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

Global warming is a significant aspect of human-induced climate change, with projected temperature increases over the 21st century comparable to the largest global changes in the past 65 million years. Biological responses to warming are already evident across various taxa. Species must adapt, tolerate, move, or face extinction as environments change. Movement is a key response, with species expected to shift their distributions to higher elevations, latitudes, or depths to maintain suitable conditions. Marine species, particularly, are highly responsive to temperature change, and can closely track isotherms. Consequently, marine species are moving poleward at an accelerated rate, with global redistributions projected for thousands of species. Polar and temperate regions are expected to act as 'sinks' while tropical regions act as 'sources'. These shifts can profoundly alter ecosystem structure and function, ultimately affecting human communities. For highly mobile marine megafauna, these hypotheses are only recently being investigated due to monitoring difficulties. Evidence suggests habitat losses, core habitat displacement, and varying responses among species with differing life histories. However, the location of many species' future habitats and the impact of climate-driven redistribution on anthropogenic threats (like ship collisions) remain unclear. Ocean climate changes may shift marine megafauna into new habitats with increased shipping activity, increasing their vulnerability to collisions, potentially exacerbated by future shipping increases. Alternatively, shifts into safer areas may provide refuge. Quantitative understanding of the interactions between wildlife movement, human activities, and climate change is crucial for conservation assessments and global strategic planning.

Several studies have explored the impacts of climate change on species distributions. Parmesan & Yohe (2003) demonstrated a globally coherent fingerprint of climate change impacts across natural systems, while Poloczanska et al. (2013) highlighted the global imprint of climate change on marine life. Research has shown that marine taxa are highly responsive to temperature change, often shifting their distributions poleward at rates exceeding those observed in terrestrial systems (Pinsky et al., 2019; Sunday et al., 2012). Studies have also projected global redistribution for many marine species (Lenoir et al., 2020). Previous research has focused on understanding potential habitat losses and shifts for specific species (Lezama-Ochoa et al., 2024; Braun et al., 2023), but the combined effect of climate-driven habitat redistribution and increased exposure to existing anthropogenic threats has received less attention. Womersley et al. (2022) identified global collision-risk hotspots for whale sharks and marine traffic, highlighting the need to consider the intersection of these factors.

This study utilized a 15-year (2005–2019) satellite-tracking dataset of whale sharks ( *Rhincodon typus*), including individuals tagged across all major oceans. The dataset comprised 348 individuals tracked for >15,000 days. This data, combined with oceanographic variables and global climate models from CMIP6, was used to develop distribution models to (1) estimate global habitat suitability and (2) project whale shark distribution in 2050 and 2100 under three SSPs (ssp126, ssp370, ssp585). These projections were then used to (3) assess habitat changes and co-occurrence with shipping traffic. Habitat suitability maps were generated using correlative distribution models based on oceanographic variables and tracked animal movements. Data preparation, model algorithms, and variable selection were rigorously implemented with sensitivity checks. Future habitat projections were based on CMIP6 data for the specified years under the three SSPs. To assess the impact of climate change on the whale shark's exposure to ship strikes, the study adapted a previously validated whale shark-ship collision risk index. This was recalculated as a ship co-occurrence index (SCI) based on predicted and projected habitat suitability and shipping traffic density. The SCI was calculated for all EEZs within the whale shark's range for the baseline period (2005–2019) and compared with future projections. The SCI increase was driven by increases in habitat suitability in new regions overlapping with high shipping activity and increases in suitable regions with lower activity.

The study's habitat suitability maps predicted circumglobal whale shark distribution in tropical, subtropical, and temperate waters (2005–2019). Future projections revealed increasing habitat suitability at the range edges of current distributions across all scenarios. However, patterns varied regionally. By 2100, under the high-emissions scenario (ssp585), the east Pacific showed habitat reduction in equatorial waters, with potential losses in some areas coinciding with expansion into new mid-latitude regions. Changes were linked to projected shifts in oceanographic conditions, such as chlorophyll *a* and temperature. The north Atlantic showed a pronounced habitat shift away from the Gulf of Mexico into equatorial waters. An independent AquaMaps model confirmed increasing habitat suitability in equatorial waters, suggesting that whale sharks may tolerate warmer oceans than they currently occupy, but not necessarily the increase in salinity. Habitat shifts increased in intensity across ssp370 and ssp585 through to 2100, suggesting that under sustainable development, habitat shifts will be less extreme. By 2050, core habitat (90th percentile current habitat suitability) was expected to decrease in some areas and increase in others under all scenarios. By 2100, greater variation between scenarios was observed, with significant increases and decreases in core habitat area projected for the north Atlantic and east Pacific, respectively. Northerly core habitat cold edges shifted at a rate of 12 km yr⁻¹ (mid-century, 15 km yr⁻¹; end of century, 9 km yr⁻¹), significantly faster than southerly cold edges. These poleward shifts align with projections for chondrichthyan fishes. Analysis of national waters revealed latitudinal shifts in both mean habitat suitability and core habitat coverage across the Atlantic, Pacific, and Indian Oceans. Many currently less suitable EEZs were projected to show increased mean habitat suitability in the future, while reductions were apparent in some Pacific EEZs currently supporting suitable habitats. The seasonal patterns were also projected to change in several regions. Global redistribution is predicted, with a shift from current centers to range-edge habitats. Medium-importance areas (mean habitat suitability 0.05–0.5) are projected to become more suitable, whereas high-importance areas (>0.5) will become less suitable. Under the high-emissions scenario, 57.5% of EEZs will experience suitable habitat losses >50% by 2100, while 65.5% will gain core habitat coverage of >50% under ssp126. The SCI increased in all future scenarios, even when ship numbers were held constant. Under the high-emissions scenario, the SCI was >15,000 times greater by 2100 compared with present-day habitats. Under sustainable development, this increase was reduced, but still substantial.

This study's projections of whale shark habitat changes and increased ship co-occurrence highlight the significant impacts of climate change on this endangered species. The observed poleward shifts and habitat redistribution emphasize the importance of incorporating climate change into conservation strategies. The findings suggest a potential for substantial core habitat losses in some national waters and increased risk of ship strikes in regions with expanding whale shark habitat and high shipping density. The complex consequences of habitat shifts for whale shark ecology and life history need to be considered, including the potential impacts on key aggregation sites and breeding areas. These shifts could have broader consequences for countries reliant on whale shark tourism. The methods developed in this study can be applied to other species to inform conservation efforts and help design protection networks encompassing future habitats. Future research should further explore the vertical dimension of collision risk by examining diving behavior and how that interacts with changing ocean conditions and shipping activity.

This study projects significant changes in whale shark habitats and a substantial increase in their overlap with global shipping under future climate scenarios. The results highlight the urgent need for climate-informed conservation strategies to mitigate the potential for increased mortality through ship collisions. The methodologies employed provide a valuable framework for assessing the impacts of climate change on other vulnerable marine megafauna and for incorporating these projections into broader conservation initiatives. Future research should focus on more detailed species-specific models incorporating other compound stressors such as deoxygenation and ocean acidification and improved modelling of ship traffic to more accurately predict future collision risk.

The study's reliance on correlative models means the projections are based on correlations between environmental variables and whale shark occurrences. The accuracy of the projections depends on the accuracy of future climate models. The SCI does not account for the dynamic movements of both ships and sharks, nor does it incorporate factors beyond spatial overlap (such as whale shark diving behavior), and therefore does not represent an absolute collision risk assessment. The study's focus on a single species limits the generalizability of the findings to other marine megafauna, although the methodologies used could be readily adapted for other species.