Samantha Reynolds

Aggregations are key events, supporting critical ecological and biological functions in many species. For highly mobile and elusive species, aggregations often provide the only feasible opportunities for research. Whale sharks (Rhincodon typus) form at least 30 consistent seasonal aggregation sites globally, yet none have been documented in the Coral Sea, despite sporadic sightings of solitary individuals and groups. This study aimed to identify and characterise the first whale shark aggregation on Australia's east coast by predicting potential sites through a data layering approach and confirming their presence through targeted field expeditions. A combination of historical sightings data, expert and anecdotal knowledge, and scientific knowledge from other whale shark aggregation sites led to the identification of Wreck Bay, situated at the far northern Great Barrier Reef, as potential aggregation habitat. An initial field expedition in 2019 confirmed the aggregation, and three subsequent voyages in 2021-2024 gathered further demographic and movement data. A total of 59 individuals were identified, with a strong male bias (3.5:1) and all classified as immature sharks ranging from 3.5 to 8.0 m in estimated total length. Satellite tracking revealed a mean residence time of approximately 3 weeks (21.6 days +/- 10.1 SD; range: 7-43 days), with some individuals revisiting the aggregation in subsequent years. The peak aggregation period occurs from late November to late December, with movements concentrated along the continental shelf before dispersing into the Coral Sea. Tracked sharks (n = 18) exhibited wide-ranging movements, with a mean track duration of 144 days (range: 3-770 days) and a mean total track length of 1463 km (range: 19-11,355 km). This study provides the first evidence of a whale shark aggregation in the Coral Sea and highlights Wreck Bay as key habitat for this iconic and globally endangered species.

Aim
To understand how natural geomorphological features and oil and gas platforms (OG platforms) influence the habitat use and seascape connectivity of the whale shark (Rhincodon typus).

Location
East-Indian Ocean and North-West Australia.

Methods
We compiled a satellite tracking dataset of 78 whale sharks tagged across a 14-year period at Ningaloo Reef and Shark Bay World Heritage Areas in Western Australia to develop spatial networks for the regions of the East-Indian Ocean and North-West Australia. We then applied a Bayesian modelling framework to assess the effects of natural features and OG platforms on spatial patterns and habitat connectivity.

Results
Geomorphological features such as pinnacles, canyons, and seamounts promoted habitat connectivity and strongly influenced the habitat use of whale sharks across both regional (1000's km; East-Indian Ocean) and local (100's km; North-West Australia) spatial scales. In the North-West of Australia, OG platforms had similar effects on habitat use as natural feature types and also enhanced habitat connectivity. The OG platforms most visited by whale sharks were situated close to the edge of the continental shelf and near natural geomorphological features that likely enhance productivity.

Main Conclusion
Our work identified natural geomorphological features that promoted habitat use and connectivity for whale sharks across oceanic and coastal seascapes. Sharks routinely visited OG platforms, which acted as migratory stepping stones that further enhanced habitat connectivity. Protection of natural feature types that promote habitat use and connectivity could assist conservation management of whale sharks. We suggest that the influence of OG platforms on their movement and habitat use beyond individual structures, should be considered in environmental impact assessments during operation and decommissioning phases.

Climate change is shifting animal distributions. However, the extent to which future global habitats of threatened marine megafauna will overlap existing human threats remains unresolved. Here we use global climate models and habitat suitability estimated from long-term satellite-tracking data of the world’s largest fish, the whale shark, to show that redistributions of present-day habitats are projected to increase the species’ co-occurrence with global shipping. Our model projects core habitat area losses of >50% within some national waters by 2100, with geographic shifts of over 1,000 km (∼12 km yr −1 ). Greater habitat suitability is predicted in current range-edge areas, increasing the co-occurrence of sharks with large ships. This future increase was ∼15,000 times greater under high emissions compared with a sustainable development scenario. Results demonstrate that climate-induced global species redistributions that increase exposure to direct sources of mortality are possible, emphasizing the need for quantitative climate-threat predictions in conservation assessments of endangered marine megafauna.

Responses of organisms to climate warming are variable and complex. Effects on species distributions are already evident and mean global surface ocean temperatures are likely to warm by up to 4.1 °C by 2100, substantially impacting the physiology and distributions of ectotherms. The largest marine ectotherm, the whale shark Rhincodon typus, broadly prefers sea surface temperatures (SST) ranging from 23 to 30 °C. Whole-species distribution models have projected a poleward range shift under future scenarios of climate change, but these models do not consider intraspecific variation or phenotypic plasticity in thermal limits when modelling species responses, and the impact of climate warming on the energetic requirements of whale sharks is unknown. Using a dataset of 111 whale shark movement tracks from aggregation sites in five countries across the Indian Ocean and the latest Earth-system modelling produced from Coupled Model Intercomparison Project Phase 6 for the Intergovernmental Panel on Climate Change, we examined how SST and total zooplankton biomass, their main food source, may change in the future, and what this means for the energetic balance and extent of suitable habitat for whale sharks. Earth System Models, under three Shared Socioeconomic Pathways (SSPs; SSP1–2.6, SSP3–7.0 and SSP58.5), project that by 2100 mean SST in four regions where whale shark aggregations are found will increase by up to 4.9 °C relative to the present, while zooplankton biomass will decrease. This reduction in zooplankton is projected to be accompanied by an increase in the energetic requirements of whale sharks because warmer water temperatures will increase their metabolic rate. We found marked differences in projected changes in the extent of suitable habitat when comparing a whole-species distribution model to one including regional variation. This suggests that the conventional approach of combining data from different regions within a species' distribution could underestimate the amount of local adaptation in populations, although parameterising local models could also suffer from having insufficient data and lead to model mis-specification or highly uncertain estimates. Our study highlights the need for further research into whale shark thermal tolerances and energetics, the complexities involved in projecting species responses to climate change, and the potential importance of considering intraspecific variation when building species distribution models.