Earlier this summer, a friend of mine from South Africa shared with me the splendour of their annual sardine run:
Each year, massive shoals of sardines (Sardinops sagax, the South African pilchard, to be precise) move down the western coast, attracting predators of all shapes and sizes, including an estimated 18,000 bottlenose dolphins. Apart from direct economic impacts from selling and consuming the fish themselves, this event provides a spectacle for divers and tourists, injecting an estimated 1.48 million USD into local economies.
As always, I couldn’t help but pose the climate change question…what will happen as temperatures in the Indian Ocean continue to rise? Could this event, so bountiful for marine animals and humans alike, be impacted?
Roberts et al. (2010) explain the occurrence of this phenomenon as the formation of an envelope, of sorts, of suitable-temperature water that can move along the eastern South Africa coast under certain conditions – those conditions being a ‘natal pulse’ which acts as a gateway for this pocket of water to migrate upwards:
[Figure from Roberts et al. 2010]
How could climate change factor in? As these fish are ectotherms (their body temperature is controlled by the ambient water temperature) this migration could very well be dependent upon sea temperatures. The sardines prefer waters that are under ~21°C , so it has been postulated that with continued warming of ocean waters, the sardines may not travel as far up the coast, and thus provide less fuel for predator activity and socio-economic sustenance. As the surface waters of the Indian Ocean have already warmed an average of 0.11 °C per decade over 1950-2009 (Hoegh-Guldberg et al. 2014), there is justifiable reason to monitor this event for changes that could be tied to temperature. Although the sardine migration might seem relatively small in terms of it’s economic impact, it plays an undeniable role in the lives of oceanic predators and coastal South African citizens.
Although the impact of environmental shifts on fish migrations seemed fairly intuitive, my curiosity was piqued – could migrations on land also be impacted? Indeed, the annual trek of the wildebeest is at threat from another facet of climatic change – shifts in precipitation patterns. This is in addition to the poaching, habitat loss, and blocked migratory corridors that have placed a strain on populations throughout Asia and Africa.
As massive herds move across the continent, their voracious appetite for grass modifies vegetation communities, and predators’ similar enthusiasm for wildebeest as prey feeds into ecosystems throughout Africa. On the monetary side, tourism has yielded 1.2 billion USD in Kenya (2012) and 1.3 billion USD in Tanzania (2011).
An evolutionary modeling study by Boone et al. (2006) showed that vegetation (i.e. food supply) was the dominant influence in determining how the path of the migration came to be (75%), followed by precipitation (25%). The extent of vegetation can be hit hard by drought, as in 2009 when the wildebeest population of Amboseli declined by 85%.
With precipitation patterns projected to become more ‘extreme’ – meaning longer dry spells interspersed with high-intensity rain events, which often lead to flooding – there is a clear need to facilitate these animals so they have the greatest chance of success in the years to come. Proactive protections focusing on preserving wildebeest food sources, preventing urban encroachment on migration corridors, and enforcing poaching prevention, look to be necessary to conserve both the wildebeest populations and the species they interact with on their trek – humans included.
Boone, R. B., Thirgood, S. J., & Hopcraft, J. G. C. (2006). Serengeti wildebeest migratory patterns modeled from rainfall and new vegetation growth. Ecology,87(8), 1987-1994.
Hoegh-Guldberg, O., R. Cai, E.S. Poloczanska, P.G. Brewer, S. Sundby, K. Hilmi, V.J. Fabry, and S. Jung, 2014: The Ocean. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1655-1731.
Roberts, M. J., Van der Lingen, C. D., Whittle, C., & Van den Berg, M. (2010). Shelf currents, lee-trapped and transient eddies on the inshore boundary of the Agulhas Current, South Africa: their relevance to the KwaZulu-Natal sardine run. African Journal of Marine Science, 32(2), 423-447.