Abstract
The impacts on marine species from secular warming and heatwaves are well demonstrated; however, the impacts of extreme cold events are poorly understood. Here we link the death of organisms from 81 species to an intense cold upwelling event in the Agulhas Current, and show trends of increasing frequency and intensification of upwelling in the Agulhas Current and East Australian Current. Using electronic tagging, we illustrate the potential impacts of upwelling events on the movement behaviour of bull sharks Carcharhinus leucas, including alterations of migratory patterns and maintenance of shallower dive profiles when transiting through upwelling cells. Increasing upwelling could result in ‘bait and switch’ situations, where climate change expands subtropical species’ distribution, while simultaneously exposing climate migrants to an increased risk of cold-mortality events at poleward distributional limits. This shows the potential impacts of increased cold events, an understudied aspect of climate change research, and highlights the complexities of climate change effects on marine ecosystems.
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Data availability
Both the OSTIA and the CCI satellite SST product were accessed at https://marine.copernicus.eu/. The AVHRR Night satellite SST product was accessed at https://coastwatch.pfeg.noaa.gov/erddap/griddap/erdPH53sstnmday_Lon0360.html, while the AVHRR Day satellite SST product was accessed at https://coastwatch.pfeg.noaa.gov/erddap/griddap/erdPH53sstdmday_Lon0360.html. The level-3 Aqua MODIS satellite SST product was accessed at https://coastwatch.pfeg.noaa.gov/erddap/griddap/erdMH1sstd1dayR20190SQ.html. NetCDF-files used to map the progression of the lethal upwelling event were downloaded from the OceanColor webserver (https://oceancolor.gsfc.nasa.gov/l3/). Long-term wind data for South Africa can be requested from the South African Weather Service at https://www.weathersa.co.za/. Wind data for key dates in the paper are supplied in Supplementary Information. Data associated with bull sharks tagged in Sydney Harbour (temperature, depth, acoustic detections) are publicly available from the animal tracking facility at IMOS https://animaltracking.aodn.org.au/ (see Supplementary Information for tagging dates of sharks). Acoustic tracking data for bull sharks tagged in Southern Africa are available from the South African Acoustic Tracking Array Platform, https://www.saiab.ac.za/atap.htm. Mooring temperature data for Sydney are publicly available at the Australian Ocean Data Network at https://portal.aodn.org.au/ (mooring code SYD100). In situ temperature data collected by a logger at Port Alfred are supplied in Supplementary Information. PSAT temperature and depth data for bull sharks tagged in southern Africa (n = 15) are available at https://doi.org/10.5061/dryad.xd2547dn1 together with an example file for the OSTIA satellite SST product and example code66.
Code availability
Example code for modelling of upwelling variables over time can be accessed at https://doi.org/10.5061/dryad.xd2547dn1 (ref. 66). No public code was used. All code was produced in the statistical software R (https://www.R-project.org/)64. The main packages used were heatwaveR60 and Dharma, which is available at http://florianhartig.github.io/DHARMa/.
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Acknowledgements
We thank the Acoustic Tracking Array Platform (ATAP) hosted by the National Research Foundation - South African Institute for Aquatic Biodiversity, the Ocean Tracking Network, the Department of Science and Innovation−Shallow Marine and Coastal Research Infrastructure programme, and the NRF-South African Environmental Observation Network Elwandle Node for providing acoustic telemetry hardware and in situ ocean temperature recorders that facilitated data collection for this study. We thank the Save Our Seas Foundation and the African Coelacanth Ecosystem Programme for funding running expenses for the ATAP. Additional temperature logger (UTR) data were used from the ‘SASMIA UTR Southcoast network’ (South African Squid Management Industrial Association). We thank the iSimangaliso Marine Protected Area and iSimangaliso Wetland Park management and Ezemvelo KZN Wildlife, together with the Maputo National Park and Park Warden, M. Gonçalves, for their ongoing support of this research. The Australian New South Wales component of this work was completed under NSW DPI Research Permit Section 37 (Ref. PO1/0059A-2.0) and Animal Care and Ethics permit 07/08. Additional data were sourced from Australia’s Integrated Marine Observing System (IMOS) Animal Tracking Database (https://animaltracking.aodn.org.au). IMOS is enabled by the National Collaborative Research Infrastructure Strategy (NCRIS). This project would not have been possible without the dedicated support of NSW DPI shark research team, contracted SMART drumline fishers and countless volunteers. This research was funded by the NSW Government through the NSW Department of Primary Industries Bull shark tagging research programme and NSW Shark Management Program. M.J.R. is the UK-SA Bilateral Research Chair in Ocean Science and Marine Food Security, a partnership between Nelson Mandela University and the National Oceanography Centre, University of Southampton.
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N.L. conceived the study, analysed most of the data and drafted the paper. All authors contributed significantly to the writing and editing of this paper. R.D. collected data, conceived the study and helped draft the paper. A.F.S. and P.V. collected data and helped conceive the study. P.V. and M.J.R. analysed some data and helped conceive the study. D.S.S. helped conceive the study and helped in drafting the manuscript. M. Sheaves provided supervision and guidance in writing the paper. P.D.C., L.D., F.G.F., M. Soria, V.M.P., J.D.F., P.A.B. and G.B. collected data. A.B. provided supervision, helped conceive the study and helped edit the paper.
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Extended data
Extended Data Fig. 1 Sharks stay near the surface and experience colder temperatures in upwelling zones.
a) The percentage of time at temperature and depth bins based on PSAT-tag recordings for sharks tagged in the Breede River, South Africa: when in the Breede River, the upwelling zone, and past the upwelling zone, in the subtropic/tropics. b) Percentage of time at temperature and depth bins recorded by acoustic transmitters during detections for sharks tagged in Sydney Harbour, Australia: when inside Sydney Harbour (green), outside in the vicinity of Sydney Harbour, the upwelling zone, and past the upwelling zone, in the subtropic/tropics.
Extended Data Fig. 2 Bull shark detections when sharks are seasonally present at their seasonal distributional limits.
Percentages are given for detections inside and outside Sydney Harbour for sharks tagged in Sydney Harbour and for inside and outside the Breede River Estuary for sharks tagged in the Breede River Estuary.
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Supplementary Figs. 1–22 and Tables 1–15.
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Lubitz, N., Daly, R., Smoothey, A.F. et al. Climate change-driven cooling can kill marine megafauna at their distributional limits. Nat. Clim. Chang. 14, 526–535 (2024). https://doi.org/10.1038/s41558-024-01966-8
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DOI: https://doi.org/10.1038/s41558-024-01966-8
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