction of mean maximum total length (Lmax), due to size-selective harvesting. Therefore Lmax serves as a reliable proxy for reproductive potential, and an indicator for the viability of a population. Though historical population records are scarce, the abundance of preserved trophy rostra provides an opportunity to reconstruct size trends, first requiring validation of length estimation methods. Morphometric relationships between rostrum length and total body length were evaluated across Indo-Pacific sawfish species of all life history stages, using citizen science photographs. Comparative analysis of established models confirmed accurate relationships for P. clavata and P. pristis, and for the first time a model was built for P. zijsron inclusive of adults. For Anoxypristis cuspidata, ontogenetic changes in rostrum morphology invalidated existing models for mature individuals, thus highlighting the importance of comprehensive size representation moving forward. Building on validated morphometrics, population trends within Queensland during the critical period before and after federal protection were assessed using public sightings data (1983-2023) from Sharks And Rays Australia's (SARA) national citizen science database. Shifts in species composition and distribution indicate that the faster reproducing A. cuspidata, is now more frequently encountered than the three Pristis species, particularly along the east coast. Negative correlations between body size and capture year were present across all species, and while nursery areas were identified for P. pristis and P. clavata, spatial overlap with high mortality areas highlighted ongoing threats for P. zijsron despite conservation measures. Updated species distribution maps identified priority areas for conservation and education while forewarning future declines for the larger-bodied sawfish species. Contemporary sawfish distribution assessments are complicated within areas of ongoing illegal take, as confiscated rostra lack fine-scale locality data. To address this gap, trace elemental analysis of rostral teeth via Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was evaluated as a tool to assign geographic origin, using specimens of known provenance. Results from 146 individuals across 16 estuaries demonstrated promising potential to identify capture locations at regional scales, reducing uncertainty by hundreds of kilometres. Beyond geographic assignment, elemental transects along the tooth growth axis and associated vertebrae were used to reconstruct individual environmental histories. Together, these methods provided rare insights into adult sawfish ecology where otherwise not available, despite the opportunistic nature of specimen collection. Finally, through extensive archival research incorporating newspaper records and museum collections, sawfish distributions along Queensland's east coast during the 19th-20th centuries were mapped. Historical records revealed previously unpublished population hotspots and range contractions, particularly in South East Queensland and northern New South Wales, that coincided with European settlement, population growth, and habitat modification. These records also documented the shift in public perceptions from 'monsters' to species of conservation concern. Collectively, this research demonstrates the value of integrating multiple data sources to reconstruct baseline populations, document range contractions, and identify priority conservation areas for endangered megafauna. These findings provide crucial insights for focusing management strategies for Australia’s sawfish populations, while the methodological framework offers a template for assessment of other data-deficient threatened marine species.

Threatened sawfishes of Australia: Spatial ecology and species composition through time

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