Fish and plants already reacting to climate change on SA coastline
Official stats track shifts in fish, kelp and kreef as oceans warm
They say animals don't like strange weather, and move off until it's over.
But when the weather stays strange, animals behave strangely themselves.
That's the story all along the South African coastline, where fish - and plants - are already reacting to climate change, according to the latest scientific research.
Numerous changes to coastal fauna and flora are listed in a climate-change adaptation and mitigation plan awaiting formal adoption by the department of agriculture, forestry & fisheries (Daff).
The document, which incorporates the collective wisdom of many experts, is the first overall picture of how climate change is already affecting SA's territorial waters - and what is likely to happen in future.
"There is a lot of evidence showing that species distributions have changed, and usually that is because things are warming," said Carl van der Lingen, a member of Daff's fisheries climate-change task team.
"None of us is sufficiently convinced that it is unequivocally climate change-induced. However, I would say it is likely."
Among the most significant changes are: An eastward shift in the distribution of anchovy, round herring and sardine since the mid-1990s. Durban may no longer see its massive annual sardine run;
An increase in massive algal blooms (red tides) that cause mass walkouts of west coast rock lobster (kreef) and a related eastward migration of the rock lobster, and possible decline in the species;
The spread of kelp, previously limited to the west coast and southwestern Cape but now seen as far east as the De Hoop Nature Reserve;
Subtropical and tropical fish species, such as spotted grunter, are moving south. About 30 marine and estuarine subtropical and tropical species on the east coast have expanded their distribution, suggesting a common cause;
Temperate fish are moving north into subtropical waters, possibly due to inshore cooling on the east coast; and
An increase in windy conditions in the southern Cape, causing an increase in "non- seagoing days" - which is a problem for small-scale line fishermen. Many of the documented changes are believed to be directly or indirectly linked to changes in ocean temperature, a global phenomenon believed to be caused by the ocean absorbing surplus warming energy in the earth's atmosphere.
Worldwide, surface waters (up to 700m deep) warmed by an average of 0.7ºC between 1900 and 2016. There has also been a decrease in dissolved oxygen.
Climate change has a knock-on effect on SA's fisheries economy, which directly employs 43,000 people and provides the lifeblood for many coastal communities. Impacts also have implications for conservation efforts, with vulnerable species likely to come under even more pressure.
By understanding climate change, the government can amend fishing policy to mitigate negative impacts on species diversity and the fishing industry.
Significant shifts in ocean temperature and currents coincide with a southward shift in the core of the South Atlantic high-pressure system - an atmospheric phenomenon that scientists predict will cause a decrease in rainfall for the Western Cape.
The recent influx of subtropical fish species along the east coast, and a decrease in temperate species, is another key focus. This trend has been observed in spearfishing records, which support the notion that warmer east coast temperatures are proving inviting for species from further north.
In response to these changes, Daff has developed a vulnerability index for each fishery. It rates line fish and small pelagic species as most vulnerable.
This week, several top fisheries experts said climate-change data was useful in helping the government to devise the appropriate policy response.
"Basically, there has been some success in linking distributional shifts in fish populations to climate-change effects," said University of Cape Town stalwart Doug Butterworth.
Johann Augustyn, co-author of a recent study of climate-change impacts, said recent global research suggested the situation was even more dangerous than previously thought, and "a lot of effects they thought would only happen in 50 years might happen in the next 20 years".
For Kevern Cochrane of Rhodes University, the causes of species distribution change remain elusive.
"Nevertheless, the challenges for fishing operations and management have been substantial."
Mitigation measures could include changes to the total allowable catch of certain species and support for affected fisheries and communities.