Complex and drastic changes taking place under the ocean’s surface has resulted in massive fish and shellfish mortalities along South Africa’s east and south coast.
According to the Department of Environment, Forestry and Fisheries , this is due to a marine heatwave as a result of a “very large Agulhas Current meander”.
This means that the Agulhas Current has changed course drastically, resulting in changing temperatures, ocean currents, water levels and water biochemistry, which has led to the fish mortalities.
Cape Agulhas is the southernmost tip of Africa and where the warm waters of the Indian Ocean and the icy Atlantic waters meet.
The department said when the marine heatwave first started, reports of fish swimming away from warm water were received, as well as some seaweed bleaching.
It said the “cold-water intrusion” resulted in fish and shellfish suffering from thermal shock, which stunned them in the shallows.
Some fish and shellfish found themselves too weak to avoid being washed out to shore.
The department warned coastal communities against collecting and eating the washed up fish and shellfish.
“Some of the fish may have been dead longer than you may think, some of the unfamiliar fish may be toxic and it is not yet clear whether the anomaly has also resulted in red tides of harmful algal blooms.”
An intense red tide, which resulted in the death of about 1,000kg of West Coast rock lobster, took place in the Western Cape in January.
The tide stretched along the northern shores of St Helena Bay to north of Lambert’s Bay, before moving south towards the Berg River estuary.
The marine heatwave due to changing currents in the Agulhas region resulted in water temperatures reaching 24°C.
This was accompanied by an “upswelling” of water, resulting in a 10°C to 15°C difference between the cold and warm waters of the coast.
The department said this can occur up to five times a year in the northern Agulhas current system, but rarely more than twice a year near Port Elizabeth due to the current weakening on its way south.
The latest heatwave has therefore been described as “irregular”.
More about marine heatwaves
Marine heatwaves can be caused by ocean currents, in addition to air-sea heat flux, which warms ocean surface temperatures from the atmosphere.
Winds can also play a role in marine heatwaves, as well as climate swings such as El Niño and La Niña.
South Africa has been experiencing wetter, cooler weather as a result of La Niña, which is only expected to subside in August.
The La Niña effect sees cool waters building up in the eastern Pacific, resulting in colder ocean surface temperatures, less water evaporation and widespread rainfall in certain parts of the world.