Davide Lomeo, Emma Tebbs, Nlingisisi Dombole Babayani, Michael Chadwick, Mangaliso John Gondwe, Anne Jungblut, Graham McCulloch, Anna Songhurst, Eric Morgan, Daniel Schillereff, Stefan Simis

The 2020 mass mortality of 350 African elephants (Loxodonta africana) in the Okavango Delta, Botswana sparked global concern. These deaths have been tentatively linked to cyanotoxins in watering holes (pans), but evidence remains inconclusive. In this study, we used remote sensing in combination with spatial analysis to explore the relationship between the ecohydrology of ~3,000 pans and the locations of deceased elephants. Our findings reveal a significant difference in the spatial distribution of fresh versus decayed elephant carcasses (p < 0.001), suggesting that the die-off event deviated from characteristic, regional patterns of elephant deaths. From our analysis we identified 20 pans near the sites of fresh carcasses that experienced increased phytoplankton (microalgae or cyanobacteria) bloom events in 2020 (n = 123) compared to the previous 3 years combined (n = 23). Additionally, these pans in 2020 also exhibited the highest average phytoplankton biomass of the period 2015 - 2023 (Normalised Difference Chlorophyll Index > 0.2; p < 0.001). These findings suggested an elevated risk and higher likelihood of cyanotoxins presence in these pans. Our spatial analysis also demonstrated that elephants walked an average of 16.5 km (± 6.2 km) and died within 88 hours (± 33 hours) from initial exposure, offering metrics that were previously unknown for these elephants. Our study presents important evidence that cyanobacterial toxicity could be a factor in the 2020 mass die-off, while also considering other possible causes. Moreover, we highlight the necessity of integrating spatial analysis and ecohydrological assessments to inform conservation strategies to potentially mitigate future mortality events.