Below the Surface of South Africa’s Worst Drought On Record

Guest Author: Jeremy Shelton
Editor: Patrick Cooney

Between 2015 and 2018, South Africa’s City of Cape Town experienced its worst drought on record. The City’s water supply dams dropped to an all-time low of <20%, and temperatures soared. But while the media was transfixed on the city’s struggle with securing drinking water for its population of nearly 4 million people, the freshwater life inhabiting the rivers that feed these dams was also starting to feel the heat!

Figure 1: Theewaterskloof Dam – Cape Town’s largest water supply dam at 10% of its storage capacity during April 2018 (photo by Steve Benjamin)

Cape Town falls within one of the 35 Global Biodiversity Hotspots recognised by Conservation International, the Cape Floristic Region (CFR). While best known for its diverse and unique flora called ‘Fynbos’, the freshwater fishes inhabiting the rivers flowing through the Fynbos, while less diverse, also exhibit very high levels of endemism (20 of the 23 currently described species unique to the CFR) (Ellender et al. 2017). Sadly, these fishes face a severe threat of extinction  (14 of the 20 endemics are listed as Vulnerable, Endangered or Critically Endangered in the IUCN Red List) due to deterioration in habitat quality and quantity, as well as impacts from invasive plants and animals (Shelton et al. 2017). Moreover, the region’s freshwater environments are also predicted to be particularly vulnerable to climate change, with conservative models forecasting a 2°C increase in mean water temperature and 20% reduction in flow over the next 30–50 years (Dallas and Rivers-Moore 2014), but until recently climate change impacts on these fishes had not been studied.

Figure 2: Dr Jeremy Shelton from the Freshwater Research Centre observes a school of endemic Breede River redfin (Pseudobarbus burchelli) in a headwater stream an hour outside Cape Town (photo by Steve Benjamin)

In 2014 South African non-profit the Freshwater Research Centre (FRC), in collaboration with the South African Institute for Aquatic Biodiversity (SAIAB) and local conservation authority CapeNature,  embarked on a series of studies funded by the Water Research Commission (WRC) and the Table Mountain Fund (TMF) to evaluate how these threatened freshwater fishes are likely to respond to a warmer and drier climate. The unexpected drought provided a natural experiment for investigating climate change impacts in these rivers – a window into conditions expected to become a lot more common in the decades ahead, particularly higher water temperatures and reduced flows.

Figure 3: A non-native rainbow trout in a CFR headwater stream. Trout pose a predatory threat to endemic species like the Breede River redfin (photo by Jeremy Shelton)

A combination of field surveys, thermal experiments and predictive modelling were employed to gain insights into what climate change spells for CFR fishes and their river habitats. The surveys show habitat being lost in the wild and contractions in the distributions of thermally sensitive native and non-native species as the Cape’s rivers warm (Shelton et al. 2018). Thermal experiments reveal a wide variety of thermal tolerances among the different fishes inhabiting CFR streams, with some of the native species found to be highly sensitive to even a slight increase in water temperature (Dallas et al. 2017). Finally, the species distribution models warn of a high risk of endemic species extinctions (for example the native Breede River redfin) as soon as this century under a scenario of 2°C increase in mean water temperature and 20% reduction in flow (Dallas et al. 2017).

Figure 4: Species distribution models showing contraction of Breede River redfin distribution under different climate change scenarios (figure from Dallas et al. 2017). Warmer colours (orange and red) show areas predicted by the model to be more suitable for the species.

In the face of change and uncertainty, one thing is becoming abundantly clear: balancing water for people and the environment is about to become a whole lot more complex. Now the challenge is to bring these findings to managers and policy makers to effect on-the-ground change (Bragg et al. 2017); only coordinated action from government, policy-makers and civil society will ensure that these freshwater fishes remain a part of the Western Cape’s rivers for many decades to come!

Figure 5: Dr Helen Dallas (FRC director) communicating the research findings to freshwater managers and decision-makers (photo by Jeremy Shelton)
References

Bragg CJ, Paxton BR, Shelton JM, Bovim LA and Dallas HF (2017). Freshwater Fishes of the Cape Fold Ecoregion and Climate Change: Volume 2: Policy Uptake Strategy. Prepared on behalf of the Table Mountain Fund by the Freshwater Research Centre. Pp. 14. thetablemountainfund.org.za/wp-content/uploads/2017/11/TMF-Vol-2-Policy-Uptake-Report_30-Sep-2017.pdf

Dallas HF, Shelton JM, Paxton BR & Weyl OLF (2017) Freshwater Fishes of the Cape Fold Ecoregion and Climate Change: Volume 1: Research Synthesis. Prepared on behalf of the Table Mountain Fund by the Freshwater Research Centre. Pp. 12. thetablemountainfund.org.za/wp-content/uploads/2017/11/TMF-Vol-1-Research-Synthesis_-30-Sep-2017.pdf

Dallas HF and Rivers‐Moore N A (2014). Ecological consequences of global climate change for freshwater ecosystems in South Africa. South African Journal of Science 110: 48–58 http://dx.doi.org/10.1590/sajs.2014/20130274

Shelton JM, Weyl OLF, Chakona A, Ellender BR, Esler KJ, Impson ND, Jordaan MS, Marr SM, Ngobela T, Paxton BR, Van Der Walt JA (2017) Vulnerability of Cape Fold Ecoregion freshwater fishes to climate change and other human impacts. Aquatic Conservation: Marine and Freshwater Ecosystems 28: 68-77 https://doi.org/10.1002/aqc.2849

Shelton JM, Weyl OLF, Esler KJ, Paxton BR, Impson DN and Dallas HF (2018) Temperature mediates the impact of non-native rainbow trout on native freshwater fishes in South Africa’s Cape Fold Ecoregion. Biological Invasions https://doi.org/10.1007/s10530-018-1747-7

Ellender BR, Wasserman RJ, Chakona A, Skelton PH, Weyl OLF (2017). A review of the biology and status of Cape Fold Ecoregion freshwater fishes. Aquatic Conservation: Marine and Freshwater Ecosystems 27: 867-879 https://doi.org/10.1002/aqc.2730

About the Guest Author

Dr Jeremy Shelton

Jeremy SheltonSince a child, Jeremy has been drawn to watery environments, fascinated by the wonder of the world beneath the surface. Through his studies in freshwater conservation biology at the University of Cape Town (2000-2013), he felt an increasing need to share the hidden wonders and struggles of freshwater environments with the world, and seeks to achieve this through a blend of science, education and film.

Jeremy believes that “we only care about what we love, and we will only love those things that we come to know” and is especially passionate about sharing his love of rivers with the youth of today – the future custodians of our rivers and wetlands.  “I feel lucky to share this beautiful planet with such an insane diversity of wild and wonderful creatures, but it also troubles me that we have devastated so much of our wondrous world over such a short time. Without our help the natural splendor of this place will disappear for good, and be lost from the lives of our children’s children!”

Since 2014, Jeremy has been working as a freshwater conservation biologist at the Freshwater Research Centre, a non-profit based in Kommetjie (Cape Town, South Africa). In addition to research, Jeremy is spending more and more time communicating science through film and photography, and working on the Centre’s outdoor education programme, “Living Labs”.

Website: http://frcsa.org.za

Instagram: @belowtheflow

Vimeo: https://vimeo.com/user21257282

Twitter: @freshwatersa

Facebook: @freshwatersa

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