Climate-driven global redistribution of an ocean giant predicts increased threat from shipping

nature climate change Article https://doi.org/10.1038/s41558-024-02129-5 Climate-driven global redistribution of an ocean giant predicts increased threat from shipping Received: 21 February 2024 A list of authors and their affiliations appears at the end of the paper Accepted: 21 August 2024 Published online: xx xx xxxx Check for updates 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. Global warming is one of the most pervasive facets of human-driven climate change, with the magnitude of projected temperature rises over the 21st century comparable to that of the largest global changes in the past 65 million years1,2. Biological responses to warming are already apparent across terrestrial3, freshwater4 and marine taxa5,6. As environments change, species must either adapt, tolerate, move or face extinction7–9. A series of commonly articulated hypotheses have emerged in relation to movement, whereby species are expected to shift their distributions under warming to greater elevations (altitude), higher latitudes or deeper ocean depths to remain within suitable environmental conditions10–12. Marine taxa, in particular, are highly responsive to temperature change, and can closely follow isotherms with fewer physical barriers to dispersal compared with their terrestrial counterparts5,13,14. As a result, marine species are moving poleward as much as six times faster15, with global redistributions projected for over 12,000 species16. The general expectation is that polar and temperate regions will act as ‘sinks’ and tropical regions as ‘sources’16–20. Profound alterations to ecosystem e-mail: Nature Climate Change structure and function can result from these shifts in marine socioecological systems, ultimately impacting human communities21. For highly mobile marine megafauna that can travel hundreds or thousands of kilometres annually22, these hypotheses have only recently begun to be addressed due to logistical difficulties in their monitoring23. There is some evidence for potential habitat losses, core habitat displacement and divergent responses among species with differing life histories24,25. However, the location of many species’ future habitats remains an open question. In addition, it remains unknown how climate-driven habitat redistribution will affect their exposure to existing anthropogenic threats such as collisions with ships26,27 or fishing exploitation28, even though such impacts may exacerbate population declines already occurring. Ocean climate changes may shift marine megafauna into new habitats with busier shipping activity, increasing their vulnerability to collisions and potentially compounded by predicted future increases in shipping traffic29,30. Alternatively, habitats may shift into safer areas with less activity, providing refuge. Quantitative understandings of Article the interactions between wildlife movement, human activities and climate change are now needed for incorporation into conservation assessments, as well as into global strategic planning frameworks (for example, cbd.int/cop). However, global insights based on dynamic animal movements are still lacking31. To address this, we tested whether a highly mobile, globally distributed marine megafauna species—the world’s largest fish, the whale shark (Rhincodon typus)—conforms to commonly held distributional hypotheses under climate change across its entire range (for example, ocean basin-scale poleward shifts32), while quantifying changes in co-occurrence with shipping. The whale shark serves as a model species to test these ideas for marine megafauna because of its circumtropical distribution and expected climate responses32,33, and is classified as ‘Endangered’ in the International Union for Conservation of Nature (IUCN) Red List34. Recent evidence suggests that the species is particularly vulnerable to ship collisions due to its extensive use of surface waters and the high overlap of its habitat with marine traffic26. Therefore, it is possible that relatively small climate-induced changes in distribution could have a disproportionate impact on collision vulnerability for whale sharks, and potentially other marine megafauna30. To explore potential climate responses and co-occurrence with shipping, we used a whale shark satellite-tracking dataset spanning 15 years, including tagging sites in all major oceans they inhabit (348 individuals collectively tracked for >15,000 days). Using these data, together with oceanographic variables and global climate models from the Coupled Model Intercomparison Phase 6 (CMIP6), distribution models were developed to (1) generate a first-order approximation of global habitat suitability and (2) project the distribution of whale sharks in two future decades under three mitigation scenarios. These were then used to (3) assess habitat changes and horizontal co-occurrence with shipping traffic. Results Whale shark habitat suitability maps—defined as areas where a given environment has the capacity to support whale sharks, thus determining the likelihood of their presence—were generated using a series of correlative distribution models based on a suite of oceanographic variables and tracked animal movements. Data preparation, model algorithm and oceanographic variable selection followed careful procedures with sensitivity checks (Supplementary Information 3.3, 3.4 and 4.4). Final models performed well in quantitative and qualitative validation tests and were used to explore current and projected future habitat areas for whale sharks (Supplementary Information 4.2 and 4.3). Future habitats were based on CMIP6 data for the years 2050 and 2100 under the shared socioeconomic pathways (SSPs) ssp126, ssp370 and ssp585 (Supplementary Information 3.5). Ocean-scale habitat shifts Current regions of whale shark habitat suitability were predicted circumglobally within tropical, subtropical and temperate wate (...truncated)