Rays of light: A multifaceted investigation into the distribution and ecology of globally threatened, largebodied rhino rays in Western Australia

Rebecca L Bateman

Despite their conspicuous appearance, large, shark-like batoid elasmobranchs such as sawfishes (Pristidae), wedgefishes (Rhinidae), and giant guitarfishes (Glaucostegidae) remain among the least understood chondrichthyans and have suffered sharp population declines throughout most of their range. These three groups fall under the collective term ‘rhino rays’, a broader grouping that also includes smaller-bodied species such as banjo rays and guitarfishes. Large-bodied rhino rays have been heavily impacted by overfishing, largely due to the high value of their fins, and are especially susceptible to bycatch and habitat loss due to their coastal distribution. In addition to population declines in some regions, several factors may have hindered our understanding of their distribution, ecology, and biology. These include taxonomic uncertainties within the families Pristidae, Glaucostegidae, and Rhinidae, as well as their similar morphological features and overlapping ranges which can obscure accurate species-specific knowledge. Within Western Australia, sawfishes, wedgefishes, and giant guitarfishes are known to occur throughout both remote and populated Indian Ocean coastal habitats, some of which are subject to commercial fishing, tourism pressure, and various levels of industrial development. This thesis aims to improve the general understanding of large-bodied rhino ray diversity, distribution, ecology, and habitat use within Western Australia to inform the future management and conservation of these rays. I combined citizen science observations, results from targeted research surveys, and commercial fishing records to examine the diversity and distribution of the members of the Pristidae and Glaucostegidae within the Ningaloo Reef, Exmouth Gulf, and wider Pilbara region, and the Rhinidae throughout the state of Western Australia. In addition, to investigate residency, activity, and habitat use, I used gillnet and cast net surveys to capture and acoustically tag Giant Shovelnose rays (Glaucostegus typus) in a coastal embayment of the Exmouth Gulf. Findings demonstrate that northwestern Western Australia is inhabited by all eight species of Australia’s large-bodied rhino rays. Specifically, the Exmouth Gulf and surrounding coasts provide a high diversity of these rays (7 of 8 species), harbour reproductive grounds for G. typus, and support globally important pupping areas for Green Sawfish (Pristis zijsron). The Bottlenose Wedgefish (Rhynchobatus australiae) was the most commonly identified rhinid in this region, while the Shark Ray (Rhina ancylostomus) and Eyebrow Wedgefish (Rhynchobatus palpebratus) were less frequently observed. More broadly, Western Australia was revealed as a promising global refuge for the three species of wedgefishes found within Australian waters, and R. australiae, in particular, was widely distributed along the Western Australian coastline. Several range and depth extensions are presented for specific wedgefish species. Further evidence is offered for the sexual dimorphism, and size and sexual spatial segregation of Rhynchobatus spp., along with details regarding potential pupping locations and habitat preferences of this genus. In the Bay of Rest in Exmouth Gulf, gillnet surveys indicated a high catch per unit effort of juvenile G. typus, and acoustic tracking data from tagged individuals offer several insights into this species’ fine-scale ecology. Tagged rays were generally highly resident within a small, shallow, sandy embayment. Glaucostegus typus activity peaked in the afternoon and was positively related to temperature, yet inversely related to tide height. Acoustic data suggested restricted space use and refuge behaviour by juvenile G. typus in the Bay of Rest, which appears to be a nursery for this species and therefore warrants protection. This thesis provides a comprehensive overview of the diversity and distribution of large-bodied rhino rays within Western Australia, with a particular emphasis on the northwest of the state, and builds on existing ecological and biological knowledge of these species. These findings will help fill and highlight crucial knowledge gaps essential for the future conservation and management of these globally threatened species, aiding in species conservation assessments, fisheries management plans, and marine park spatial planning.

Rays of light: A multifaceted investigation into the distribution and ecology of globally threatened, largebodied rhino rays in Western Australia

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